Sara Imari Walker is an astrobiologist and theoretical physicist at Arizona State University. As a Berggruen Institute Fellow, Walker develops theories and experiments toward characterizing the origin of life. In her recent book, “Life as No One Knows It: The Physics of Life’s Emergence” (Riverhead Books, 2024), she argues that studying the origin of life requires radical new thinking.
Claire Isabel Webb directs the Berggruen Institute’s Future Humans program that investigates the histories and futures of life, mind and outer space. She is also an associate editor of Noema Magazine.
Theoretical physicist and astrobiologist Sara Imari Walker proposes that evolution and selection can operate at a planetary scale on Earth, and perhaps worlds beyond. In her telling, a planet accretes, iterates and then — crucially — evolves, acquiring information and memory that structure material possibilities. The following is a discussion between Walker and the Berggruen Institute’s historian of science Claire Isabel Webb.
Claire Isabel Webb: The James Webb Space Telescope (JWST), launched in 2021, has revealed the magnificence of our universe as never before. A main priority of NASA’s mission is to learn more about exoplanets’ atmospheres, where evidence of extraterrestrial life might be found. What are your hopes for how this technology of perception could help astrobiologists like you characterize alien signs of life?
Sara Imari Walker: The fact that we can build instruments and technologies that allow us to see billions of years into the universe’s past is in many ways more interesting than the images and information we get from these telescopes.
Humans are part of Earth’s physical system. While what we see through our telescopes is extraordinary, it is more extraordinary that we emerged from the geochemistry of Earth and, after around four billion years of evolution, can construct telescopes and interpret what we see.
CIW: We see collective intelligence in organisms like honeybees, which waggle information to each other; starlings that murmurate; and slime molds that coordinate chemical responses to their environment despite their lack of brains. You argue that physical systems capable of intelligence can scale to planet Earth, but also, perhaps, to planets beyond.
How would thinking of planets, including all the flora and fauna they may foster, through the lens of physics, fundamentally change how we look for life beyond Earth?
SIW: We are realizing how little we know about exoplanets, even from the features we can infer, such as simple atmospheric gases. Astronomers hope that by analyzing the spectra of these gases, we might learn something about planetary chemistry and whether it indicates the presence of life. The diversity of planets is proving to be much broader than we could have naively anticipated.
CIW: Right, it was about four years ago that astronomers characterized — but have yet to confirm — a Hycean planet: a new potential world that’s ocean-covered with a thin hydrogen atmosphere that could be conducive to life emerging. There also are sub-Neptunes, Super-Earths, Mini-Neptunes and Mega-Earths. These neologisms speak to the fact that scientists are discovering many kinds of planets that don’t fit into the mold of our solar system’s planets.
SIW: Exactly. We have no priors for what we are seeing. Studying these worlds will raise many unknowns about alien environments and the potential biologies that could evolve there. To assume any of those exoplanets harbor life forms just like those on Earth enormously understates the theoretical possibility of alien life forms.
CIW: A possibility space is an arena where all plausible outcomes are considered, simulated and theorized; the concept also acknowledges the unknown lacunae of present knowledge. So, successfully detecting extraterrestrial life might mean we need to reframe how we currently conceptualize evidence for what even counts as “life” on Earth.
SIW: Yes. Historically, astronomy experiments that sought alien life forms fixated on detecting molecules rather than conceptualizing the life processes of an entire planet. That is, we are now starting to think about detecting entire biospheres.
But to do this, we need to develop new theories of life around the concept of complexity. By “complexity,” I mean the amount of information necessary to produce a particular set of structures; in other words, it is what determines what possibilities exist. And by “information,” I really mean
causation and selection, which we formalize in assembly theory as the minimum amount of contingent historical steps necessary for the observed objects to exist. How much selection and historical contingency must go into making what we observe? If the answer is significant, it suggests those features require much acquired memory and can only be produced by life.
Earth can provide a model for understanding planetary complexity. To begin to answer the question — How would one characterize our planet as a living world? — we can start with the concept that our planet has some four billion years of acquired memory. When scientists set out to characterize and then detect life, what I think we need to aim to detect is the depth in time of past states that the planet retains in its current state. This might seem like a weird way to think about it, but with it, we can then use assembly theory to follow how selection constructs entire atmospheres, and potentially detect alien life.
“To assume any of those exoplanets harbor life forms just like those on Earth enormously understates the theoretical possibility of alien life forms.”
This conceptual reorientation requires that we decouple our thinking about specific “things life constructs” (e.g., discrete units like a Lego building block) from entire systems of “life-constructing things” (e.g., an organism). There might be other information processing or intelligent systems in the universe — what we might consider “life” — and we would recognize these only because they can make things we know could not form in the absence of life. Knowing a planet’s full history over billions of years of planetary evolution is not necessary because the evolved objects themselves should be evidence of that history and whether “life” is a part of the history or not.
