What is Life

We usually think that life is easy to recognize, that it would be obvious if something is alive or not. It turns out that is not that easy. The question “what is life” is asked in very different contexts by different groups of people. For centuries, theologians have hotly debated life’s definition and relation to the beginning of human life. Does life start at the moment of conception? Or does it begin when the brain’s first response, or with the heart’s first beat? In some theological doctrines, life commences not with a physical process, but rather at the unknowable supposed instant known as “ensoulment”.

At the other end of our human journey, doctors, lawyers and politicians require a definition of life in order to deal ethically with patients with brain death. As we saw with the contentious case of Terri Schiavo (the woman who spent more than a decade in comma), lots of people have intense and emotional views on this issue.

In sharp contrast with these ethically difficult and emotionally charged issues, are the more abstract ongoing scientific efforts to define life. A must read book on the origin of life is Noam Lahav’s “Biogenesis”, which was published in 1999. Lahav’s works in the Hebrew University at Jerusalem, and he has been involved in origins research for almost 40 years. His book is filled with insights, as well as countless technical details. As part of his text, he prepared an appendix with lots of different scientific definitions of life, which are written by over 48 different authorities.

These definitions span 150 years period, from the mid 18th century to the late 20th century. It’s worthwhile thinking about a few of those:

- Alexander Oparin: he reflects the view of many authorities. Life can be defined by a combination of traits. He says: “Life may be recognized only in bodies which have particularly special characteristics. These characteristics are peculiar to living things, and are not seen in the world of the dead.” What are these characteristics? In the first place, there is a definite structure or organization. Then there is the ability of organisms to metabolize, reproduce others like themselves and the response to stimulation. The problem is that Oparin’s characteristics are not unique to life. Many non-living systems have definite structure and organization (think about your car or your PC). Oparin says that organisms obtain energy from their surroundings to grow and reproduce, but fire does that also. Many natural non-living systems, such as flowing water or drifting clouds, respond to stimulation.

- John Desmond Bernal: an influential 20th century biological theorist, who provides a longer list of characteristics. He says: “Life is a partial, continuous, progressive, multi-form, and conditionally interactive self-realization of the potentialities of atomic electron states”. I don’t know about you, but that definition seems to be hopelessly fussy and unhelpful in distinguishing life from non-life.

- Stuart Kauffman offered a more promising definition of life in 1993. He claimed: “Life is an expected collectively self-organized property of catalytic polymers”. Embedded in this statement are a couple of key ideas. Kauffman said that life is self-organized. That is, life is a collective emergent phenomenon. He also states that life relies on chemicals to promote the production of more copies of themselves. In Kauffman’s view, life might be a relatively simple collection of self-replicating chemicals. That includes much more primitive entities than modern cellular life.

- John Maynard Smith proposed a short and persuasive definition of life in 1975. He describes life as “any population of entities which have the properties of multiplication, heredity and variation”. Here Smith introduces two key ideas and thus comes closer to a useful set of criteria. First, all life possesses information that’s passed from one generation to the next. That key idea of heredity may not be unique to life, but it is certainly one of life’s most important characteristics. Second, life displays variations. In life, heredity isn’t perfect like a Xerox copy. Variation, in turn, leads to evolution by natural selection.

Lahav goes on and on citing definitions of life, and remarkably, no two definitions are the same. I think you can see this lack of agreement might represent a problem for those of us who search for signs of living organisms in other worlds, as well as for anyone interested in the origin of life. After all, how can you be sure that you discovered life, or that you figured out the process of life origin, when you can’t come close to defining what exactly life is? In spite of generations of work by hundreds of thousands of biologists, we still have no universally accepted definition.

To be continued…