At one time, life was simple, and defining it was easy. NASA defined life as: “a self-sustained chemical system capable of undergoing Darwinian evolution.” [1] In that case, what are they to make of recent findings that life’s simplest cells evolve mainly by swapping genes, and not through Darwinian competition? [2] Can they forbid teaching that in publicly funded schools – Texas Darwin lobby-style? But then …
Don Johnson, author of Probability’s Nature and the Nature of Probability offers, on the definition of life:
Although there is no universally accepted definition of life [3], it often includes characteristics like metabolism, growth, adaptation, and reproduction. The existence of a genome and the genetic code divides living organisms from nonliving matter is perhaps the most concise definition of life. [4]
Concise yes, and it offers no theory of origin. Thoughts?
[1]The wording is attributed to OOL researcher Gerald Joyce, in Foreword, Origins of Life: The Central Concepts, D.W. Deamer and G R Fleischaker, editors (Boston: Jones & Bartlett; 1994). pp. xi–xii. This article from the NASA Astrobiology Institute: Leslie Mullen, “Unfamiliar Life: Why should the particular polymer combinations of Earth reign supreme?” (April 17, 2002) The definition seems to have been accepted.
[2] “Microbes evolve predominantly by acquiring genes from other microbes, new research suggests, challenging previous theories that gene duplication is the primary driver of protein evolution in prokaryotes”, from Megan Scudellari Gene swap key to evolution: Horizontal gene transfer accounts for the majority of prokaryotic protein evolution, The Scientist ( 27th January 2011)
[3] Claus Emmeche, “DefiningLife, Explaining Emergence,” 1997.
[4] Hubert Yockey, Information Theory, Evolution, and the Origin of Life, 2005. This definition includes as living (or at least as once living) organisms those that are sterile (e.g. mules and worker ants) and those not having cells (e.g. viruses). While life uses the laws of chemistry and physics, those laws cannot define or explain life any more than the rule of grammar that were used during the preparation of this book define its content. (P. 18)