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The logo of the METI International Puerto Rico workshop. At the center is Charles Darwin, the nineteenth century British naturalist whose theory of evolution is central to assessing the likelihood and nature of extraterrestrial intelligence. To the left is the octopus, a creature that evolved sophisticated cognition and perception along an evolutionary path quite different from that of humans. To the right is the peacock, whose elaborate tail feathers evolved by sexual selection, a process that may also have been of central importance to the evolution of human intelligence. METI International, used with permisson.
In our galaxy, there may be, at least, tens of billions of habitable planets, with conditions suitable for liquid water on their surfaces. There may be habitable moons as well. On an unknown number of those worlds, life may have arisen. On an unknown fraction of life-bearing worlds, life may have evolved into complex multicellular, sexually reproducing forms.
The purposes of the newly created METI (Messaging to ExtraTerrestrial Intelligence) International include fostering multidisciplinary research in the design and transmission of interstellar messages, and building a global community of scholars from the natural sciences, social sciences, humanities, and arts concerned with the origin, distribution, and future of life in the universe.
On May 18 the organization sponsored a workshop which included presentations by biologists, psychologists, cognitive scientists, and linguists. This is the third and final installment of a series of articles about the workshop.
In previous installments, we’ve discussed some ideas about the evolution of intelligence that were featured at the workshop. Here we’ll see whether our Earthly experience can provide us with any clues about how we might communicate with aliens.
Many of the animals that we are most familiar with from daily life, like humans, cats, dogs, birds, fishes, and frogs are vertebrates, or animals with backbones. They are all descended from a common ancestor and share a nervous system organized according to the same basic plan.
Molluscs are another major group of animals that have been evolving separately from vertebrates for more than 600 million years. Although most molluscs, like slugs, snails, and shellfish, have fairly simple nervous systems, one group; the cephalopods, have evolved a much more sophisticated one. The common octopus, Octopus vulgaris, Is a cephalopod mollusc, has evolved sophisticated cognition and perception along a very different evolutionary path than have human beings and our relatives. The brain is located between the eyes. The large bulbous structure below the eyes is the mantle, a muscular organ involved in swimming. Public domain.
Cephalopods include octopuses, squids, and cuttlefishes. They show cognitive and perceptual abilities rivaling those of our close vertebrate kin. Since this nervous system has a different evolutionary history than of the vertebrates, it is organized in a way completely different from our own. It can give us a glimpse of the similarities and differences we might expect between aliens and ourselves.
David Gire, an associate professor of psychology at the University of Washington, and researcher Dominic Sivitilli gave a presentation on cephalopods at the Puerto Rico workshop. Although these animals have a sophisticated brain, their nervous systems are much more decentralized than that of familiar animals. In the octopus, sensing and moving are controlled locally in the arms, which together contain as many nerve cells, or neurons, as the brain. Dr. David Gire is an Assistant Professor in the Department of Psychology at the University of Washington and a behavioral neuroscientist. He presented at the Puerto Rico workshop on cephalopod intelligence.
The animal’s eight arms are extraordinarily sensitive. Each containing hundreds of suckers, with thousands of sensory receptors on each one. By comparison, the human finger has only 241 sensory receptors per square centimeter. Many of these receptors sense chemicals, corresponding roughly to our senses of taste and smell. Much of this sensory information is processed locally in the arms. When an arm is severed from an octopus’s body, it continues to show simple behaviors on its own, and can even avoid threats. The octopus’s brain simply acts to coordinate the behaviors of its arms.
Cephalopods have acute vision. Although their eyes evolved separately from those of vertebrates, they nonetheless bear an eerie resemblance. They have a unique ability to change the pattern and color of their skin using pigment cells that are under direct control of their nervous systems. This provides them with the most sophisticated camouflage system of any animal on Earth, and is also used for social signaling.
Despite the sophisticated cognitive abilities it exhibits in the lab, the octopus is largely solitary.
Cephalopod groups exchange useful information by observing one another, but otherwise exhibit only limited social cooperation. Many current theories of the evolution of sophisticated intelligence, like Miller’s sapiosexual hypothesis, which was featured in the second installment, assume that social cooperation and competition play a central role in the evolution of complicated brains. Since cephalopods have evolved much more impressive cognitive abilities than other molluscs, their limited social behavior is surprising.
Dominic Sivitilli is a post-baccalaureate researcher in the laboratory of David Gire, studying responses to chemical signals by the octopus. He is the co-presenter of a talk on cephalopod cogntition at the METI International Puerto Rico conference. METI International used with permission.
Maybe the limited social behavior of cephalopods really does set limits on their intelligence. However, Gire and Sivitilli speculate that perhaps “an intelligence capable of technological development could exist with minimum social acuity”, and the cephalopod ability to socially share information is enough. The individuals of such an alien collective, they suppose, might possess no sense of self or other.
Besides Gire and Sivitilli, Anna Dornhaus, whose ideas were featured in the first installment, also thinks that alien creatures might function together as a collective mind. Social insects, in some respects, actually do. She doubts, though, that such an entities could evolve human-like technological intelligence without something like Miller’s sapiosexuality to trigger a runaway explosion of intelligence.
But if non-sapiosexual alien technological civilizations do exist, we might find them impossible to comprehend. Given this possible gulf of incomprehension about social structure, Gire and Stivitilli suppose that the most we might aspire to accomplish in terms of interstellar communication is an exchange of mutually useful and comprehensible astronomical information.
Workshop presenter Alfred Kracher, a retired staff scientist at the Ames Laboratory of the University of Iowa, supposes that “the mental giants of the Milky Way are probably artificially intelligent machines… It would be interesting to find evidence of them, if they exist”, he writes, “but then what?” Kracher supposes that if they have emancipated themselves and evolved away from their makers, “they will have nothing in common with organic life forms, human or extraterrestrial. There is no chance of mutual understanding”. We will be able to understand aliens, he maintains, only if “it turns out that the evolution of extraterrestrial life forms is highly convergent with our own”.
Peter Todd, a professor of psychology from Indiana University, holds out hope that such convergence may actually occur. Earthly animals must solve a variety of basic problems that are presented by the physical and biological world that they inhabit.
They must effectively navigate through a world of surfaces, barriers and objects, finding food and shelter, and avoiding predators, parasites, toxins. Extraterrestrial organisms, if they evolve in an Earth-like environment, would face a generally similar set of problems. They may well arrive at similar solutions, just as the octopus evolved eyes similar to ours.
In evolution here on Earth, Todd notes, brain systems originally evolved to solve these basic physical and biological problems appear to have been re-purposed to solve new and more difficult problems, as some animals evolved to solve the problems of living and finding mates as members of societies, and then as one particular ape species went on to evolve conceptual reasoning and language. For example, disgust at bad food, useful for avoiding disease, may have been become the foundation for sexual disgust to avoid bad mates, moral disgust to avoid bad clan mates, and intellectual disgust to avoid dubious ideas.
If alien brains evolved solutions similar to the ones our brains did for negotiating the physical and biological world, they they might also have been re-purposed in similar ways. Alien minds might not be wholly different from ours, and thus hope exists for a degree of mutual understanding.
In the early 1970’s the Pioneer 10 and 11 spacecraft were launched on the first exploratory missions to the planet Jupiter and beyond. When their missions were completed, these two probes became the first objects made by humans to escape the sun’s gravitational pull and hurtle into interstellar space.
Because of the remote possibility that the spacecraft might someday be found by extraterrestrials, a team of scientists and scholars lead by Carl Sagan emplaced a message on the vehicle, etched on a metal plaque. The message consisted, in part, of a line drawing of a man and a woman. Later, the Voyager 1 and 2 spacecraft carried a message that consisted, in part, of a series of 116 digital images encoded on a phonographic record. The use of images in interstellar communication. In 1977, NASA launched the Voyager 1 and 2 spacecraft on a mission to explore the outer solar system. Destined to wander interstellar space forever following the completion of their mission, each spacecraft carried an interstellar message encoded on a phonographic record. The message, designed by SETI pioneers Carl Sagan and Frank Drake and their collaborators, included 116 digital images. This image is intended to show extraterrestrials how human beings eat and drink. Will extraterrestrials understand such images? The limited quality of the image reflects the state of digital imaging technology in the 70’s National Astronomy and Ionosphere Center, public domain.
The assumption that aliens would see and understand images seems reasonable, since the octopus evolved an eye so similar to our own. And that’s not all. The evolutionary biologists Luitfried Von Salvini-Plawen and Ernst Mayr showed that eyes, of various sorts, have evolved forty separate times on Earth, and vision is typically a dominant sense for large, land dwelling animals. Still, there are animals that function without it, and our earliest mammalian ancestors were nocturnal. Could it be that there are aliens that lack vision, and could not understand a message based on images?
In his short story, The Country of the Blind, the great science fiction writer H. G. Wells imagined an isolated mountain village whose inhabitants had been blind for fifteen generations after a disease destroyed their vision.
A lost mountain climber, finding the village, imagines that with his power of vision, he can easily become their king. But the villagers have adapted thoroughly to a life based on touch, hearing, and smell. Instead of being impressed by their visitor’s claim that he can ‘see’, they find it incomprehensible. They begin to believe he is insane. And when they seek to ‘cure’ him by removing two strange globular growths from the front of his head, he flees. The Mexican blind cavefish (Astyanax mexicanus) has lived in the total darkness of a cave system in central Mexico for more than a million years, and has evolved the loss of its eyes. Astyanax possess a sense that land dwelling animals lack. The lateral line sense, which is present in all fishes, allows these animals to sense their near surroundings based on pressure differences in fields of water flow around their bodies. They also have an acute sense of taste, with taste receptors on their bodies as well as in their mouths. The evolution of cave dwelling intelligent life is probably unlikely, since large brains are metabolically expensive, and food is scarce in caves. On the surface, plants capture energy from sunlight and form the base of the food chain. State Museum of Natural History, Karlsruhe.
