American Scientist

LIFE AS NO ONE KNOWS IT: The Physics of Life’s Emergence. Sara Imari Walker. 272 pp. Riverhead Books, 2024. $29.00.

If you’ve ever yearned for a fresh take on the age-old questions “Are we alone?” and “Where did we come from?” then look no further than Sara Imari Walker’s new book, Life as No One Knows It: The Physics of Life’s Emergence.

For Walker, a theoretical physicist and astrobiologist, life is not something to be defined but rather a phenomenon that must be explained by a new theoretical framework. (Think, perhaps, of how we don’t seek to define gravity; we have a theory of it instead.) Walker begins the book by introducing the conundrum of defining life through the varied viewpoints of other established researchers. To each, she has visceral reactions: “I was aghast in my seat,” she writes, upon hearing one physicist’s take. “His idea left me extremely perplexed,” she says of an encounter with a biologist. Through these responses on the first pages of the book, she immediately posits herself as an intellectual maverick. Moreover, she makes it abundantly clear from the start that there is no scientific consensus on what life is, why it originates, or how to identify it in the universe.

“What is life?” is a daunting question. In the first third of the book, Walker builds toward her answer by layering in key concepts of information, selection, and causality before arriving at her punchline: The phenomenon of life cannot be explained with our current laws of physics. To understand what it means to be alive, we require a new conceptual framework that captures the causal power of information and selection. She calls this new physics Assembly Theory, expounding upon it in her lengthiest chapter, which spans the middle third of Life as No One Knows It. Walker then spends the remainder of the book explaining how Assembly Theory equips scientists with new tools for searching for life in the universe, solving the origins of life, and understanding the future trajectory of life on Earth.

At the core of Assembly Theory is the idea that the universe builds objects of increasing complexity. For astrobiologists, molecules are the objects of interest. The complexity of a molecule can be measured by the number of unique transformations required to construct that molecule from simpler building blocks. If a transformation is repeated, it does not add to the assembly index of the final product. This way of counting is key, because it differentiates between large molecules made of repeating parts (with low assembly indices) from equally large molecules of much greater diversity (with high assembly indices). Another piece of Assembly Theory is an object’s copy number—the number of final products that exists. A single complex molecule might be a fluke, but millions of exact copies could only result from a reliable, generative process.

Together, the assembly index and the copy number combine to form an object’s assembly. Every object in the universe can be recast as coordinates in assembly space. Walker’s working hypothesis is that life is the universe’s way of making high-assembly objects.

In other words, life turns rare events into everyday occurrences. Walker illustrates this point memorably through the example of what's called anti-accretion. “Accretion is the process of planetary formation, where matter starts to clump together to form small rocky bodies called planetesimals,” Walker explains. “Anti-accretion, by contrast, occurs on only one planet we know of as I write. That planet is the one we live on, and it is happening because of us.” Why is this so? Because life—specifically, human life—has accrued enough information to understand the laws of physics and construct the high-assembly contraptions we call rockets, satellites, and Mars rovers. She continues, “A one-off ejection event can happen anytime, for example, if a meteor hit the surface of the Earth and ejected debris into space. But the type of anti-accretion I’m talking about is repeatable in a programmable sense. … We can launch satellites to space every day if we choose.”

Throughout the book, Walker makes a careful and elegant case that metrics of Assembly Theory offer scientists a way to quantify the amount of information—of memory—that is required to build an object. The mind-bending thing about Assembly Theory is that it is not just about what we observe here and now, but how it all came to be. Everything we see around us—including ourselves—is part of a shared history, a persistence of memory. “We are each just one temporary instance of life on this planet,” writes Walker, “a pattern of information structuring matter across billions of years.”

As a fellow astrobiologist, I’ve followed Walker’s work for years. I wasn’t surprised to find myself nodding vigorously at many passages, such as her conviction that the search-for-life and origins-of-life communities should seek to collaborate more regularly. After all, any effort to make life from scratch eventually turns into a life-detection problem once you want to know, "Did it work? Did I actually make life?" On the flipside, say you detect a strange gas in the atmosphere of a faraway exoplanet. Your confidence that you’re seeing the burps of alien life hinges upon the likelihood that life got started on that world sometime between its fiery formation and now. If you’re in the dark regarding how life emerges, then you’re not well-equipped to know if you’ve found it in the cosmos.

For me, the most impressive part of the book is how many diverse intellectual threads Walker weaves into her narrative. Not only does she review key issues in astrobiology—from chemist Stanley Miller’s origins-of-life experiment from the 1950s to the contemporary debate over whether the compound phosphine in Venus’s atmosphere constitutes a biosignature—but she also shows how epistemology, philosophy of mind, and the foundations of computer science all contributed to the development of Assembly Theory. For example, Walker rewrites the famous Sagan-ism “extraordinary claims require extraordinary evidence” as “extraordinary claims require extraordinary explanations,” to emphasize the fact that theoretical understanding is just as important as cold, hard data to pushing science forward. Assembly Theory, Walker posits, may just be the explanation for life that astrobiologists need.

In practice, Walker hopes that Assembly Theory will help astrobiologists hunt for signs of life beyond Earth and help identify the origins of life in the lab. Interestingly, she is bearish on the former and bullish on the latter. “Our best bet for making contact with an alien life-form in the near term may be to evolve it, from scratch, in the lab,” she says. In her penultimate chapter, she outlines an exciting (if bold) vision for a Large Hadron Collider–scale origins-of-life project. Walker describes a genesis engine of unprecedented scale that explores as many corners of chemical space as possible. Originally pitched to funders as a machine that can “3D-print any molecule,” Walker hopes that this ambitious effort will also spawn new life right here on Earth. Although a genesis engine sounds like something from Star Trek, Walker reminds us that the idea of creating life from nonlife is not mere science fiction. The origin of life happened at least once, billions of years ago. There is, in principle, no reason why we cannot coax it into happening again.

Assembly Theory is Walker’s avenue toward solving the emergence of life. But it’s not so much a new way of making life as it is a new way of perceiving it. “Unless we confront the problem of what life is head-on, we will not be able to discover alien life or solve our own origins,” she warns. “We will not know it when we see it.”