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How we think about what it means to be alive will always depend on what questions we ask

Life's Edge: The Search for What It Means to Be Alive

Carl Zimmer
368 pp.
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Carl Zimmer’s Life’s Edge is a departure from his previous work in that it is a book that is as much about what scientists have so far failed to understand as what they have come to understand. As its subtitle suggests, this book is about how life is defined, how life arose, and how we tell life from nonlife. These topics seem as though they would be of great concern to the field of biology—biology being, after all, the study of (“ology”) life (“bio”)—but they have rarely received much formal attention, occupying the scientific margins for hundreds of years and appearing on center stage every so often only to quickly retreat.

Zimmer begins with a story from the early 1900s in which a physicist named John Butler Burke synthesized “highly organized bodies” that resembled microbial colonies using radium and sterilized beef broth. Newspapers buzzed with exciting headlines, proclaiming that Burke had discovered the “secret of life.” But the scientist’s fame and success were short-lived, his discovery a false start. The book is full of such false starts, including the notable period during which the biologist Thomas Huxley became convinced that life evolved from a kind of primal slime that coats the bottom of the sea. (Spoiler alert: It did not.)

Superficially, the question “What is life?” seems resolvable. After all, as Zimmer points out in the book’s first chapter, “experiments on animals,” including chickens, “have revealed they can make some of the same distinctions between the living and the nonliving that we do.” If poultry can make sense of the boundary between the living and the nonliving, how difficult could it be? Very difficult, it turns out.

At every boundary, life is blurry. When does the life of one generation begin and that of the previous generation end? Is a bacterial spore that is not metabolizing alive or dead or something else? If a human body is partially human cells and partially bacterial cells, and the bacterial cells go on living after the human cells have died, has the organism died? If some of the human cells go on living and dividing, has the human died? Zimmer shows that the more one searches for answers to these questions, the more such answers retreat.

Throughout the book, Zimmer illustrates how our behavior and our conceptions of birth, death, and organismal boundaries are very human-centric. For each species, these criteria are different, sometimes substantially so. The “bodies” of slime molds, for instance, can break apart, dry out, and drift in the wind when times are tough, only to reunite again under better circumstances.

One has the feeling, while reading this book, of fumbling through the unknown. In a section called “The Quickening,” for example, Zimmer transitions from a careful discussion of the biological details of fertilization, to studies of species such as tardigrades that can enter life stages in which they are quiescent and neither dead nor fully alive, to research on when early human ancestors began to afford the dead special status by burying them. Meanwhile, the poems of Erasmus Darwin are set alongside Mary Shelley’s Frankenstein and the chemistry of urea, to fascinating effect.

I found myself feeling very grateful that Zimmer had drawn connections among these disparate themes. We biologists are often necessarily narrow in our perspective. “To become an expert on just one kind of life can demand an entire career,” Zimmer acknowledges. His breadth reveals more of the whole, however blurry, than would otherwise be available to the specialist.

There were also plenty of sections that made me wish that we were living in a time when dinner parties were possible, so that some of Zimmer’s observations might be readily shared: details about the sex lives and intelligence of slime molds, the possibility that tardigrades are currently living on the Moon, and his descriptions of the expandable hearts of some snakes, for example.

By the end of this book, I felt challenged as a biologist to pull together my colleagues to talk about the big issues related to the limits of life, the origins of life, and the margins of life. We do not have these conversations often, probably partly because we are all so specialized, but also likely because the beginnings of life and the origins of life have become politicized.

To this latter point, Zimmer reminds readers that how we think about the boundaries of life will always depend on what questions we ask. Quoting the biologist Joshua Lederberg, he writes, “The question of when life begins is answered according to the purposes for which we ask it.” By the end of the book, Zimmer had fully convinced me that the question of what it means to be alive is also best answered according to the purposes for which we ask—and that such inquiries will yield different outcomes depending on how we ask them.

About the author

The reviewer is at the Department of Applied Ecology, North Carolina State University, Raleigh, NC, USA, and the Center for Evolutionary Hologenomics, University of Copenhagen, Copenhagen, Denmark, and is the co-author of Delicious: The Evolution of Flavor and How It Made Us Human (Princeton University Press, 2021).