Introduction

C an life be defined? This question has taken on increasing significance because it now seems likely that in the next few years someone will claim to have assembled a version of artificial life in the laboratory. To make this claim convincingly, it will be necessary to show that the system has a set of properties that fall within an accepted definition of life. The problem is that no definition is generally accepted by biologists. Even the simplest microorganisms are extraordinarily complex, and dictionary-style definitions do not easily encompass such complexity.

Because life is a complex phenomenon, one approach to a definition is to state a minimal set of properties associated with the living state. Here is a list of properties of terrestrial life that, taken together, exclude anything that is not alive.

The machinery of life is composed of polymers, very long molecules composed of subunits called monomers. The primary polymers of life are nucleic acids and proteins, often called biopolymers.

Given this list of properties, we can now test whether it can be used as a definition. There is no doubt that a claim of synthetic artificial life would be convincing if the system incorporated all the above properties. However, if the properties are deleted one by one, the definition becomes blurred and the claim weaker. Suppose the system reproduced perfectly so that evolution could not occur. Would it still be considered to be alive? Most would say yes, so the ability to evolve might not be an essential property of life. But consider another system in which all the nutrients required for growth were present in the medium so that no metabolism was required. This system would resemble a virus that requires the cytoplasm of living cells to reproduce. Viruses, however, can evolve, so they seem to exist in the border between life and non-life.

In a second test of a definition of life, imagine that a future Mars rover discovers what appears to be a frozen pond at the bottom of a crater. The rover has been designed so that it can melt some of the ice and examine the resulting liquid with a microscope. Surprisingly, large numbers of tiny spherical structures can be seen in the images sent back to Earth. Are the particles a form of microbial life? How many of the properties in the above list would need to be confirmed to convince skeptics that life exists on Mars? Can a robotic Mars lander be developed that has the ability to test all the properties? If we are to design such a lander, a definition of life is clearly desirable in order to guide the choices of instrument packages on board.

As an approach to answering such challenging questions, we invited three authors to explore a variety of ways to define life. Mark Bedau is a professor of Humanities and Philosophy at Reed College, Antonio Lazcano is Professor in the School of Sciences at the National Autonomous University of Mexico, and Steven Benner is the director of the Foundation for Applied Molecular Evolution in Gainesville, Florida. Professor Lazcano invited two colleagues, Stephane Tirard and Michel Morange, to join him in writing his essay. Some representative publications of the authors are listed in the references below.

A fourth author, Sergey Tsokolov, was a Ukrainian scientist who died in Germany in 2009. He published an earlier paper in Astrobiology (Tsokolov, 2009), which outlined some of the ideas presented here. His essay was adapted from a book manuscript he was writing at the time of his death.

—David Deamer, Research Professor of Biomolecular Engineering, University of California, Santa Cruz.