Life is a self-sustained chemical system capable of undergoing evolution. (definition of life by Richard Underwood)
This definition of life includes everything we intuitively know is alive, and excludes everything we know is not alive.
There are close to eight billion people living in the world. Most of those people are intelligent, many are clever, and a few are extremely clever. So, it is surprising nobody has yet come up with a definition of life to which everyone else can agree. It is not as though there’s a shortage of research material.
All adults have lived longer than it takes to complete a PhD, so when it comes to the subject of life, we should all be experts. Unfortunately, despite the accumulated years of study, ‘life’ is a subject which has few experts, and what experts there are disagree about almost every facet of the subject.
Marcello Barbieri, an Italian theoretical biologist and professor of embryology at the University of Ferrara, Italy, and Radu Popa, an associate professor of biology at Portland State University, U.S.A. have both independently published books containing definitions of life. Between them, Barbieri and Popa have listed over 120 different definitions. The problem is not that there is no definition of life, but that we can’t agree on which definitions are right. At a workshop in 2003 every member of the International Society for the Study of the Origin of Life was asked to give his or her definition of life, and between them, they gave 78 different definitions.
Even the NASA working definition of life includes the phrase ‘Darwinian Evolution’, but there was a lot of debate about whether the word ‘Darwinian’ was necessary, or whether there may be other types of evolution than Darwinian evolution. I believe there are.
We all know that we are alive, and we intuitively know what things are alive and what things are not alive. The problem, is that almost all of the definitions of life do not agree with what we intuitively know. They all seem to include things which we would intuitively know are not living, and to exclude things that we intuitively know are alive. For example, any definition of life which includes an ability to reproduce, would automatically exclude any woman over child bearing age and any adult male who is sterile.
I believe my definition of life that Life is a self-contained system capable of undergoing evolution defines all possible life forms, including all those life forms which are problematical for other definitions. The definition covers viruses, plasmids, viroids, prions, hybrids, chimera, artificial life, the last living specimen of a near extinct species, new forms of life that may exist only within the confines of a laboratory, and life that may (or may not) exist in the furthest reaches of the universe.
My book, ‘A Brief History of Life‘ looks at this subject in depth, and examines all the contentious life forms or near life forms that have proved problematical for other definitions of life. Without exception, my definition of life agrees with what we intuitively know is true about what collection of fundamental particles are alive and which collection are not alive.
Definition of life: Life is a self-contained system capable of undergoing evolution.
