How to Search for Life as we Don’t Know it

The fields of extrasolar planet studies and astrobiology have come a long way in recent years. To date, astronomers have confirmed the existence of 4,935 exoplanets in 3,706 star systems, with another 8,709 candidates awaiting confirmation. With so many planets to study, next-generation instruments, and improved data analysis, the focus is transitioning from discovery to characterization. With the James Webb Space Telescope now deployed, these fields are about to advance much farther!

In particular, scientists anticipate that the characterization of planetary atmospheres may lead to the discovery of “biosignatures” – signs we associate with life and biological processes. The challenge will be how to recognize signatures that don’t conform to “life as we know it.” In a recent study, researchers from the School of Earth and Space Exploration (SESE) at Arizona State University (ASU) investigate possible tools for searching for life “as we don’t know it.”

For the sake of their study, the team looked to the various processes that we associate with life here on Earth and attempted to identify the universal patterns that don’t appear to depend on specific molecules. On Earth, life emerges from the interplay of hundreds of chemical compounds and reactions, some of which are shared by all organisms. This “universal biochemistry” characterizes all life on Earth but raises problems regarding astrobiology (the study of life beyond Earth).

In other planetary environments, the emergence and evolution of life may come down to different chemical elements altogether. Instead of carbon, the basic building block of life could be silicon or germanium. Instead of water, organisms could metabolize solvents like methane or ammonia. However, certain biological processes associated with life could be shared between life on Earth and elsewhere in the Universe.

In other words, future astrobiological surveys could find evidence of life beyond Earth by focusing on what it does rather than what it is. As co-author Sara Imari Walker, an associate professor with SESE and ASU’s School of Complex Adaptive Systems and the deputy director of ASU’s Beyond Center, said in an ASU News release:

“We want to have new tools for identifying and even predicting features of life as we don’t know it. To do so, we are aiming to identify the universal laws that should apply to any biochemical system. This includes developing quantitative theory for the origins of life, and using theory and statistics to guide our search for life on other planets.”

“We are not just the molecules that are part of our bodies; we, as living things, are an emergent property of the interactions of the many molecules we are made of. What our work is doing is aiming to develop ways of turning that philosophical insight into testable scientific hypotheses.”

Dylan Gagler, an ASU graduate and current bioinformatics analyst at New York University’s (NYU) Langone Medical Center, was the study’s lead author. Along with their colleagues, Walker and Gagler decided to focus on enzymes, the functional drivers of biochemistry. Using the Integrated Microbial Genomes and Microbiomes (IMGM) database (maintained by the U.S. Department of Energy (DOE) and the Joint Genome Institute), they investigated the enzymatic makeup of bacteria, archaea, and eukarya.

Whereas the former are single-celled organisms (prokaryotes) found everywhere on Earth, eukarya are cells that have a nucleus enclosed within a nuclear envelope – which includes everything from protists and fungi to plants and animals. Through this approach, the team examined the majority of Earth’s biochemistry and identified statistical patterns in the biochemical function of enzymes shared across these many lifeforms.

In so doing, they verified that statistical patterns originated from functional principles that cannot be explained by the common set of enzyme functions used by all known life and identified scaling relationships associated with general types of functions. As co-author Hyunju Kim, an assistant research professor at SESE and ASU’s Beyond Center, said:

“We identified this new kind of biochemical universality from the large-scale statistical patterns of biochemistry and found they are more generalizable to unknown forms of life compared to the traditional one described by the specific molecules and reactions that are common to all life on Earth. This discovery enables us to develop a new theory for the general rules of life, which can guide us in the search for novel examples of life.”

The ASU-led team is part of the Interdisciplinary Consortia for Astrobiology Research (ICAR) program, funded through the NASA Astrobiology Program. This program, inaugurated last year, selected eight interdisciplinary research teams to investigate topics ranging from cosmic origins and planetary system formation to the origins and evolution of life and the search for life beyond Earth. The ASU-led team focuses on “Planetary Systems Biochemistry,” of which Walker is the principal investigator (PI).

In addition to researchers with SESE, the team included members from the Santa Fe Institute (SFI) in New Mexico, Oberlin College, the ASU’s Beyond Center for Fundamental Concepts in Science, the ASU-SFI Center for Biosocial Complex Systems, and the Blue Marble Space Institute for Science (BMSIS). The paper that describes their findings recently appeared in the Proceedings of the National Academy of Sciences.

Further Reading: ASU News, PNAS