Artist's impression of the Archean Eon. Credit: Tim Bertelink/Wikimedia
The search for extrasolar planets is currently undergoing a seismic shift. With the deployment of the Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS), scientists discovered thousands of exoplanets, most of which were detected and confirmed using indirect methods. But in more recent years, and with the launch of the James Webb Space Telescope (JWST), the field has been transitioning toward one of characterization. In this process, scientists rely on emission spectra from exoplanet atmospheres to search for the chemical signatures we associate with life (biosignatures).
However, there’s some controversy regarding the kinds of signatures scientists should look for. Essentially, astrobiology uses life on Earth as a template when searching for indications of extraterrestrial life, much like how exoplanet hunters use Earth as a standard for measuring “habitability.” But as many scientists have pointed out, life on Earth and its natural environment have evolved considerably over time. In a recent paper, an international team demonstrated how astrobiologists could look for life on TRAPPIST-1e based on what existed on Earth billions of years ago.
J. William Schopf and colleagues from UCLA and the University of Wisconsin analyzed the microorganisms with cutting-edge technology called secondary ion mass spectroscopy. Credit: John Vande Wege/UCLA
Life on Earth has had a long and turbulent history. Scientists estimate that roughly 4 billion years ago, just 500 million years after planet Earth formed, the first single-celled lifeforms arose. By the Archean Eon (4 to 2.5 billion years ago), multi-celled lifeforms are believed to have emerged. While the existence of such organisms (Archaea) has been inferred from carbon isotopes found in ancient rocks, fossil evidence has remained elusive.
All of that has changed, thanks to a recent study performed by a team of researchers from UCLA and the University of Wisconsin–Madison. After examining ancient rock samples from Western Australia, the team determined that they contained the fossilized remains of diverse organisms that are 3.465 billion years old. Combined with the recent spate of exoplanet discoveries, this study strengthens the theory that life is plentiful in the Universe.
Apex chert in Western Australia, where the 3.465 billion year old fossils were found. Credit: John Valley/UW-Madison
These 11 fossils were diverse in nature and the researchers divided them into five species groups based on their apparent biological functions. Whereas two of the fossil samples appear to have performed a primitive form of photosynthesis, another apparently produced methane gas. The remaining two appear to have been methane-consumers, which they used to build and maintain their cell walls (much like how mammals use fat).
As J. William Schopf – a professor of paleobiology in the UCLA College and the lead author on the study – indicated in a UCLA Newsroom press release:
“By 3.465 billion years ago, life was already diverse on Earth; that’s clear — primitive photosynthesizers, methane producers, methane users. These are the first data that show the very diverse organisms at that time in Earth’s history, and our previous research has shown that there were sulfur users 3.4 billion years ago as well.
This study, which is the most detailed ever conducted on microorganisms preserved as ancient fossils, builds on work that Schopf and his associates have been performing for over two decades. Back in 1993, Schopf and another team of researchers conducted a study that first described these types of fossils. This was followed in 2002 by another study which substantiated their biological origin.
In this latest study, Schopf and his team established what kind of organisms they are and how complex they are. To do this, they analyzed the microorganisms using a technique called Secondary Ion Mass Spectroscopy (SIMS), which reveals the ratio of carbon-12 to carbon-13. Whereas carbon-12 is stable and the most common type found in nature, carbon-13 is a less common but similarly stable isotope that is used in organic chemistry research.
A microorganism analyzed by the researchers. Credit: J. William Schopf/UCLA
By separating the carbon from each fossil into its constituent isotopes and determining their ratios, the team was able to conclude how long ago the microorganisms lived, as well as how they lived. This task was performed by the Wisconsin researchers, who were led by professor John Valley. “The differences in carbon isotope ratios correlate with their shapes,” said Valley. “Their C-13-to-C-12 ratios are characteristic of biology and metabolic function.”
According to the current scientific consensus, advanced photosynthesis had not yet evolved and oxygen would not appear on Earth until 500 million years later. By 2 billion year ago, concentrations of oxygen gas began increasing rapidly. This means that these fossils, being around roughly 1 billion years after Earth formed, would have lived at a time when their was little oxygen in the atmosphere.
Given that oxygen would be poisonous to these types of primitive photosynthesizers, they are quite rare today. In truth, they can only be found in places where there is sufficient light but no oxygen, something which is rarely found in combination. What’s more, the rocks themselves were a source of great interest since the average lifespan of rock exposed to the surface of Earth is only about 200 million years.
When Shopf first began his career, the oldest-known rock samples were 500 million years old. This means that the fossil-bearing rocks he and his team examined are as old as rocks on Earth can get. To find fossilized life in such ancient samples demonstrates that diverse organisms and a life cycle had already evolved on Earth by the early Archaen Eon, something which scientists only suspected up until this point.
In the future, SIMS technology could be used to look for signs of fossilized life on Mars. Credit: NASA/JPL)
These findings naturally have implications for the study of how and when life emerged on Earth. Beyond Earth, the study also has implications since it demonstrates that life emerged when Earth was still very young and in a primitive state. It is therefore not unlikely that a similar process has been taking place elsewhere in the Universe. As Schopf explained:
“This tells us life had to have begun substantially earlier and it confirms that it was not difficult for primitive life to form and to evolve into more advanced microorganisms. But, if the conditions are right, it looks like life in the universe should be widespread.”