Could their really be an alien country of the blind whose inhabitants function without vision? Workshop presenter Dr. Sheri Wells-Jensen, an associate professor of Linguistics at Bowling Green State University, doesn’t need to imagine the country of the blind, because, in a sense, she lives there. She is blind, and believes that creatures without vision could achieve a level of technology sufficient to send interstellar messages. “Sighted people”, she writes, “tend to overestimate the amount and quality of information gathered by vision alone”. Dr. Sheri Wells-Jensen is an associate professor of linguistics at Bowling Green State University. She presented at talk at the Puerto Rico workshop on alternative perceptual systems and interstellar communications. METI International, used with permission.
Bats and dolphins image their dimly lit environments with a kind of naturally occurring sonar called echolocation. Blind human beings can also learn to echolocate, using tongue clicks or claps as emitted signals and analyzing the returning echoes by hearing. Some can do so well enough to ride a bicycle at a moderate pace through an unfamiliar neighborhood. A human can develop the touch sensitivity needed to read braille in four months. A blind marine biologist can proficiently distinguish the species of mollusc shells by touch.
Wells-Jensen posits a hypothetical civilization which she calls the Krikkits, who lack vision but possess sensory abilities otherwise similar to those of human beings. Could such beings build a technological society? Drawing on her knowledge of the blind community and a series of experiments, she thinks they could.
Finding food would present few special difficulties, since blind naturalists can identify many plant species by touch. Agriculture could be conducted as modern blind gardeners do it, by marking crops using stakes and piles of rock, and harvesting by feel. The combination of a stick used as a cane to probe the path ahead and echolocation make traveling by foot effective and safe. A loadstone compass would further aid navigational abilities. The Krikkits might use snares rather than spears or arrows to trap animals, making tools by touch.
Mathematics is vital to building a technological society. For most human beings, with our limited memory, a paper and pencil or a blackboard are essential for doing math. Krikkits would need to find other such aids, such as tactual symbols on clay tablets, abacus-like devices, or patterns sewn on hides or fabric.
Successful blind mathematicians often have prodigious memories, and can perform complex calculations in their heads. One of history’s greatest mathematicians, Leonard Euler, was blind for the last 17 years of his life, but remained mathematically productive through the use of his memory.
The obstacles to a blind society developing technology may not be insurmountable. Blind people are capable of handling fire and even working with molten glass. Krikkits might therefore use fire for cooking, warmth, to bake clay vessels, and smelt metal ores. Initially there only astronomical knowledge would be of the sun as a source of heat. Experiments with loadstones and metals would lead to a knowledge of electricity.
Eventually, the Krikkits might imitate their sonar with radio waves, inventing radar. If their planet possessed a moon or moons, radar reflections from them might provide their first knowledge of astronomical objects other than their sun. Radar would also enable them to learn for the first time that their planet is round.
The Krikkits might learn to detect other forms of radiation like X-rays and ‘light’. The ability to detect this second mysterious form of radiation might allow them to discover the existence of the stars and develop an interest in interstellar communication.
What sorts of messages might they send or understand? Well-Jensen believes that line drawings, like the drawing of the man and the woman on the Pioneer plaque, and other such pictorial representations might be an impenetrable mystery to them. On the other hand, she speculates that Krikkits might represent large data sets through sound, and that their counterpart to charts and graphs might be equally incomprehensible to us.
Images might pose a challenge for the Krikkits, but perhaps, Wells-Jensen concedes, not an impossible one. There is evidence that bats image their world using echolocation. Kikkits might be likely to evolve similar abilities, though Wells-Jensen believes they would not be essential for making tools or handling objects.
Perhaps humans and Krikkits could find common ground by transmitting instructions for three dimensional printed objects that could be explored tactually. Wells-Jensen thinks they might also understand mathematical or logical languages proposed for interstellar communication.
The diversity of cognition and perception that we find here on Earth teaches us that if extraterrestrial intelligence exists, it is likely to be much more alien than much of science fiction has prepared us to expect. In our attempt to communicate with aliens, the gulf of mutual incomprehension may yawn as wide as the gulf of interstellar space. Yet this is a gulf we must somehow cross, if we wish ever to become citizens of the galaxy.
The peahen (at left) and the peacock (at right). The peacock’s elaborate plumage and many other similar animal ornaments posed a troubling difficulty for Charles Darwin in his development of the theory of evolution, since they seemed to have no value for survival. The peacocks that were everywhere present in English gardens were a frustrating and ever-present reminder of the difficulty. “The sight of a feather in a peacock’s tail”, Darwin wrote, “whenever I gaze at it, makes me sick!”. Darwin solved the problem with his theory of sexual selection, which posits that such ornaments evolved because they help animals to woo mates, and thereby pass the trait into the next generation. Since mating in peafowl is by female choice, elaborate tail feathers are vital to the peacock, but unnecessary to the peahen. Sexual selection is increasingly recognized as a central evolutionary process by modern biologists. Evolutionary psychologist Dr. Geoffrey Miller posits that the enormous human brain evolved by sexual selection through the choices of both genders. Understanding how human intelligence evolved here on Earth is critical to understanding whether or not alien civilizations are likely to exist. Picture is from the Miho Museum, Shiga, Japan, 1781, public domain.
In the first instalment of this series, we saw that intelligence, of various sorts, is widespread across the animal kingdom. Workshop presenter Anna Dornhaus, who studies collective decision-making in insects as an associate professor at the University of Arizona, showed that even insects, with their diminutive brains, exhibit a surprising cognitive sophistication. Intelligence, of various sorts, is a likely and probable evolutionary product.
Animals evolve the cognitive abilities that they need to meet the demands of their own particular environments and lifestyles. Sophisticated brains and cognition have evolved many times on Earth, in many separate evolutionary lineages. But, of the millions of evolutionary lineages that have arisen on Earth in the 600 million years since complex life appeared, only one, that which led to human beings, produced the peculiar combination of cognitive traits that led to a technological civilization. What this tells us is that technological civilization is not the inevitable product of a long term evolutionary trend, it is rather the quirky and contingent product of particular circumstances. But what might those circumstances have been, and just how special and improbable were they?
Dr. Geoffery Miller is an associate professor of psychology at the University of New Mexico, and is the author of a 2001 book, The Mating Mind, where he explains his theory that human intelligence evolved by sexual selection to a general audience. He presented at the METI Institute conference in Puerto Rico, in May 2016. Picture used with permission.
Workshop presenter Geoffrey Miller is an associate professor of psychology at the University of New Mexico. Miller thinks he has an answer to the question of what the special circumstances that produced human evolution were. Our protohuman ancestors inhabited the African savanna. But so do many other mammals that don’t need enormous brains to survive there. The evolutionary forces driving the production of our large brains, Miller surmises, can’t be due to the challenges of survival. He thinks instead that human evolution was guided by an intelligence. But Miller is no creationist, nor does he have the alien monolith from the 1960’s science fiction classic 2001: A Space Odyssey in mind. Miller’s guiding intelligence is the intelligence that our ancestors themselves used when they selected their mates.
Miller’s theory harkens back to the ideas of the founder of modern evolutionary theory, the nineteenth century British naturalist Charles Darwin. Darwin proposed that evolution works through a process of natural selection. Animal offspring vary one from another, and are produced in too great of numbers for all of them to survive. Some starve, some are eaten by predators, others fall prey to the numerous other hazards of the natural world. A few survive to produce offspring, thereby passing on the traits that allowed them to survive. Down the generations, traits that aided survival become more elaborate and useful and traits that did not, vanished. Charles Darwin published his theory of evolution, in his book, The Origin of Species, in 1859. The theory was inspired, in part, by observations he made during his five year voyage as a naturalist on board the HMS Beagle and has become the central principle of much of modern biology. Picture by George Richmond (1830’s) public domain.
But Darwin was troubled by a serious problem with his theory. He knew that many animals have prominent traits that don’t seem to contribute to their survival, and are even counterproductive to it. The bright colors of many insects, the colors, elaborate plumage, and songs of birds, the huge antlers of elk, were all prominent and costly traits that couldn’t be explained by his theory of natural selection. Peacocks, with their elaborate tail feathers were everywhere in English gardens, and came to torment him.
At last, Darwin found the solution. To produce offspring, an animal must do more than just survive, it must find a partner to mate with. All the traits which worried Darwin could be explained if they served to make their bearers sexier and more beautiful to prospective mates than other competing members of their own gender. If peahens like elaborate plumage, then in each generation, they will choose to mate with the males with the most elaborate tail feathers, and reject the rest. Through the competition for mates, peacock tails will become more and more elaborate down the generations. Darwin called his new theory sexual selection.
Many subsequent evolutionary biologists regarded sexual selection as of limited importance, and lumped it in with natural selection, which was said to favor traits conducive to survival and reproductive success. However, in recent decades evolutionary biologists have come to view sexual selection in a much more favorable light. Geoffrey Miller proposed that the human brain evolved through sexual selection. Human beings, he supposes, are sapiosexual; that is, they are sexually attracted by intelligence and its products. The preference for selecting intelligent mates produced greater intelligence, which in turn allowed our ancestors to become more discerning in selecting more intelligent mates, producing a kind of amplifying feedback loop, and an explosion of intelligence.
