The Origin, Extension, and Future of the "NASA Definition" of Life

The "NASA definition" enjoys broad popularity among scientists, particularly in the astrobiology community: life is "a self-sustained chemical system capable of Darwinian evolution." Imprecise and difficult to implement, it nevertheless captures important features of life interesting to researchers and public audiences. A product of the Exobiology Discipline Working Group, it received a four-sentence introduction in a book foreword by Gerald Joyce. The only subsequent defense can be found in an online interview of Joyce. Additional commentary is limited, mostly critical, and occasionally incorrect. This article looks at the history of the definition and how it differs from earlier definitions of life as replication plus metabolism. The NASA definition focuses attention on information, molecular mechanisms, and "Darwinian evolution" through generalizations of replication. It retains traditional ambiguity about metabolism related to concepts of "self," "self-sustaining," and "capable." Understood in this context, the NASA definition identifies the current state of biology and suggests an agenda for future research. A "NASA working definition" may be more usefully constructed as follows: NASA will recognize life in a self-sustained chemical system demonstrating evolution by natural selection.