DEE WILSON CONSULTING
Book Review:
A Thoughtful Discussion of Life
Life's Edge: The Search for What it Means to be Alive
Carl Zimmer (2021)
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This Carl Zimmer work is a wide ranging discussion of life's characteristics (e.g., reproduction, metabolism, homeostasis), the mystery of life's beginning and competing theories regarding its origin, along with numerous anomalies that cloud the boundary between living and non-living things and between life and death among biological species. One of the most important and consequential discussions concerns definitions of life, all of which appear problematic, and of whether a consensus definition of life is necessary or counter- productive for scientific inquiry. Interestingly, it turns out that philosophers have something insightful to say on the function and limits of definitions in science.
Zimmer comments that living things such as trees, snakes, bats, slime molds, and bacteria have "hallmarks": "They all reproduce and evolve, they all make decisions, turn food into energy, and maintain an internal balance. ... If all living things share certain hallmarks, can they tell us what life is?
That question - What is life? - may seem like it's the first and foremost question biologists should answer. And yet it remains unanswered and, perhaps, ultimately unanswerable." In recent decades, NASA's definition of life, "a self-sustained chemical system capable of undergoing Darwinian evolution" has been widely used by scientists. However, this definition is open to obvious objections in that it excludes viruses (not self sustaining, require host cells), red blood cells that lack genes and mitochondria that "cannot exist outside their host cells, ... cannot find their own food, ... cannot build genes or proteins on their own," yet "our own lives depend on them."
Any specific "hallmark" of life has exceptions, which have left some scientists baffled or frustrated, and led some to abandon the search for a satisfactory definition in favor of studying weird chemistry in extreme environments such as Enceladus, one of Saturn's moons, without feeling a need to classify all unusual or surprising chemical reactions as living or non- living. One of these scientists, Laurie Barge, in response to Zimmer's question, "Is there a definition of life that is guiding your work?" asserts "Not really, and I actually try not to have one ... I've been really surprised and impressed with what organic chemistry can do when you remove life from the system. And I don't know how far it can go, honestly."
Some philosophers have utilized Wittgenstein's ideas regarding family resemblances among games to develop a working definition of life. Zimmer states: "They ( philosophers and scientists at Lund University in Sweden) propose that we can call something alive if it has a number of properties that are associated with being alive. It doesn't have to have all of those properties, nor does it even need exactly the same set found in any other living thing. Family resemblances are enough."
In my view, the philosopher cited by Zimmer who has the most insightful perspective regarding the limits of definitions in science is Carol Cleland. She maintains that "there's no point in searching for a definition of life or even a convenient stand-in for one. It's actually bad for science ... because it keeps us from reaching a deeper understanding about what it means to be alive. "Definitions," Cleland wrote, "are not the proper tool for answering the scientific question 'what is life'?" Cleland argues that "Definitions serve to organize our concepts. .. because they have a narrow job to do, we can't revise them through scientific investigation. (Life) is not the sort of thing that can be defined simply by linking together concepts. .. it's futile to search for a laundry list of features that will turn out to be the real definition of life." Cleland asserts: "We don't want to know what the word 'life' means to us. We want to know what life is. ..." And, she asserts "The best thing that scientists can do right now, ... is work towards a theory that explains life." Zimmer comments: "I've met many scientists who agree with Cleland on this point. They do not have a theory of life yet. They're confident that someday a theory will emerge, but for now they can only guess what it will be."
To explain Cleland's perspective, Zimmer uses the example of attempts to define water before the age of modern chemistry, i.e., " by putting together a list of its qualities." "The quandary was so deep that it left even Leonardo da Vinci confused. ... Crafting a new definition of water wouldn't have freed Leonardo from his ignorance. The difficulty lay elsewhere: in just how little he and everyone else in the Renaissance knew about chemistry. It took three centuries for a mature theory of chemistry to emerge, one that explained that the universe is made up of atoms belonging to many elements, which it can bond together to form different molecules."
Zimmer discusses a couple of theories of life, one developed by Stuart Kauffman that describes networks of molecules that become more abundant by joining together "into loops of chemical reactions." "Kaufmann named these networks autocatalytic sets." Kaufmann discovered these molecules through computer modeling, yet "Autocatalytic sets are not just mathematical dreams. But that doesn't mean they are common in nature, "Zimmer states, in part because they are prone to collapse unless they have a "resilient structure." Inquiry into the properties of autocatalytic sets is a promising approach to developing a theory of life, Zimmer states, but "Scientists will have to settle questions such as these ( i.e. relating to structure) before autocatalytic sets can become part of a mature theory of life."
Kauffman and other scientists interested in developing a theory of life are looking for a way to explain how large numbers of organic molecules can join together and replicate. They are searching for rules of assembly. Zimmer writes: "Life ...is a state of matter that can spontaneously make things with a lot of assembly steps." Some scientists have proposed that what allows life to do this is the special way in which information flows through it. "In life, information is able to control matter."
What these theories do not explain is how non- living matter can create entities that seek a preferential state conducive to surviving and flourishing. It is the valuing inherent in life that requires explanation, not just how genes, metabolism, homeostasis and goal directed behavior serve this end.
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-- Dee Wilson
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