On this account, language, music, dancing, humor, art, literature, and perhaps even morality and ethics exist because those who were good at them were deemed sexier, or more trustworthy and reliable, and were thus more successful in securing mates than those who weren’t. The elaborate human brain is like the elaborate peacock’s tail. It exists for wooing mates and not for survival. There are some important ways in which protohumans were different from peafowl. Both males and females are choosy and both have large brains. Protohumans, unlike peafowl, probably formed monogamous pair bonds. Miller’s theory has complexities that space won’t permit us to explore here. To show that his theory can work, Miller needed to develop a computer model.
The evolution of protohuman intelligence through geography and time. Homo egaster lived in the early Pleistocene between 1.9 and 1.4 million years ago and had a brain about half the size of modern Homo sapiens. It developed advanced stone tools, and may have domesticated fire. It was closely related to Homo erectus. Homo antecessor lived from 1.2 million to 800,000 years ago and spread from Africa into Europe. It’s brain was also about half as large as that of ours. Homo rhodesiensis lived about 120,000 to 300,000 years ago. Our species, Homo sapiens, arose in Africa about 200,000 years ago and spread throughout much of the world. Homo neanderthalensis had a brain capacity somewhat larger than that of modern humans, and its larger eye sockets suggest keener vision. They disappeared about 30,000 years ago, and may have died out, in part, through competition with Homo sapiens and cooling of the climate. Public Library of Science 2003.
If Miller is right, then just how probable is the evolution of a technological civilization, and how likely is it that we will find them elsewhere in the galaxy? Miller thinks that if complex life exists on other planets or moons, it is likely to evolve reproduction through sex, just as has happened here on Earth. For complex organisms that depend on a large and complicated body of genetic information, most mutations will be neutral or harmful. In sexual reproduction half the genes of one’s offspring come from each parent. Without this mixing of genes from other individuals, asexual lineages are likely to falter and go extinct due to an accumulation of harmful mutations. Unless sexually reproducing creatures choose their mates purely at random, sexual selection is an inevitability. So, the basic conditions for runaway sexual selection to produce a brain suited to language and technology probably exists on other worlds with complex life.
One problem, though, that Anna Dornhaus pointed out, is that in sexual selection, the trait that gets exaggerated is essentially arbitrary. There are many bird species with elaborate plumage, but none exactly like the peacock. There are many species where brains and cognitive traits matter for mating success, like the singing ability of nightingales and many other birds, or gibbons, or whales. Male bower birds build complicated structures, called bowers, out of found items, like sticks and leaves and stones and shells, to attract a female. Chimpanzees engage in complex power struggles that involve negotiation, grooming, and fighting their way to the top.
But despite the selective success of cognition and braininess in many species, our specific human sort of intelligence, with language and technology, has happened only once on Earth, and therefore might be rare in the universe. If our ancestors had found big noses rather than big brains sexy, then we might now have enormous noses rather than enormous radio telescopes capable of signaling to other worlds.
Miller is more optimistic. “It’s a rare accident” he writes, in the sense that mate preferences only rarely turn ‘sapiosexual’, focused so heavily on conspicuous displays of general intelligence… On the other hand, I think it’s likely that in any biosphere, sexual selection would eventually stumble into sapiosexual mate preferences, and then you’d get human-level intelligence and language of some sort. It might only arise in 1 out of every 100 million species though,…I suspect that in any biosphere with sexually reproducing complex organisms and a wide variety of species, you’d quite likely get at least one lineage stumbling into the sapiosexual niche within a billion years”.
A planet or moon is currently deemed potentially habitable if it orbits its parent star within the right distance range for liquid water to exist on its surface. This distance range is called the habitable zone. Since stars evolve with time, the duration of habitability is limited. Such matters can be explored through climate modeling, informed by what we know of the climates of Earth and other worlds within our solar system, and about the evolution of stars.
Current thinking is that Earth’s total duration of habitability is 6.3 to 7.8 billion years, and that our world may remain habitable for another 1.75 billion years. Since complex life has already existed on Earth for 600 million years, this seems a generous amount of time for complex life on a similar planet to stumble upon Miller’s sapiosexual niche. Stars of smaller mass than the sun are stable on longer timescales, some perhaps capable of sustaining worlds with liquid water for a hundred billion years. If Miller’s estimates are reasonable, then there may be worlds enough and time for an abundance of sapiosexual alien civilizations in our galaxy.
A central message of the METI Institute workshop is that, animals evolve whatever sort of intelligence is necessary for them to survive and reproduce under the circumstances in which they find themselves. Human-style intelligence, with language and technology, is a peculiar quirk of particular and improbable evolutionary circumstances. But we don’t know just how improbable. Given the vastness of time and number of worlds potentially available for the roll of the evolutionary dice, alien civilizations might be reasonably abundant, or they might be once-in-a-billion galaxies rare. We just don’t know. Better knowledge of the evolution of life and intelligence here on Earth might allow us to improve our estimates.
If alien civilizations do exist, what can life on Earth tell us about what their minds and senses are likely to be like? Are they, like us, visually oriented creatures, or might they rely on other senses? Can we expect that their minds might be similar enough to ours to make meaningful communication possible? These intriguing questions will be the subject of the third and final installment of this series.
The face of a dimorphic jumping spider (Maevia inclemens). Spiders have a very different evolutionary history from more familiar animals with backbones, and function in a different regime of body sizes. Their sensory endowment is thus evocative of what we might find in aliens. Spiders typically have a total of eight eyes, In this image, four eyes are clearly visible as shiny black globes, and two additional eyes are partially visible around the side of the head. The large frontal eyes provide the acute vision needed to recognize and capture prey. The other eyes provide the spider with a broad field of view, extending even behind the head which allows it to detect a potential meal, and to avoid predators. This image was taken in 2005 by an author named ‘Opoterser’ for open use.
Is it likely that human level intelligence and technological civilization has evolved on other worlds? If so, what kinds of sensory and cognitive systems might extraterrestrials have? This was the subject of the workshop ‘The Intelligence of SETI: Cognition and Communication in Extraterrestrial Intelligence’ held in Puerto Rico on May 18, 2016. The conference was sponsored by the newly founded METI International (Messaging to ExtraTerrestrial Intelligence). One of the organization’s central goals is to build an interdisciplinary community of scholars concerned with designing interstellar messages that can be understood by non-human minds.
METI International
At present, the only clues we have to the nature of extraterrestrial minds and perception are those that can be garnered by a careful study of the evolution of mind and perception here on Earth. The workshop included nine speakers from universities in the United States and Sweden, specializing in biology, psychology, cognitive science, and linguistics. It had sessions on the evolution of cognition and the likely communicative and cognitive abilities of extraterrestrials.
Doug Vakoch, a psychologist and the founder and president of METI International, notes that astronomers and physicists properly concern themselves largely with the technologies needed to detect alien intelligence. However, finding and successfully communicating with aliens may require attention to the evolution and possible nature of alien intelligence. “The exciting thing about this workshop”, Vakoch writes, “is that the speakers are giving concrete guidelines about how to apply insights from basic research in biology and linguistics to constructing interstellar messages”. In this, the first installment dealing with the conference, we’ll focus on the question of whether the evolution of technological societies on other planets is likely to be common, or rare. Dr. Douglas Vakoch is a Professor of clinical psychology and the founder and president of METI International. Photo by Mara Lavitt, used with permission.
We now know that most stars have planets, and rocky planets similar to or somewhat larger than the Earth or Venus are commonplace. Within this abundant class of worlds, there are likely to be tens of billions with conditions suitable for sustaining liquid water on their surfaces in our galaxy. We don’t yet know how likely it is that life will arise on such worlds. But suppose, as many scientists suspect, that simple life is abundant. How likely is it that alien civilizations will appear; civilizations with which we could communicate and exchange ideas, and which could make their presence known to us by signaling into space? This was a central question explored at the conference.
In addressing such questions, scientists have two main sets of clues to draw on. The first comes from the study of the enormous diversity of behavior and nervous and sensory systems of the animal species that inhabit our Earth; an endeavor that has been called cognitive ecology. The second set of clues come from modern biology’s central principle; the theory of evolution. Evolutionary theory can provide scientific explanations of how and why various senses and cognitive systems have come to exist here on Earth, and can guide our expectations about what might exist elsewhere.
Artist’s impression of three newly-discovered exoplanets orbiting an ultracool dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).The basics of the electrochemical signalling that make animal nervous systems possible have deep evolutionary roots. Even plants and bacteria have electrochemical signalling systems that share some basic features with those in our brains. Conference presenter Dr. Anna Dornhaus studies how social insects make decisions collectively as an associate professor at the University of Arizona. She defines cognitive ability as the ability to solve problems with a nervous system, and sometimes also by social cooperation. An animal is more ‘intelligent’ if its problem solving abilities are more generalized. Defined this way, intelligence is widespread among animals. Skills traditionally thought to be the sole province of primates (monkeys and apes, including human beings) have now been shown to be surprisingly common. Dr. Anna Dornhaus is an Associate Professor of Ecology and Evolutionary Biology at the University of Arizona, and a presenter at the Puerto Rico conference
For example, cognitive skills like social learning and teaching, generalizing from examples, using tools, recognizing individuals of one’s species, making plans, and understanding spatial relationships have all been shown to exist in arthropods (an animal group consisting of insects, spiders, and crustaceans). The evidence shows the surprising power of the diminutive brains of insects, and indicates that we know little of the relationship between brain size and cognitive ability.
But different animals often have different sets of cognitive skills, and if a species is good at one cognitive skill, that doesn’t necessarily mean it will be good at others. Human beings are special, not because we have some specific cognitive ability that other animals lack, but because we possess a wide range of cognitive abilities that are more exaggerated and highly developed than in other animals.