CIW: By a planet’s “acquired memory” then, you mean almost a Gordian knot of chemical, biological, and physical data that enfolds over eons.
SIW: Yes. Because biological systems are constantly reproducing and building new structures, we tend not to realize how old some forms of life are. The interior structure of the ribosome has changed less than most rocks on this planet in the last four billion years. The lineage of sharks has been around longer than Saturn’s rings. Given the continual evolution of biological systems and the fact that scientists seek evidence of life through their physical traces, the question for astrobiologists and exoplanetary astronomers then becomes: “How do we infer processes that are deep in time simply from the structure of a planet’s atmosphere?”
We build optical telescopes because we think they are the best technologies to infer molecules that life processes produce. But the challenge is that we won’t directly “see” certain structures existing in the universe unless we know what to look for — in this case, we need to recognize an alien biosphere filtered through the lens of an atmosphere and then a telescope. To do this kind of inference, we need to better conceive of what life is, so we know what to look for.
CIW: Technologies must catch up with theories. Geologist Eduard Suess, writing in 1875, conceptualized Earth as a series of layered, interlocking spheres. The biosphere, or “selbständige,” as he coined it, was a layer that enshelled all life on Earth. Soviet scientist Vladimir Vernadsky, about 60 years later, developed Suess’s concept. He described the biosphere to be in a state of momentous transition: an emerging noösphere, or “the energy of human culture.” There were glimmerings of humans’ impact
on Earth in Vernadsky’s writing, and it was only in the consequential decades that technologies — computers, satellite images, climate models — rendered in great scientific clarity the extent of that impact. How we look for climate change is through the technologies we’ve built to look at climate change. Of course, there is always room for surprise and serendipity.
SIW: To use an analogy, how we should look for life is somewhat akin to how we discovered gravitational waves. In 1916, Albert Einstein’s theory of general relativity predicted that the collisions of supermassive objects like black holes would create ripples in the very fabric of spacetime. Humans did not know how to build an instrument to measure this — an interferometer — nor did they possess the technological tools necessary to confirm the existence of gravitational waves. It took a century for us to develop the technology to make the detection. We made “first contact” in 2015 — confirming Einstein’s prediction of these waves — almost exactly 100 years later. Technology and exoplanetary insights can only work together. We did not have the technologies of perception to see gravitational waves in 1916. In 1916, cars were barely on the road!
CIW: Your analogy reminds me of my work with radio astronomers who search for extraterrestrial intelligence (like those at the SETI Institute). They make a distinction between biosignatures, such as planetary atmospheres that would indicate some form of life, and technosignatures, which are artifacts of intelligent alien technologies. Even if one is generous with the parameters of life or even “intelligence” existing beyond Earth, there’s no way to say with certainty that humans would be able to notice — let alone receive, let alone translate — a directed, intentional and meaningful communication. The interoperability — or ability of human and speculative alien transmissions to communicate effectively — is not guaranteed.
Gravity waves, I think you’re saying, represent a different kind of epistemic endeavor. Einstein’s prediction of gravity waves led to a century of theoretical research that allowed physicists to precisely predict the shape of the “chirp” of two black holes colliding — they characterized the disturbance in spacetime at the length of one-ten-thousandth of the diameter of a proton! Theory came first. Experiments to support such theory followed. In SETI, astronomers are developing experiments of expectation where the object is not guaranteed, let alone characterized with any theoretical clarity.
“The interior structure of the ribosome has changed less than most rocks on this planet in the last four billion years.”
SIW: Yes, this is exactly the challenge. Compared to life processes, predicting and detecting gravitational waves is a fairly simple problem. We do not have the right abstractable concept or theory to talk about extraterrestrial life, let alone alien intelligence. How are we going to possibly know we have the right technology or framework to see complex biological features in the universe or perceive alien signals? The coupling between how we build and use technology and how we conceptualize life is fundamental, yet unanswered.
So, some of JWST’s data might already indicate biosignatures in the composition of atmospheric chemistry. But I propose a conceptual reframing of how we even begin to interpret that data: We need to understand molecules’ presence as products of the collective evolution of living worlds, not as individual units. That’s where assembly theory, an explanation for life first developed by chemist Leroy (Lee) Cronin, comes in. It allows unfolding analysis of structures of molecular bonds and the recurrence of certain bonds’ structures as indications of how much minimal acquired memory is necessary for a given chemical system to emerge.
CIW: Can you walk me through an example of how that works at the molecular level?