This study was made possible thanks to funding provided by the NASA Astrobiology Institute. Looking to the future, Schopf indicated that the same technology used to date these fossils will likely be used to study rocks brought back by NASA’s crewed mission to Mars. Scheduled for the 2030s, this mission will entail retrieving samples obtained by the Mars 2020 Rover and bringing them back to Earth for analysis.
Fossilized remains found in Greenland have been dated to 3.7 billion years ago, 220 million years older than when life is believed to have emerged. Credit: A.P. Nutman et al./Nature
Fossilized remains are a fascinating thing. For paleontologists, these natural relics offer a glimpse into the past and a chance to understand what kind of lifeforms lurked there. But for astronomers, fossils are a way of ascertaining precisely when it was that life first began here on our planet – and perhaps even the Solar System.
And thanks to a team of Australian scientists, the oldest fossils to date have been uncovered. These fossilized remains have been dated to 3.7 billion years of age, and were of a community of microbes that lived on the ancient seafloor. In addition to making scientists reevaluate their theories of when life emerged on Earth, they could also tell us if there was ancient life on Mars.
The fossil find was made in what is known as the Isua Supracrustal Belt (ISB), an area in southwest Greenland that recently became accessible due to the ice melting in the area. According to the team, these fossils – basically tiny humps in rock measuring between one and four centimeters (0.4 and 1.6 inches) tall – are stromatolites, which are layers of sediment packed together by ancient, water-based bacterial colonies.
The Australian team searching for fossilized remains in the Isua supracrustal belt (ISB) in southwest Greenland. Credit: uow.edu.au
According to the team’s research paper, which appeared recently in Nature Communications, the fossilized microbes grew in a shallow marine environment, which is indicated by the seawater-like rare-earth elements and samples of sedimentary rock that were found with them.
They are also similar to colonies of microbes that can be found today, in shallow salt-water environments ranging from Bermuda to Australia. But of course, what makes this find especially interesting is just how old it is. Basically, the stone in the ISB is dated back to the early Archean Era, which took place between 4 and 3.6 billion years ago.
Based on their isotopic signatures, the team dated the fossils to 3.7 billion years of age, which makes them 220 million years older than remains that had been previously uncovered in the Pilbara Craton in north-western Australia. At the time of their discovery, those remains were widely believed to be the earliest fossil evidence of life on Earth.
As such, scientists are now reconsidering their estimates on when microbial life first emerged on planet Earth. Prior to this discovery, it was believed that Earth was a hellish environment 3.7 billion years ago. This was roughly 300 million years after the planet had finished cooling, and scientists believed it would take at least half a billion years for life to form after this point.
4.5 billion years ago, during the Hadean Eon, Earth had a much different environment than it does today. Credit: NASA
But with this new evidence, it now appears that life could have emerged faster than that. As Allen P. Nutman – a professor from the University of Wallongong, Australia, and the study’s lead author – said in a university press release:
“The significance of stromatolites is that not only do they provide obvious evidence of ancient life that is visible with the naked eye, but that they are complex ecosystems. This indicates that as long as 3.7 billion years ago microbial life was already diverse. This diversity shows that life emerged within the first few hundred millions years of Earth’s existence, which is in keeping with biologists’ calculations showing the great antiquity of life’s genetic code.”
When life emerged is a major factor when it comes to Earth’s chemical cycles. Essentially, Earth’s atmosphere during the Hadean was believed to be composed of heavy concentrations of CO² atmosphere, hydrogen and water vapor, which would be toxic to most life forms today. During the following Archean era, this primordial atmosphere slowly began to be converted into a breathable mix of oxygen and nitrogen, and the protective ozone layer was formed.
The emergence of microbial life played a tremendous role in this transformation, allowing for the sequestration of CO² and the creation of oxygen gas through photosynthesis. Therefore, when it comes to Earth’s evolution, the question of when life arose and began to affect the chemical cycles of the planet has always been paramount.
Could fossilized remains of microbes be found underneath Mars’ cold, dry landscape? Credit: NASA/JPL-Caltech
“This discovery turns the study of planetary habitability on its head,” said associate Professor Bennett, one of the study’s co-authors. “Rather than speculating about potential early environments, for the first time we have rocks that we know record the conditions and environments that sustained early life. Our research will provide new insights into chemical cycles and rock-water-microbe interactions on a young planet.”
The find has also inspired some to speculation that similar life structures could be found on Mars. Thanks to the ongoing efforts of Martian rovers, landers and orbiters, scientists now know with a fair degree of certainty that roughly 3.7 billion years ago, Mars had a warmer, wetter environment.
As a result, it is possible that life on Mars had enough time to form before its atmosphere was stripped away and the waters in which the microbe would have emerged dried up. As Professor Martin Van Kranendonk, the Director of the Australian Centre for Astrobiology at UNSW and a co-author on the paper, explained:
“The structures and geochemistry from newly exposed outcrops in Greenland display all of the features used in younger rocks to argue for a biological origin. This discovery represents a new benchmark for the oldest preserved evidence of life on Earth. It points to a rapid emergence of life on Earth and supports the search for life in similarly ancient rocks on Mars.”
Another thing to keep in mind is that compared to Earth, Mars experiences far less movement in its crust. As such, any microbial life that existed on Mars roughly 3.7 billion years ago would likely be easier to find.
This is certainly good news for NASA, since one of the main objectives of their Mars 2020 rover is to find evidence of past microbial life. I for one am looking forward to seeing what it leaves for us to pickup in its cache of sample tubes!