Termite mounds demonstrate that architecture and agriculture are not unique to humans. Housing one to two million inhabitants, they can reach 5 meters (17 feet) or more in height, and also extend beneath the surface of the ground. They are organized to ensure that appropriate levels of oxygen, moisture, and temperature are maintained. Although the inhabitants of a termite mound collectively weigh only 15 kilograms (33 lb), a typical mound will, in an average year, move a quarter of a metric ton (550 lb) of soil, and several tons of water. Using carefully prepared plant materials, termites “farm” a species of fungus that occupies eight times more space in the mound than they do. Photo taken by Brain Voon Yee Yap of cathedral termite mounds in the Northern Territories of Australia for open use.
Although the Earth, as a planet, has existed for 4.6 billion years, complex animals with hard body parts don’t appear in the fossil record until 600 million years ago, and complex life didn’t appear on land until about 400 million years ago. Looking across the animal kingdom as a whole, three groups of animals, following separate evolutionary paths, have evolved especially complex nervous systems and behaviors. We’ve already mentioned arthropods, and the sophisticated behaviors mediated by their diminutive yet powerful brains.
Molluscs, a group of animals that includes slugs and shellfish, have also produced a group of brainy animals; the cephalopods. The cephalopods include octopuses, squids, and cuttlefish. The octopus has the most complex nervous system of any animal without a backbone. As the product of a different evolutionary path, the octopus’s sophisticated brain has a plan of organization that is completely alien to that of more familiar animals with backbones.
The third group to have produced sophisticated brains are the vertebrates; animals with backbones. They include fishes, amphibians, reptiles, birds, and mammals, including human beings. Although all vertebrate brains bear a family resemblance, complex brains have evolved from simpler brains many separate times along different paths of vertebrate evolution, and each such brain has its own unique characteristics.
Along one path, birds have evolved a sophisticated forebrain, and with it, a flexible and creative capacity to make and use tools, an ability to classify and categorize objects, and even a rudimentary understanding of numbers. Following a different path, and based on a different plan of forebrain organization, mammals have also evolved sophisticated intelligence. Three groups of mammals; elephants, cetaceans (a group of aquatic mammals including dophins, porpoises, and whales), and primates (monkeys and apes, including human beings) have evolved the most complex brains on Earth.
Given the evidence that intelligent problem solving skills of various sorts have evolved many times over, along many different evolutionary pathways, in an amazing range of animal groups, one might suspect that Dornhaus believes that human-style cognitive abilities and civilizations are widespread in the universe. In fact, she doesn’t. She thinks that humans with their exaggerated cognitive abilities and unique ability to use language to express complex and novel sorts of information are a quirky and unusual fluke of evolution, and might, for all we know, be wildly improbable. Her argument that alien civilizations probably aren’t widespread resembles one stated by the imminent and influential American evolutionary biologist Ernst Mayr in his 1988 book Towards a New Philosophy of Biology.
There are currently more than 10 million different species of animals on Earth. All but one have failed to evolve the human level of intelligence. This makes the chance of evolving human intelligence less than one in 10 million. Over the last six hundred million years since complex life has appeared on Earth, there have been tens of million different animal species, each existing for roughly 1-10 million years. But, so far as we know, only one of them, Homo sapiens, ever produced a technological society. The human lineage diverged from that of other great ape species about 8 million years ago, but we don’t see evidence of distinctly human innovation until about 50,000 years ago, which is, perhaps, another indication of its rarity.
Despite the apparent improbability of human level intelligence evolving in any one lineage, Earth, as a whole, with its vast array of evolutionary lineages, has nonetheless produced a technological civilization. But that still doesn’t tell us very much. For the present, Earth is the only habitable planet that we know much of anything about. And, since Earth produced us, we are working with a biased sample. So we can’t be at all confident that the presence of human civilization on Earth implies that similar civilizations are likely to occur elsewhere.
For all we know, the quirky set of events that produced human beings might be so wildly improbable that human civilization is unique in a hundred billion galaxies. But, we don’t know for sure that alien civilizations are wildly improbable either. Dornhaus freely concedes that neither she nor anybody has a good idea of just how improbable human intelligence might be, since the evolution of intelligence is still so poorly understood.
Most current evolutionary thinking, following in the footsteps of Mayr and others, holds that human civilization was not the inevitable product of a long-term evolutionary trend, but rather the quirky consequence of a particular and improbable set of evolutionary events. What sort of events might those have been, and just how improbable were they? Dornhaus supports a popular theory proposed by Dr. Geoffrey Miller, an evolutionary psychologist who is an associate professor in the Department of Psychology at the University of New Mexico and who also spoke at the METI institute workshop.
In our next installment we’ll explore Miller’s theories in a bit more detail, and see why the abundance of extraterrestrial civilizations might depend on whether or not aliens think big brains are sexy.
The left image shows a mosaic of images of Titan taken by the Cassini spacecraft in near infrared light. Titan’s polar seas are visible as sunlight glints off of them. The right image is a radar image of Kraken Mare. Credit: NASA Jet Propulsion Laboratory.
Could there be life on Saturn’s large moon Titan? Asking the question forces astrobiologists and chemists to think carefully and creatively about the chemistry of life, and how it might be different on other worlds than it is on Earth. In February, a team of researchers from Cornell University, including chemical engineering graduate student James Stevenson, planetary scientist Jonathan Lunine, and chemical engineer Paulette Clancy, published a pioneering study arguing that cell membranes could form under the exotic chemical conditions present on this remarkable moon.
In many ways, Titan is Earth’s twin. It’s the second largest moon in the solar system and bigger than the planet Mercury. Like Earth, it has a substantial atmosphere, with a surface atmospheric pressure a bit higher than Earth’s. Besides Earth, Titan is the only object in our solar system known to have accumulations of liquid on its surface. NASA’s Cassini space probe discovered abundant lakes and even rivers in Titan’s polar regions. The largest lake, or sea, called Kraken Mare, is larger than Earth’s Caspian Sea. Researchers know from both spacecraft observations and laboratory experiments that Titan’s atmosphere is rich in complex organic molecules, which are the building blocks of life.
All these features might make it seem as though Titan is tantalizingly suitable for life. The name ‘Kraken’, which refers to a legendary sea monster, fancifully reflects the eager hopes of astrobiologists. But, Titan is Earth’s alien twin. Being almost ten times further from the sun than Earth is, its surface temperature is a frigid -180 degrees Celsius. Liquid water is vital to life as we know it, but on Titan’s surface all water is frozen solid. Water ice takes on the role that silicon-containing rock does on Earth, making up the outer layers of the crust.
The liquid that fills Titan’s lakes and rivers is not water, but liquid methane, probably mixed with other substances like liquid ethane, all of which are gases here on Earth. If there is life in Titan’s seas, it is not life as we know it. It must be an alien form of life, with organic molecules dissolved in liquid methane instead of liquid water. Is such a thing even possible?
The Cornell team took up one key part of this challenging question by investigating whether cell membranes can exist in liquid methane. Every living cell is, essentially, a self-sustaining network of chemical reactions, contained within bounding membranes. Scientists think that cell membranes emerged very early in the history of life on Earth, and their formation might even have been the first step in the origin of life.
Here on Earth, cell membranes are as familiar as high school biology class. They are made of large molecules called phospholipids. Each phospholipid molecule has a ‘head’ and a ‘tail’. The head contains a phosphate group, with a phosphorus atom linked to several oxygen atoms. The tail consists of one or more strings of carbon atoms, typically 15 to 20 atoms long, with hydrogen atoms linked on each side. The head, due to the negative charge of its phosphate group, has an unequal distribution of electrical charge, and we say that it is polar. The tail, on the other hand, is electrically neutral.
Here on Earth, cell membranes are composed of phospholipid molecules dissolved in liquid water. A phospholipid has a backbone of carbon atoms (gray), and also contains hydrogen (sky blue), phosphorus (yellow), oxygen (red), and nitrogen (blue). Due to the positive charge associated with the nitrogen containing choline group, and the negative charge associated with the phosphate group, the head is polar, and attracts water. It is therefore hydrophilic. The hydrocarbon tail is electrically neutral and hydrophobic. The structure of a cell membrane is due these electrical properties of phospholipids and water. The molecules form a double layer, with the hydrophilic heads facing outward, towards water, and the hydrophobic tails facing inward, towards one another. Credit: Ties van Brussel
These electrical properties determine how phospholipid molecules will behave when they are dissolved in water. Electrically speaking, water is a polar molecule. The electrons in the water molecule are more strongly attracted to its oxygen atom than to its two hydrogen atoms. So, the side of the molecule where the two hydrogen atoms are has a slight positive charge, and the oxygen side has a small negative charge. These polar properties of water cause it to attract the polar head of the phospholipid molecule, which is said to be hydrophilic, and repel its nonpolar tail, which is said to be hydrophobic.
When phospholipid molecules are dissolved in water, the electrical properties of the two substances work together to cause the phospholipid molecules to organize themselves into a membrane. The membrane closes onto itself into a little sphere called a liposome. The phospholipid molecules form a bilayer two molecules thick. The polar hydrophilic heads face outward towards the water on both the inner and outer surface of the membrane. The hydrophobic tails are sandwiched between, facing each other. While the phospholipid molecules remain fixed in their layer, with their heads facing out and their tails facing in, they can still move around with respect to each other, giving the membrane the fluid flexibility needed for life.
Phospholipid bilayer membranes are the basis of all terrestrial cell membranes. Even on its own, a liposome can grow, reproduce and aid certain chemical reactions important to life, which is why some biochemists think that the formation of liposomes might have been the first step towards life. At any rate, the formation of cell membranes must surely been an early step in life’s emergence on Earth.
