What does it mean to be alive? Think about the question for more than a minute and you find a definition nearly impossible. We know it when we see it, we believe, but perhaps the best we can say about life is what the eighteen century French doctor Xavier Bichat said about it: “Life consists in the sum of the functions, by which death is resisted.” (42) In other words, as long as something can be classified as ‘not dead’ it might be considered alive.
But, as Carl Zimmer notes in his new book, Life’s Edge: The Search for What it Means to Be Alive, the boundaries are fuzzy. He quotes Edgar Allan Poe from his story “The Premature Burial”: “The boundaries which divide life from death are at best shadowy and vague…Who shall say where the one ends, and where the other begins?” (48-9)
Zimmer is a veteran science columnist for The New York Times and his stock in trade is taking difficult scientific research and turning it into something comprehensible. Even so, what he’s attempting in this book is difficult for the lay reader. It’s not just that the research fields, from geology to microbiology to neurology, are so technical, it’s also that the tools the scientists are using, for all their abstruse precision, are still just blunt instruments for getting at the central question. Defining life turns out to be hard work.
There is so much to explore in the borderlands that Zimmer wants to illuminate. For instance, if information gathering and consciousness are a hallmark of life, how to explain slime molds—a single-celled, brainless monstrosity of yellow goo that somehow can make its way through mazes to get at a source of food? And if reproduction is another hallmark, what do we do with Amazon mollies, a type of fish that is only found as females, the result of a particular kind of interbreeding between two other species? And yet the Amazon mollies have survived for some 280,000 years as a kind of companion to the two other fish species. “It is as if the survival of the species depended on their company,” Zimmer notes. (214)
Then there are viruses, like our current nightmare variety, SARS-CoV-2, which don’t qualify as life if you count things like metabolism and decision-making as essential markers, but which can certainly reproduce and evolve, as we are seeing. “Can we exile all this biological diversity from life? To exile viruses also means we have to discount how intimately woven they are into life’s ecological web.” (210)
As Zimmer takes us back in time to the primordial ooze and volcanic pools from which life may have emerged, you admire how far we have come in such a relatively short time in understanding the building blocks of life. Yet you also sense the mystery that still remains in the heart of the question.
One of the remarkable scientists that Zimmer profiles is a Hungarian physiologist named Albert Szent-Györgyi who spent much of his life, despite difficult experiences in both world wars, pursuing the ‘meaning of life’ question. He was wary of using the hallmarks of life as an inviolable standard. As he once told an MIT audience, “One rabbit could never reproduce itself…And if life is characterized by self-reproduction, one rabbit cannot be called alive at all.” (180)
Near the end of his life, after pursuing the question in a number of different fields, Szent-Györgyi concluded: “I moved from anatomy to the study of tissues, then to electron microscopy and chemistry, and finally to quantum mechanics. This downward journey through the scale dimensions has its irony, for in my search for the secret of life, I ended up with atoms and electrons, which had no life at all. Somewhere along the line, life has run through my fingers.” (181)
As someone with much more affinity for the theological pursuit of this question, I’m not too bothered by that conclusion. Life is always something that has to be understood by an eccentric (turned outward) orientation since the attempt to define it from within always falters. And surely science will offer up more tantalizing tidbits in the search for a definition. But I’m glad for the continuing mystery if only to hold on to what small shreds of humility we still retain.