SIW: Basically, we take the molecule apart and we try to rebuild it by taking those constituent parts and joining them back together. Our goal is to discover the shortest possible route, only reusing parts we have already made. One can imagine doing something similar with Lego. Say one had a Lego castle, smashed it to pieces, and then asked how many steps are necessary to rebuild it — with the stipulation that the builder can only use things the builder has already built. This constraint bounds the minimum causation necessary for evolution to discover the object. And our hypothesis, which so far stands up to experimental testing for assembly theory as applied to molecules in the lab, is that some objects have sufficient minimum causation to mean that they are only producible by life.
CIW: So, when a system gains sufficient complexity, life can assemble itself.
SIW: To see the holistic structure of what we really think “life” is, we need new ways of seeing. We might not use existing, familiar technologies of perception to detect extraterrestrial life because we don’t yet understand the full complexity of a planet’s total life processes.
We also need to be very careful not to overestimate the connection between the materials of life processes and the technologies that life processes produce. As Lee [Cronin] likes to point out, the social media app TikTok will not exist anywhere else in the universe; we don’t expect a technology that evolved on Earth to have evolved on every planet with life. This is because we implicitly recognize that humans are embedded in a particular technological space, which is a product of our biology, which itself is a product of geochemical events that happened on our planet an estimated 3.8 billion years ago — it is all contingent. But for some reason, when we look at biochemistry, we choose to talk about life’s complex processes in a way that implies these processes emerge linearly as individual objects out of a singular planetary condition rather than realizing biochemistry is a complex, iterative, interactive invention of deeply complex planetary systems.
CIW: Good to know that Earth is the only planet with TikTok! But you’re saying we should think of TikTok as a result of Earth’s complex systems that can be unwound to the molecular building blocks of life as we know it. Given the awesome number of combinations that can be made on an atomic level and the many events that led to humans making TikTok, telescopes and satellites, the number of possible systems that created intelligent life is enormous.
SIW: Yes it is enormous! And that is why I am excited about assembly theory because it allows us to formalize how big the space is that must select for a given object to exist. The mystery that I and other astrobiologists are trying to sort out is how signs of life not only might have initially emerged out of a particular planetary geochemistry, but how the awesome diversification of the structures of life have been elaborated on over billions of years.
Of course, there are constraints. Everything follows the laws of physics, and we expect those laws to impose universal constraints on how biologies and technologies get invented. For instance, we can presume that all flying creatures will have a winglike structure — but the particular details of those structures, like what they are made of, their precise shape and how they emerged and then evolved among species, varied enormously based on the specific historical context under which they emerged.
“We need to understand molecules’ presence as products of the collective evolution of living worlds, not as individual units.”
CIW: Convergent evolution describes how a pterodactyl and a bat both have wings, but those structures were born out of completely different evolutionary pathways.
SIW: Yes. In the same way, conceptualizing a planet’s acquired memory means expanding the definitions of what signifies life and the tools necessary to find it. Astrobiology needs to move beyond analyzing the details of molecular structures to analyzing macro-scale patterns that might really be universal signatures of life.
CIW: What you’re saying is that we need to understand chemistry in an exoplanetary atmosphere as the result of a global system — not as just some atoms bonding together. A bird’s wings are the result of a great chain of processes that have complexified themselves over billions of years. Wings are a phenomenon we see on Earth because they’re the result of evolution and selection that emerged from a planetary system of life. So, selection processes on Earth produced life, which produced intelligence and then technology.
SIW: And that intelligence is not only observable at an individual level, like a human doing math, but at a collective level, like humans producing AIs. That process of complexification scales to planetary scale living and intelligent processes.
CIW: Intelligence is a trace of complex objects that can be observed at the planetary level. The planet embeds a material lineage that tells us how complex objects — life — can assemble themselves into other complex objects, reflexively and recursively iterating. Earth has had enough time to build a memory that includes life, and this life includes technologies.
I am curious: What future observations might be evidence of planetary scale knowledge — its acquired memory? SETI scientists I worked with were generally leery of indulging in detailed speculation about the natures of alien beings. Given our limited knowledge, it’s fun, but perhaps not scientifically useful, to imagine if aliens have fur, or 10 eyes or can operate in the sixth dimension. We can only use the present technologies to search very narrowly for a range of radio frequencies that would indicate alien technology — not some cosmological brain-scanning device that would detect alien “intelligence.”
But indulge me for a moment: How might one design a futuristic successor instrument to the JWST that would search not merely for the presence of molecules but also be equipped to search for a concept such as intelligence by assessing an entire planet’s geological, biological and chemical structures?
SIW: I think in terms of radical abstraction about the nature of life. If we could build new technologies of perception that would see the world in terms of causal structure, it would be very easy to pick out objects and entities that would be “alive” — possessing a deeper causal structure of “liveliness.”
A planet that evolved a technosphere — a series of distinct and integrated systems, like satellites and spacecraft — is more “alive” than one with a biosphere. That’s because the amount of causation that goes into assembling a technosphere is much higher. It has a much larger causal depth and, therefore, exists as an object that is deeper in time on a planet.