At the left, water, consisting of hydrogen (H) and oxygen (O), is a polar solvent. Oxygen attracts electrons more strongly than hydrogen does, giving the hydrogen side of the molecule a net positive charge and the oxygen side a net negative charge. The delta symbol ( ) indicates that the charge is partial, that is less than a full unit of positive or negative charge. At right, methane is a non-polar solvent, due to the symmetrical distribution of hydrogen atoms (H) around a central carbon atom (C). Credit: Jynto as modified by Paul Patton.
If some form of life exists on Titan, whether sea monster or (more likely) microbe, it would almost certainly need to have a cell membrane, just like every living thing on Earth does. Could phospholipid bilayer membranes form in liquid methane on Titan? The answer is no. Unlike water, the methane molecule has an even distribution of electrical charges. It lacks water’s polar qualities, and so couldn’t attract the polar heads of phospholipid molecule. This attraction is needed for the phospholipids to form an Earth-style cell membrane.
Experiments have been conducted where phospholipids are dissolved in non-polar liquids at Earthly room temperature. Under these conditions, the phospholipids form an ‘inside-out’ two layer membrane. The polar heads of the phospholipid molecules are at the center, attracted to one another by their electrical charges. The non-polar tails face outward on each side of the inside-out membrane, facing the non-polar solvent.
At left, phospholipids are dissolved in water, a polar solvent. They form a bilayer membrane, with their polar, hydrophilic heads facing outward towards water, and their hydrophobic tails facing each other. At right, when phospholipids are dissolved in a non-polar solvent at Earthly room temperature, they form an inside-out membrane, with the polar heads attracting one another, and the non-polar tails facing outwards towards the non-polar solvent. Based on figure 2 from Stevenson, Lunine, and Clancy (2015). Credit: Paul Patton
Could Titanian life have an inside out phospholipid membrane? The Cornell team concluded that this wouldn’t work, for two reasons. The first is that at the cryogenic temperatures of liquid methane, the tails of phospholipids become rigid, depriving any inside-out membrane that might form of the fluid flexibility needed for life. The second is that two key ingredients of phospholipids; phosphorus and oxygen, are probably unavailable in the methane lakes of Titan. In their search for Titanian cell membranes, the Cornell team needed to probe beyond the familiar realm of high school biology.
Although not composed of phospholipids, the scientists reasoned that any Titanian cell membrane would nevertheless be like the inside-out phospholipid membranes created in the lab. It would consist of polar molecules clinging together electrically in a solution of non-polar liquid methane. What molecules might those be? For answers the researchers looked to data from the Cassini spacecraft and from laboratory experiments that reproduced the chemistry of Titan’s atmosphere.
Titan’s atmosphere is known to have a very complex chemistry. It is made mostly of nitrogen and methane gas. When the Cassini spacecraft analyzed its composition using spectroscopy it found traces of a variety of compounds of carbon, nitrogen, and hydrogen, called nitriles and amines. Researchers have simulated the chemistry of Titan’s atmosphere in the lab by exposing mixtures of nitrogen and methane to sources of energy simulating sunlight on Titan. A stew of organic molecules called ‘tholins’ is formed. It consists of compounds of hydrogen and carbon, called hydrocarbons, as well as nitriles and amines.
The Cornell investigators saw nitriles and amines as potential candidates for their Titanian cell membranes. Both are polar molecules that might stick together to form a membrane in non-polar liquid methane due to the polarity of nitrogen containing groups found in both of them. They reasoned that candidate molecules must be much smaller than phospholipids, so that they could form fluid membranes at liquid methane temperatures. They considered nitriles and amines containing strings of between three and six carbon atoms. Nitrogen containing groups are called ‘azoto’ –groups, so the team named their hypothetical Titanian counterpart to the liposome the ‘azotosome’.
Synthesizing azotosomes for experimental study would have been difficult and expensive, because the experiments would need to be conducted at the cryogenic temperatures of liquid methane. But since the candidate molecules have been studied extensively for other reasons, the Cornell researchers felt justified in turning to the tools of computational chemistry to determine whether their candidate molecules could cohere as a flexible membrane in liquid methane. Computational models have been used successfully to study conventional phospholipid cell membranes.
Acrylonitrile has been identified as a possible basis for cell membranes in liquid methane on Titan. It is known to be present in Titan’s atmosphere at a concentration of 10 parts per million and has been produced in laboratory simulations of the effects of energy sources on Titan’s nitrogen-methane atmosphere. As a small polar molecule capable of dissolving in liquid methane, it is a candidate substance for the formation of cell membranes in an alternative biochemistry on Titan. Light blue: carbon atoms, dark blue: nitrogen atom, white: hydrogen atoms. Credit: Ben Mills as modified by Paul Patton. Polar acrylonitrile molecules align ‘head’ to ‘tail’ to form a membrane in non-polar liquid methane. Light blue: carbon atoms, dark blue: nitrogen atoms, white: hydrogen atoms. Credit: James Stevenson.
The group’s computational simulations showed that some candidate substances could be ruled out because they would not cohere as a membrane, would be too rigid, or would form a solid. Nevertheless, the simulations also showed that a number of substances would form membranes with suitable properties. One suitable substance is acrylonitrile, which Cassini showed is present in Titan’s atmosphere at 10 parts per million concentration. Despite the huge difference in temperature between cryogenic azotozomes and room temperature liposomes, the simulations showed them to exhibit strikingly similar properties of stability and response to mechanical stress. Cell membranes, then, are possible for life in liquid methane.
Computational chemistry simulations show that acrylonitrile and some other small polar nitrogen containing organic molecules are capable of forming ‘azotosomes’ when they are dissolved on liquid methane. Azotosomes are small membrane bounded spherules like the liposomes formed by phospholipids when they are dissolved in water. The simulations show that acrylonitrile azotosomes would be both stable and flexible in cryogenically cold liquid methane, giving them the properties they need to function as cell membranes for hypothetical Titanian life, or for life on any world with liquid methane on its surface. The azotosome shown is 9 nanometers in size, about the size of a virus. Light blue: carbon atoms, dark blue: nitrogen atoms, white: hydrogen atoms. Credit: James Stevenson.
The scientists from Cornell view their findings as nothing more than a first step towards showing that life in liquid methane is possible, and towards developing the methods that future spacecraft will need to search for it on Titan. If life is possible in liquid methane, the implications ultimately extend far beyond Titan.
When seeking conditions suitable for life in the galaxy, astronomers typically search for exoplanets within a star’s habitable zone, defined as the narrow range of distances over which a planet with an Earth-like atmosphere would have a surface temperature suitable for liquid water. If methane life is possible, then stars would also have a methane habitable zone, a region where methane could exist as a liquid on a planet or moon, making methane life possible. The number of habitable worlds in the galaxy would be greatly increased. Perhaps, on some worlds, methane life evolves into complex forms that we can scarcely imagine. Maybe some of them are even a bit like sea monsters.
Halleys Comet, as seen in May 1986. Credit and copyright: Bob King.
The idea of panspermia — that life on Earth originated from comets or asteroids bombarding our planet — is not new. But new research may have given the theory a boost. Scientists from Japan say their experiments show that early comet impacts could have caused amino acids to change into peptides, becoming the first building blocks of life. Not only would this help explain the genesis of life on Earth, but it could also have implications for life on other worlds.
Dr. Haruna Sugahara, from the Japan Agency for Marine-Earth Science and Technology in Yokahama, and Dr. Koichi Mimura, from Nagoya University said they conducted “shock experiments on frozen mixtures of amino acid, water ice and silicate (forsterite) at cryogenic condition (77 K),” according to their paper. “In the experiments, the frozen amino acid mixture was sealed into a capsule … a vertical propellant gun was used to [simulate] impact shock.”
They analyzed the post-impact mixture with gas chromatography, and found that some of the amino acids had joined into short peptides of up to 3 units long (tripeptides).
Based on the experimental data, the researchers were able to estimate that the amount of peptides produced would be around the same as had been thought to be produced by normal terrestrial processes (such as lighting storms or hydration and dehydration cycles).
Artists concept of the stardust spacecraft flying throug the gas and dust from comet Wild 2. Credit: NASA/JPL“This finding indicates that comet impacts almost certainly played an important role in delivering the seeds of life to the early Earth,” said Sugahara. “It also opens the likelihood that we will have seen similar chemical evolution in other extraterrestrial bodies, starting with cometary-derived peptides.”
The earliest known fossils on Earth are from about 3.5 billion years ago and there is evidence that biological activity took place even earlier. But there’s evidence that early Earth had little water and carbon-based molecules on the Earth’s surface, so how could these building blocks of life delivered to the Earth’s surface so quickly? This was also about the time of the Late Heavy Bombardment, and so the obvious answer could be the collision of comets and asteroids with the Earth, since these objects contain abundant supplies of both water and carbon-based molecules.
A view of NASA’s Deep Impact probe colliding with comet Tempel 1, captured by the Deep Impact flyby spacecraft’s high-resolution instrument.
Space missions to comets are helping to confirm this possibility. The 2004 Stardust mission found the amino acid when it collected particles from Comet Wild 2. When NASA’s Deep Impact spacecraft crashed into Comet Tempel 1 in 2005, it discovered a mixture of organic and clay particles inside the comet. One theory about the origins of life is that clay particles act as a catalyst, allowing simple organic molecules to get arranged into more and more complex structures.
The news from the current Rosetta mission to comet 67P/Churyumov-Gerasimenko also indicates that comets are a rich source of materials, and more discoveries are likely to be forthcoming from that mission.
