In Reinventing the Sacred, Stuart Kaufman, theoretical biologist and emeritus professor of biochemistry at the University of Pennsylvania, who studies the origin of life on Earth, observes: “There is a world beyond physics. Given continuous spacetime, there are a second-order infinity of possible histories of the biosphere. We are agents who alter the unfolding of the universe.”
If general principles exist that can explain properties common to all life on Earth, scientists hypothesize, then they may be universal to all life, even life on other planets. If a “universal biology” exists, it would have important implications for the search for life beyond Earth, for engineering synthetic life in the lab, and for solving the origin of life, enabling scientists to predict properties of alien life.
In astrobiology, there is an increasing interest in whether life as we know it is a quirk of the particular evolutionary history of the Earth or, instead, if life might be governed by more general organizing principles. When we think of life on Earth, we might think of individual examples ranging from animals to bacteria. When astrobiologists study life, however, they have to consider not only individual organisms, but also ecosystems, and the biosphere as a whole.
“To understand the general principles governing biology, we must understand how living systems organize across levels, not just within a given level,” says biologist Hyunju Kim.
Previous research in this area has primarily focused on specific levels of organization within biology such as individual organisms or ecological communities. These levels form a hierarchy where individuals are composed of interacting molecules and ecosystems are composed of interacting individuals.
An interdisciplinary team of researchers at Arizona State University (ASU) went beyond focusing on individual levels in this hierarchy to study the hierarchy itself, focusing on the biosphere as a whole.
Through this 2018 study, the team found that biochemistry, both at the level of organisms and ecosystems, is governed by general organizing principles.
“This means there is a logic to the planetary-scale organization of biochemistry,” says co-lead author Harrison Smith of ASU’s School of Earth and Space Exploration. “Scientists have talked about this type of logic for a long time, but until now they have struggled to quantify it. Quantifying it can help us constrain the way that life arises on a planet.”
For this research, the team constructed biochemical networks using a global database of 28,146 annotated genomes and metagenomes and 8,658 cataloged biochemical reactions. In so doing, they uncovered scaling laws governing biochemical diversity and network structure that are shared across levels of organization from individuals to ecosystems, to the biosphere as a whole.
Network representation of global biochemistry is shown in the image above. This graph represents the biosphere, ecosystems and individual organisms’ biochemistry as connecting molecules participating in shared reactions. It reveals that various scaling laws are common across different levels of biological organization.
How Life Operates as a Planetary Process
“Quantifying general principles of life—not restricted to a domain on the tree of life, or a particular ecosystem—is a challenge,” says Smith. “We were able to do that by combining tools from network science and scaling theory, while simultaneously leveraging large genomic datasets that researchers have been cataloging
The research team, led by Kim and Smith under supervision of astrobiologist Sara Walker of the ASU School of Earth and Space Exploration and the Beyond Center, also includes Cole Mathis of the Beyond Center and the ASU Department of Physics (now at the University of Glasgow), and Jason Raymond of the School of Earth and Space Exploration.
“Understanding the organizing principles of biochemistry at a global scale better enables us to understand how life operates as a planetary process” says Walker. “The ability to more rigorously identify universal properties of life on Earth will also provide astrobiologists with new quantitative tools to guide our search for alien life—both in the lab and on other worlds.”
The Last Word –Detecting Life on Exoplanets
When asked about universal properties of life that could provide astrobiologists with quantitative tools to guide our search for alien life, Ian Macara, Chair of the Dept. of Cell and Developmental Biology at Vanderbilt University School of Medicine replied in an email: “Trying to understand the organization of the Earth’s biosphere is an interesting and worthwhile approach, because we still lack a clear understanding of interactions between organisms, and between geological processes and organisms, on a global scale. Whether such interactions will provide guidance to detect life on exoplanets, however, is to me unclear.
“Certainly, there are attributes to life as we know it on Earth, some of which might be common throughout the universe because physics and chemistry are the same everywhere; but the idea that the organizing principles found for life on earth will be similar on other planets seems to me to be an unsupported assumption,” Macara explained. “Strikingly, the fundamental components of life as found on Earth seem to be commonplace in interstellar molecular gas clouds, planetary atmospheres of gas giants, and in carbonaceous meteorites. This fact is quite astonishing for what it implies – that on many planets of similar size and temperature to the earth, living organisms likely have many of the same basic attributes as here – an information storage system; a system to convert energy to work (ultimately from sunlight or chemical reactions); and some way to separate these systems from the environment. Presumably, life of this type would create a homeostatic, non-equilibrium ecosphere, which might be detectable by astronomers, but the actual biochemistry of these systems could be totally different, and global interactions would probably, in my opinion, be structured quite differently than on Earth.
“There may be many other ways than DNA/RNA to create efficient and replicable information storage molecules, other mechanisms than photosynthesis to capture solar energy, and other structures than membranes to encapsulate these processes, Macra notes. “Unfortunately, it will be a very long time before we can know if this is the case – unless chemists can create such novel molecular machines here on Earth.”
Image at top of the page: Stanley Kubrick’s 2001: A Space Odyssey. Photo: Warner Bros.