CIW: And one can calculate that causal depth using assembly theory.
SIW: Yes. Assembly theory is a mathematical description of life and its objects. But we hope to generalize assembly theory to all kinds of materials structured by life. It is not clear how speculative instruments might translate to measuring complexity through direct physical observations, but I think a key step will be inventing a new technology (e.g., a theory, like assembly theory in this case) that can help us see causal structure.
Detecting planetary life processes — either through direct observation or a conceptual framework — is difficult. From space telescopes, we are only getting photons from exoplanets rendered as spectra. While this can tell us a lot about a planet’s size and even atmospheric composition, building an instrument to characterize a planet’s living complexity is not straightforward. What kinds of measurements would we even take? Can we do so remotely? We are making a lot of headway on this, thinking about how to make inferences based on our observations of the diversity of bonds and elements in an atmosphere, which tell us something about its assembly.
CIW: So, it is not really a question of gathering enough information to eventually be able to count it as a planet with deep, acquired memory, or compiling spectra to understand that data in this new way.
“If we see a sufficiently complex atmosphere — one that required a sufficient amount of time to produce — that might be the smoking gun of a biosignature.”
SIW: Right. We cannot just use information theory or computational language to detect life, because those depend on human-derived data labeling systems. We need a new paradigm where the contingency in the matter we observe and the computation of its complexity are the same. In assembly theory, we do this by treating objects as “informational”: Objects are made up of the operations the universe uses to build them as an intrinsic property, meaning different objects require different amounts of memory and, consequently, have different depths in time. Therefore, we should expect to require varying amounts of acquired memory for these to ever appear at a given time in the universe. To detect this from atmospheric data, we require a leap to the perspective of thinking about an atmosphere as a complex system assembled by evolution and selection.
CIW: Let us bring the complexity question of exoplanets back to the familiar context of Earth — indeed, the only place in the universe where we know life exists. Would one have to compile millions of years of atmospheric spectra over time, generating different timestamps for the evidence of evolving biological processes? Would one also have to journey to the bottom of the ocean to calculate the assembly index of sharks’ teeth to plug into a holistic theory of the emergence of life on Earth?
SIW: Right now, I am just not sure how much we can infer about life from atmospheric data. That is because the objects we are interested in inferring exist at such a large temporal scale, and most of the molecules in an atmosphere are not deep in time objects.
On the other hand, objects that life uniquely produces are objects that are immensely large in time. In what we call assembly time, we can stack all objects by the minimal number of physical operations necessary to build them and define a boundary between what can be produced anywhere and what requires a living (evolving) trajectory. But this requires us to assume time is an intrinsic feature of all objects. Humans are very large in time. Plants and humans are 3.8 billion years in clock time. Everything living on this planet has parts that extend that far back.
CIW: I have never heard anyone describe objects as being “large” (a physical phenomenon) in “time” (an immaterial phenomenon).
SIW: Sometimes doing new physics requires defining what is material in new ways. To talk about planetary atmospheres in an explanatory way that allows us to theorize about life, we might measure how large the atmosphere is in time. So, if we see a sufficiently complex atmosphere — one that required a sufficient amount of time to produce — that might be the smoking gun of a biosignature. It would be a definitive sign indicating that the planet possessed some kind of life.
But the problem is that volatile gases — ones present in the atmosphere of Earth, where we know life exists, and areas where we think life cannot — tend to be composed of very simple molecules. So, to detect “life,” we have to make many other inferences, like observing how molecules interact and how they together indicate complex processes of life. That is, one might see that there is evidence in the total set’s composition that indicates an object much larger in time than any number of individual molecules.
I am proposing that we look at the whole composition of a system. That will allow us to understand a planet’s memory depth — evidence of its evolutionary history — that would have produced a total atmospheric composition we can observe.
Many people are not optimistic that we have a scientific pathway for inferring the presence of life on exoplanets. I am not sure where I land on that question. But what I am doing now with Lee [Cronin] is working toward a large-scale project that will allow us to observe the emergence of alien life in the lab — e.g., generate an origin of life event from scratch.
We need to do this by building a “planet simulator.” It cannot be a computational experiment. It must be physical for two reasons: (1) the computations to simulate life are more efficient when implemented in the real universe, and (2) we do not know all the relevant physics to simulate them, so we must run the experiments in reality, using chemistry. The technology now exists to do this at scale, and we have a theory that will allow us to guide our search. The best way for us to demonstrate the principles that will allow us to discover alien life is to do the right kinds of theory-driven experiments here on Earth.
The profound question I want to answer in my lifetime is this: Can we evolve truly alien life — and perhaps intelligence — in the laboratory?
Editor’s Note: This interview has been edited for clarity and length.