Jets of gas and dust are blasting from the active neck of comet 67P/Churyumov-Gerasimenko in this photo mosaic assembled from four images taken on 26 September 2014 by the European Space Agency’s Rosetta spacecraft at a distance of 26.3 kilometers (16 miles) from the center of the comet. Credit: ESA/Rosetta/NAVCAM/Marco Di Lorenzo/Ken Kremer/kenkremer.com
“Two key parts to this story are how complex molecules are initially generated on comets and then how they survive/evolve when the comet hits a planet like the Earth,” said Professor Mark Burchell from the University of Kent in the UK, commenting on the new research from Japan. “Both of these steps can involve shocks which deliver energy to the icy body… building on earlier work, Dr. Sugahara and Dr. Mimura have shown how amino acids on icy bodies can be turned into short peptide sequences, another key step along the path to life.”
“Comet impacts are normally associated with mass extinction on Earth, but this works shows that they probably helped kick-start the whole process of life in the first place,” said Sugahara. “The production of short peptides is the key step in the chemical evolution of complex molecules. Once the process is kick-started, then much less energy is needed to make longer chain peptides in a terrestrial, aquatic environment.”
The scientists also indicated that similar “kickstarting” could have happened in other places in our Solar System, such as on the icy moons Europa and Enceladus, as they likely underwent a similar comet bombardment.
Artist's impression of The Milky Way Galaxy. Based on current estimates and exoplanet data, it is believed that there could be tens of billions of habitable planets out there. Credit: NASA
Welcome back to our Fermi Paradox series, where we take a look at possible resolutions to Enrico Fermi’s famous question, “Where Is Everybody?” Today, we examine the possibility that the reason we’ve found no evidence of alien civilizations is because there are none out there.
It’s become a legend of the space age. The brilliant physicist Enrico Fermi, during a lunchtime conversation at Los Alamos National Laboratory in 1950, is supposed to have posed a conundrum for proponents of the existence of extraterrestrial civilizations.
If space traveling aliens exist, so the argument goes, they would spread through the galaxy, colonizing every habitable world. They should then have colonized Earth. They should be here, but because they aren’t, they must not exist.
This is the argument that has come to be known as “Fermi’s paradox”. The problem is, as we saw in the first installment, Fermi never made it. As his surviving lunch companions recall (Fermi himself died of cancer just four years later, and never published anything on the topic of extraterrestrial intelligence), he simply raised a question, “Where is everybody?” to which there are many possible answers.
Nuclear physicist Enrico Fermi won the 1938 Nobel Prize for a technique he developed to probe the atomic nucleus. He led the team that developed the world's first nuclear reactor, and played a central role in the Manhattan Project that developed the atomic bomb during World War II. In the debate over extraterrestrial intelligence, he is best known for posing the question 'Where is everybody?' during a lunchtime discussion at Los Alamos National Laboratory. His question was seen as the basis for the "Fermi Paradox". Credit: Smithsonian Institution Archives.
Welcome back to our Fermi Paradox series, where we take a look at possible resolutions to Enrico Fermi’s famous question, “Where Is Everybody?” Today, we examine the lunchtime conversation that started it all!
It’s become a kind of legend, like Newton and the apple or George Washington and the cherry tree. One day in 1950, the great physicist Enrico Fermi sat down to lunch with colleagues at the Fuller Lodge at Los Alamos National Laboratory in New Mexico and came up with a powerful argument about the existence of extraterrestrial intelligence, the so-called “Fermi paradox”.
But like many legends, it’s only partly true. Robert Gray explained the real history in a recent paper in the journal Astrobiology. Enrico Fermi was the winner of the 1938 Nobel Prize for physics, led the team that developed the world’s first nuclear reactor at the University of Chicago, and was a key contributor to the Manhattan Project that developed the atomic bomb during World War II. The Los Alamos Lab where he worked was founded as the headquarters of that project.
The prospect of alien invasion has sent shivers down the spines of science fiction fans ever since H. G. Wells published his classic “The War of the Worlds” in 1897. Drawing on the science of his times, Wells envisioned Mars as an arid dying world, whose inhabitants coveted the lush blue Earth. Wells’ portrayal of Martian imperialism had a political message. As an opponent of British colonialism, he wanted his countrymen to imagine what colonialism would be like from the other side. Although opponents of METI seldom explicitly invoke the spectre of alien invasion, some do view the human history of colonialism as a possible model for how aliens might treat us. The eminent physicist Stephen Hawking warned that “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn our well for the Native Americans”. The illustration from Well’s novel shows a Martian fighting machine attacking the British warship HMS Thunderchild.
(credit: Henrique Alvim Correa, 1906, for the novel “The War of the Worlds”)
Should we beam messages into deep space, announcing our presence to any extraterrestrial civilizations that might be out there? Or, should we just listen? Since the beginnings of the modern Search for Extraterrestrial Intelligence (SETI), radio astronomers have, for the most part, followed the listening strategy.
In 1999, that consensus was shattered. Without consulting with other members of the community of scientists involved in SETI, a team of radio astronomers at the Evpatoria Radar Telescope in Crimea, led by Alexander Zaitsev, beamed an interstellar message called ‘Cosmic Call’ to four nearby sun-like stars. The project was funded by an American company called Team Encounter and used proceeds obtained by allowing members of the general public to submit text and images for the message in exchange for a fee.
Similar additional transmissions were made from Evpatoria in 2001, 2003, and 2008. In all, transmissions were sent towards twenty stars within less than 100 light years of the sun. The new strategy was called Messaging to Extraterrestrial Intelligence (METI). Although Zaitsev was not the first to transmit an interstellar message, he and his associates where the first to systematically broadcast to nearby stars. The 70 meter radar telescope at Evpatoria is the second largest radar telescope in the world.
In the wake of the Evpatoria transmissions a number of smaller former NASA tracking and research stations collected revenue by making METI transmissions as commercially funded publicity stunts. These included a transmission in the fictional Klingon language from Star Trek to promote the premier of an opera, a Dorito’s commercial, and the entirety of the 2008 remake of the classic science fiction movie “The Day the Earth Stood Still”. The specifications of these commercial signals have not been made public, but they were most likely much too faint to be detectable at interstellar distances with instruments comparable to those possessed by humans.
Zaitsev’s actions stirred divisive controversy among the community of scientists and scholars concerned with the field. The two sides of the debate faced off in a recent special issue of the Journal of the British Interplanetary Society, resulting from a live debate sponsored in 2010 by the Royal Society at Buckinghamshire, north of London, England.
Alexander L. Zaitsev- Chief scientist of the Russian Academy of Science’s Institute of Radio Engineering and Electronics, and head of the group that transmitted interstellar messages using the Evpatoria Planetary Radar telescope. (credit: Rumin)
Modern SETI got its start in 1959, when astrophysicists Giuseppe Cocconi and Phillip Morrison published a paper in the prestigious scientific journal Nature, in which they showed that the radio telescopes of the time were capable of receiving signals transmitted by similar counterparts at the distances of nearby stars. Just months later, radio astronomer Frank Drake turned an 85 foot radio telescope dish towards two nearby sun-like stars and conducted Project Ozma, the first SETI listening experiment. Morrison, Drake, and the young Carl Sagan supposed that extraterrestrial civilizations would “do the heavy lifting” of establishing powerful and expensive radio beacons announcing their presence. Humans, as cosmic newcomers that had just invented radio telescopes, should search and listen. There was no need to take the risk, however small, of revealing our presence to potentially hostile aliens.
Drake and Sagan did indulge in one seeming exception to their own moratorium. In 1974, the pair devised a brief 1679 bit message that was transmitted from the giant Arecibo Radar Telescope in Puerto Rico. But the transmission was not a serious attempt at interstellar messaging. By intent, it was aimed at a vastly distant star cluster 25,000 light years away. It merely served to demonstrate the new capabilities of the telescope at a rededication ceremony after a major upgrade.
In the 1980’s and 90’s SETI researchers and scholars sought to formulate a set of informal rules for the conduct of their research. The First SETI Protocol specified that any reply to a confirmed alien message must be preceded by international consultations, and an agreement on the content of the reply. It was silent on the issue of transmissions sent prior to the discovery of an extraterrestrial signal.
David Brin- Space scientist, futurist consultant, and science fiction writer (credit: Glogger)A Second SETI Protocol was to have addressed the issue, but, somewhere along the way, critics charge, something went wrong. David Brin, a space scientist, futurist consultant, and science fiction writer was a participant in the protocol discussion. He charged that “collegial discussion started falling apart” and “drastic alterations of earlier consensus agreements were rubber-stamped, with the blatant goal of removing all obstacles from the path of those pursuing METI”.
Brin accuses “the core community that clusters around the SETI Institute in Silicon Valley, California”, including astronomers Jill Tartar and Seth Shostak of “running interference for and enabling others around the world- such as Russian radio astronomer Dr. Alexander Zaitsev” to engage in METI efforts. Shostak denies this, and claims he simply sees no clear criteria for regulating such transmissions.
Brin, along with Michael A. G. Michaud, a former U.S. Foreign Service Officer and diplomat who chaired the committee that formulated the first and second protocol, and John Billingham, the former head of NASA’s short lived SETI effort, resigned their memberships in SETI related committees to protest the alterations to the second protocol.
The founders of SETI felt that extraterrestrial intelligence was likely to be benign. Carl Sagan speculated that extraterrestrial civilizations (ETCs) older than ours would, under the pressure of necessity, become peaceful and environmentally responsible, because those that didn’t would self-destruct. Extraterrestrials, they supposed, would engage in interstellar messaging because of a wish to share their knowledge and learn from others. They supposed that ETCs would establish powerful omnidirectional beacons in order to assist others in finding them and joining a communications network that might span the galaxy. Most SETI searches have been optimized for detecting such steady constantly transmitting beacons.
Over the fifty years since the beginnings of SETI, searches have been sporadic and plagued with constant funding problems. The space of possible directions, frequencies, and coding strategies has only barely been sampled so far. Still, David Brin contends that whole swaths of possibilities have been eliminated “including gaudy tutorial beacons that advanced ETCs would supposedly erect, blaring helpful insights to aid all newcomers along the rocky paths”. The absence of obvious, easily detectable evidence of extraterrestrial intelligence has led some to speak of the “Great Silence”. Something, Brin notes, “has kept the prevalence and visibility of ETCs below our threshold of observation”. If alien civilizations are being quiet, could it be that they know something that we don’t know about some danger?
Alexander Zaitsev thinks that such fears are unfounded, but that other civilizations might suffer from the same reluctance to transmit that he sees as plaguing humanity. Humanity, he thinks, should break the silence by beaming messages to its possible neighbors. He compares the current state of humanity to that of a man trapped in a one-man prison cell. “We”, he writes “do not want to live in a cocoon, in a ‘one –man cell’, without any rights to send a message outside, because such a life is not INTERESTING! Civilizations forced to hide and tremble because of farfetched fears are doomed to extinction”. He notes that in the ‘60’s astronomer Sebastian von Hoerner speculated that civilizations that don’t engage in interstellar communication eventually decline through “loss of interest”.
METI critics maintain that questions of whether or not to send powerful, targeted, narrowly beamed interstellar transmissions, and of what the content of those transmissions should be needs to be the subject of broad international and public discussion. Until such discussion has taken place, they want a temporary moratorium on such transmissions.
Seth Shostak- SETI Institute radio astronomer (credit: B D Engler)On the other hand, SETI Institute radio astronomer Seth Shostak thinks that such deliberations would be pointless. Signals already leak into space from radio and television broadcasting, and from civilian and military radar. Although these signals are too faint to be detected at interstellar distances with current human technology, Shostak contends that with the rapid growth in radio telescope technology, ETCs with technology even a few centuries in advance of ours could detect this radio leakage. Billingham and Benford counter that to collect enough energy to tune in on such leakage; an antenna with a surface area of more than 20,000 square kilometers would be needed. This is larger than the city of Chicago. If humans tried to construct such a telescope with current technology it would cost 60 trillion dollars.
Shostak argues that exotic possibilities might be available to a very technologically advanced society. If a telescope were placed at a distance of 550 times the Earth’s distance from the sun, it would be in a position to use the sun’s gravitational field as a gigantic lens. This would give it an effective collecting area vastly larger than the city of Chicago, for free. If advanced extraterrestrials made use of their star’s gravitational field in this way, Shostak maintains “that would give them the capacity to observe many varieties of terrestrial transmissions, and in the optical they would have adequate sensitivity to pick up the glow of street lamps”. Even Brin conceded that this idea was “intriguing”.
Civilizations in a position to do us potential harm through interstellar travel, Shostak contends, would necessarily be technologically advanced enough to have such capabilities. “We cannot pretend that our present level of activity with respect to broadcasting or radar usage is ‘safe’. If danger exists, we’re already vulnerable” he concludes. With no clear means to say what extraterrestrials can or can’t detect, Shostak feels the SETI community has nothing concrete to contribute to the regulation of radio transmissions.
Could extraterrestrials harm us? In 1897 H. G. Wells published his science fiction classic “The War of the Worlds” in which Earth was invaded by Martians fleeing their arid, dying world. Besides being scientifically plausible in terms of its times, Wells’ novel had a political message. An opponent of British colonialism, he wanted his countrymen to imagine what imperialism was like from the other side. Tales of alien invasion have been a staple of science fiction ever since. Some still regard European colonialism as a possible model for how extraterrestrials might treat humanity. The eminent physicist Steven Hawking thinks very advanced civilizations might have mastered interstellar travel. Hawking warned that “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans”.
Though dismissing Hawking’s fears of alien invasion as an “unlikely speculation”, David Brin notes that interstellar travel by small automated probes is quite feasible, and that such a probe could potentially do harm to us in many ways. It might, for example, steer an asteroid onto a collision course with Earth. A relatively small projectile traveling at one tenth the speed of light could wreak terrible damage by simply colliding with our planet. “The list of unlikely, but physically quite possible scenarios is very long” he warns.
Diplomat Michael Michaud warns that “We can all understand the frustration of not finding any signals after fifty years of intermittent searching” but “Impatience with the search is not a sufficient justification for introducing a new level of potential risk for our entire species”.
METI critics David Brin, James Benford, and James Billingham think that the current lack of results from SETI warrants a different sort of response than METI. They call for a reassessment of the search strategy. From the outset, SETI researchers have assumed that extraterrestrials will use steady beacons transmitting constantly in all directions to attract our attention. Recent studies of interstellar radio propagation and the economics of signaling show that such a beacon, which would need to operate on a vast timescale, is not an efficient way to signal.
Instead, an alien civilization might compile a list of potentially habitable worlds in its neighborhood and train a narrowly beamed signal on each member of the list in succession. Such brief “ping” messages might be repeated, in sequence, once a year, once a decade, or once a millennium. Benford and Billingham note that most SETI searches would miss this sort of signal.
The SETI Institute’s Allen telescope array, for example, is designed to target narrow patches of sky (such as the space around a sun-like star) and search those patches in sequence, for the presence of continuously transmitting beacons. It would miss a transient “ping” signal, because it would be unlikely to be looking in the right place at the right time. Ironically, the Evpatoria messages, transmitted for less than a day, are examples of such transient signals.
Benford and Billingham propose the construction of a new radio telescope array designed to constantly monitor the galactic plane (where stars are most abundant) for transient signals. Such a telescope array, they estimate, would cost about 12 million dollars, whereas a serious, sustained METI program would cost billions.
The METI controversy continues. On February 13, the two camps debated each other at the American Association for the Advancement of Science conference in San Jose, California. At that conference David Brin commented “It’s an area where opinion rules, and everyone has a fierce opinion”. In the wake of the meeting a group of 28 scientists, scholars, and business leaders issued a statement that “We feel the decision whether or not to transmit must be based on a worldwide consensus, and not a decision based on the wishes of a few individuals with access to powerful communications equipment”.
The James Webb Space Telescope, scheduled for launch in 2018 may be the first to be capable of detecting biomarker gasses in the atmospheres of extrasolar planets. When an exoplanet passes between its star and Earth, an event called a transit, light that has passed through the planet’s atmosphere can be detected from a vantage point near Earth. When light passes through the exoplanet’s atmosphere, some wavelengths are absorbed and others transmitted. By analyzing the transmitted light spectrum, astronomers can learn the composition of the planet’s atmosphere. Astrobiologists hope to find biomarker gasses indicating the metabolic waste products of life. The oxygen in Earth’s atmosphere is a waste product of photosynthesis in plants and bacteria. The Webb telescope may be capable of conducting this test for planets larger than Earth (super-earths) transiting small stars. Space telescopes capable of conducting such research on a larger scale have been delayed by budget cuts.
Credit: NASA
In the movie “Avatar”, we could tell at a glance that the alien moon Pandora was teeming with alien life. Here on Earth though, the most abundant life is not the plants and animals that we are familiar with. The most abundant life is simple and microscopic. There are 50 million bacterial organisms in a single gram of soil, and the world wide bacterial biomass exceeds that of all plants and animals. Microbes can grow in extreme environments of temperature, salinity, acidity, radiation, and pressure. The most likely form in which we will encounter life elsewhere in our solar system is microbial.
Astrobiologists need strategies for inferring the presence of alien microbial life or its fossilized remains. They need strategies for inferring the presence of alien life on the distant planets of other stars, which are too far away to explore with spacecraft in the foreseeable future. To do these things, they long for a definition of life, that would make it possible to reliably distinguish life from non-life.
Unfortunately, as we saw in the first installment of this series, despite enormous growth in our knowledge of living things, philosophers and scientists have been unable to produce such a definition. Astrobiologists get by as best they can with definitions that are partial, and that have exceptions. Their search is geared to the features of life on Earth, the only life we currently know.
In the first installment, we saw how the composition of terrestrial life influences the search for extraterrestrial life. Astrobiologists search for environments that once contained or currently contain liquid water, and that contain complex molecules based on carbon. Many scientists, however, view the essential features of life as having to do with its capacities instead of its composition.
In 1994, a NASA committee adopted a definition of life as a “self-sustaining chemical system capable of Darwinian evolution”, based on a suggestion by Carl Sagan. This definition contains two features, metabolism and evolution, that are typically mentioned in definitions of life.
Metabolism is the set of chemical processes by which living things actively use energy to maintain themselves, grow, and develop. According to the second law of thermodynamics, a system that doesn’t interact with its external environment will become more disorganized and uniform with time. Living things build and maintain their improbable, highly organized state because they harness sources of energy in their external environment to power their metabolism.
Plants and some bacteria use the energy of sunlight to manufacture larger organic molecules out of simpler subunits. These molecules store chemical energy that can later be extracted by other chemical reactions to power their metabolism. Animals and some bacteria consume plants or other animals as food. They break down complex organic molecules in their food into simpler ones, to extract their stored chemical energy. Some bacteria can use the energy contained in chemicals derived from non-living sources in the process of chemosynthesis.
In a 2014 article in Astrobiology, Lucas John Mix, a Harvard evolutionary biologist, referred to the metabolic definition of life as Haldane Life after the pioneering physiologist J. B. S. Haldane. The Haldane life definition has its problems. Tornadoes and vorticies like Jupiter’s Great Red Spot use environmental energy to sustain their orderly structure, but aren’t alive. Fire uses energy from its environment to sustain itself and grow, but isn’t alive either.
Despite its shortcomings, astrobiologists have used Haldane definition to devise experiments. The Viking Mars landers made the only attempt so far to directly test for extraterrestrial life, by detecting the supposed metabolic activities of Martian microbes. They assumed that Martian metabolism is chemically similar to its terrestrial counterpart.
One experiment sought to detect the metabolic breakdown of nutrients into simpler molecules to extract their energy. A second aimed to detect oxygen as a waste product of photosynthesis. A third tried to show the manufacture of complex organic molecules out of simpler subunits, which also occurs during photosynthesis. All three experiments seemed to give positive results, but many researchers believe that the detailed findings can be explained without biology, by chemical oxidizing agents in the soil.
In 1976, two Viking spacecraft landed on Mars. The image is of a model of the Viking lander, along with astronomer and pioneering astrobiologist Carl Sagan. Each lander was equipped with life detection experiments designed to detect life based on its metabolic activities. These activities were assumed to be chemically similar to those of Earthly organisms. The three experiments included: 1) The labeled release experiment, in which radioactively labeled organic nutrients were added to Martian soil. If organisms were present, it was assumed that their metabolism would involve breaking down the nutrients for their energy content and releasing labeled carbon dioxide as a waste product. 2) The gas exchange experiment, in which Martian soil was provided with nutrients and light and monitored for the release of oxygen. On Earth, organisms that capture the energy of sunlight through the process of photosynthesis, like plants and some bacteria, release oxygen as a waste product. 3) The pyrolytic release experiment, in which Martian soil was placed in a chamber with radioactively labeled carbon dioxide. If there were organisms in the soil that photosynthesized like those on Earth, their metabolic processes would take up the gas and use the energy of sunlight to manufacture more complex organic molecules. Radioactive carbon would be given off when those more complex molecules were broken down by heating the sample. All three experiments produced what seemed like positive results. However, most scientists rejected this interpretation because the details of many of the results could be explained by supposing that there were chemical oxidizing agents in the soil instead of life, and because Viking failed to detect organic materials in Martian soil. This interpretation, especially for the labeled release experiment, remains controversial to this day and may need to be revisited based on recent findings. Credits: NASA/Jet Propulsion Laboratory, Caltech
Some of the Viking results remain controversial to this day. At the time, many researchers felt that the failure to find organic materials in Martian soil ruled out a biological interpretation of the metabolic results. The more recent finding that Martian soil actually does contain organic molecules that might have been destroyed by perchlorates during the Viking analysis, and that liquid water was once abundant on the surface of Mars lend new plausibility to the claim that Viking may have actually succeeded in detecting life. By themselves, though, the Viking results didn’t prove that life exists on Mars nor rule it out.
Most of the methane in Earth’s atmosphere is released by living organisms or their remains. Subterranean bacterial ecosystems that use chemosynthesis as a source of energy are common, and they produce methane as a metabolic waste product. Unfortunately, there are also non-biological geochemical processes that can produce methane. So, once more, Martian methane is frustratingly ambiguous as a sign of life.
Extrasolar planets orbiting other stars are far too distant to visit with spacecraft in the foreseeable future. Astrobiologists still hope to use the Haldane definition to search for life on them. With near future space telescopes, astronomers hope to learn the composition of the atmospheres of these planets by analyzing the spectrum of light wavelengths reflected or transmitted by their atmospheres. The James Webb Space Telescope scheduled for launch in 2018, will be the first to be useful in this project. Astrobiologists want to search for atmospheric biomarkers; gases that are metabolic waste products of living organisms.
Once more, this quest is guided by the only example of a life-bearing planet we currently have; Earth. About 21% of our home planet’s atmosphere is oxygen. This is surprising because oxygen is a highly reactive gas that tends to enter into chemical combinations with other substances. Free oxygen should quickly vanish from our air. It remains present because the loss is constantly being replaced by plants and bacteria that release it as a metabolic waste product of photosynthesis.
Traces of methane are present in Earth’s atmosphere because of chemosynthetic bacteria. Since methane and oxygen react with one another, neither would stay around for long unless living organisms were constantly replenishing the supply. Earth’s atmosphere also contains traces of other gases that are metabolic byproducts.
In general, living things use energy to maintain Earth’s atmosphere in a state far from the thermodynamic equilibrium it would reach without life. Astrobiologists would suspect any planet with an atmosphere in a similar state of harboring life. But, as for the other cases, it would be hard to completely rule out non-biological possibilities.
Besides metabolism, the NASA committee identified evolution as a fundamental ability of living things. For an evolutionary process to occur there must be a group of systems, where each one is capable of reliably reproducing itself. Despite the general reliability of reproduction, there must also be occasional random copying errors in the reproductive process so that the systems come to have differing traits. Finally, the systems must differ in their ability to survive and reproduce based on the benefits or liabilities of their distinctive traits in their environment. When this process is repeated over and over again down the generations, the traits of the systems will become better adapted to their environment. Very complex traits can sometimes evolve in a step-by-step fashion.
Mix named this the Darwin life definition, after the nineteenth century naturalist Charles Darwin, who formulated the theory of evolution. Like the Haldane definition, the Darwin life definition has important shortcomings. It has trouble including everything that we might think of as alive. Mules, for example, can’t reproduce, and so, by this definition, don’t count as being alive.
Despite such shortcomings, the Darwin life definition is critically important, both for scientists studying the origin of life and astrobiologists. The modern version of Darwin’s theory can explain how diverse and complex forms of life can evolve from some initial simple form. A theory of the origin of life is needed to explain how the initial simple form acquired the capacity to evolve in the first place.
The chemical systems or life forms found on other planets or moons in our solar system might be so simple that they are close to the boundary between life and non-life that the Darwin definition establishes. The definition might turn out to be vital to astrobiologists trying to decide whether a chemical system they have found really qualifies as a life form. Biologists still don’t know how life originated. If astrobiologists can find systems near the Darwin boundary, their findings may be pivotally important to understanding the origin of life.
Can astrobiologists use the Darwin definition to find and study extraterrestrial life? It’s unlikely that a visiting spacecraft could detect to process of evolution itself. But, it might be capable of detecting the molecular structures that living organisms need in order to take part in an evolutionary process. Philosopher Mark Bedau has proposed that a minimal system capable of undergoing evolution would need to have three things: 1) a chemical metabolic process, 2) a container, like a cell membrane, to establish the boundaries of the system, and 3) a chemical “program” capable of directing the metabolic activities.
Here on Earth, the chemical program is based on the genetic molecule DNA. Many origin-of-life theorists think that the genetic molecule of the earliest terrestrial life forms may have been the simpler molecule ribonucleic acid (RNA). The genetic program is important to an evolutionary process because it makes the reproductive copying process stable, with only occasional errors.
Both DNA and RNA are biopolymers; long chainlike molecules with many repeating subunits. The specific sequence of nucleotide base subunits in these molecules encodes the genetic information they carry. So that the molecule can encode all possible sequences of genetic information it must be possible for the subunits to occur in any order.
Steven Benner, a computational genomics researcher, believes that we may be able to develop spacecraft experiments to detect alien genetic biopolymers. He notes that DNA and RNA are very unusual biopolymers because changing the sequence in which their subunits occur doesn’t change their chemical properties. It is this unusual property that allows these molecules to be stable carriers of any possible genetic code sequence.
DNA and RNA are both polyelectrolytes; molecules with regularly repeating areas of negative electrical charge. Benner believes that this is what accounts for their remarkable stability. He thinks that any alien genetic biopolymer would also need to be a polyelectrolyte, and that chemical tests could be devised by which a spacecraft might detect such polyelectrolyte molecules. Finding the alien counterpart of DNA is a very exciting prospect, and another piece to the puzzle of identifying alien life.
Deoxyribonucleic acid (DNA) is the genetic material for all known life on Earth. DNA is a biopolymer consisting of a string of subunits. The subunits consist of nucleotide base pairs containing a purine (adenine A, or guanine G) and a pyrimidine (thymine T, or cytosine C). DNA can contain nucleotide base pairs in any order without its chemical properties changing. This property is rare in biopolymers, and makes it possible for DNA to encode genetic information in the sequence of its base pairs. This stability is due to the fact that each base pair contains phosphate groups (consisting of phosphorus and oxygen atoms) on the outside with a net negative charge. These repeated negative charges make DNA a polyelectrolyte. Computational genomics researcher Steven Benner has hypothesized that alien genetic material will also be a polyelectrolyte biopolymer, and that chemical tests could therefore be devised to detect alien genetic molecules. Credit: Zephyris
In 1996 President Clinton, made a dramatic announcement of the possible discovery of life on Mars. Clinton’s speech was motivated by the findings of David McKay’s team with the Alan Hills meteorite. In fact, the McKay findings turned out to be just one piece to the larger puzzle of possible Martian life. Unless an alien someday ambles past our waiting cameras, the question of whether or not extraterrestrial life exists is unlikely to be settled by a single experiment or a sudden dramatic breakthrough. Philosophers and scientists don’t have a single, sure-fire definition of life. Astrobiologists consequently don’t have a single sure-fire test that will settle the issue. If simple forms of life do exist on Mars, or elsewhere in the solar system, it now seems likely that that fact will emerge gradually, based on many converging lines of evidence. We won’t really know what we’re looking for until we find it.
S. Tirard, M. Morange, and A. Lazcano, (2010), The definition of life: A brief history of an elusive scientific endeavor, Astrobiology, 10(10):1003-1009.
