Life is short
LIFE: the condition that distinguishes organisms from inorganic objects and dead organisms, being manifested by growth through metabolism, reproduction, and the power of adaptation to environment through changes originating internally.
The deﬁnitions [of life, IF] are more than often in conﬂict with one another. Undeniably, however, most of them do have a point, one or another or several, and common sense suggests that, probably, one could arrive to a consensus, if only the authors, some two centuries apart from one another, could be brought together. One thing, however, can be done – sort of voting in absentia – asking which terms in the deﬁnitions are the most frequent and, thus, perhaps, reﬂecting the most important points shared by many. Such analysis is offered below, revealing those most frequent terms that may be used for tentative formulation of the consensus.
By word count, seems like life has mainly to do with living. Well, no surprises there, but somewhat tautological and less-than-informative. However, rejoice O system biologists, for SYSTEM is the second most frequent keyword grouping. Then we have organic stuff, CHEMICAL, COMPLEXITY, REPRODUCTION with ENERGY and ABILITY trailing. Trifonov continues:
Thus, the consensus of the life definition patched from these nine definientia would be: Life is [System, Matter, Chemical (Metabolism), Complexity (Information), (Self-)Reproduction, Evolution (Change), Environment, Energy, Ability,…] where the square brackets correspond to some compact expression containing the words listed within. For example, one possibility is:
Life is metabolizing material informational system with
ability of self-reproduction with changes (evolution),
which requires energy and suitable environment.
(I added the underlines.) Hm. Actually, not bad for a definition culled from a simple exercise in word counting. Of course, to put these definientia together one would need some knowledge of life, this the exercise is not completely automatic and unbiased, nor does it profess to be so.
But Trifonov wants to condense this definition even more. To quote Hemingway: “boil it down; know what to leave out; tell a story in six words”. Is there still some redundancy in the definientia themselves that would let us boil it down and tell the story in only six words? Trifonov argues that metabolism implies the existence of energy and materials. Whereas the existence of materials already implies a suitable environment. But self-reproduction subsumes all the above, as it requires metabolism, energy, materials and environment. However, variations and self-reproduction are actually mutually exclusive. Both must be noted. The boiled down, Hemingwayan definition would therefore be:
Life is self-reproduction with variations.
And, to top it off, six words! Hemingway achievement unlocked.
Of course, this succinct definitions renders all sorts of problems. trifonov admits to that:
One unforeseen property of the minimalistic definition is its generality. It can be considered as applicable not just to “earthly” life but to any forms of life imagination may offer, like extraterrestrial life, alternative chemistry forms, computer models, and abstract forms. It suggests a unique common basis for the variety of lives: all is life that copies itself and changes.
Here is where I think things go a bit too far: is self-reproducing (and mutating) software alive? Is the Weasel program alive? Are viruses alive? All of those examples fulfill, at least technically, the above definition. In a previous post, I talked about going from life to non-life on a scale roughly correlating to size and thus the amount of information and sustaining materials life can carry with it. The difference between life and non-life seems to be not only in self-reproduction with variations, but the ability to do so at some level of autonomy. When adding the caveat of autonomy, viruses are not alive, since they require the transcriptional and translational machinery of their host cells. Neither are organelles such as mitochondria, since most of their proteins are encoded by the nucleus. But requiring autonomy raises another problem, which I find hard to solve: how far does this requirement of autonomy go? After all, all heteretrophs are, to some degree non-autonomous, as they require basic materials produced only by autotrophs. So the definition becomes fuzzy again: humans are alive, although they cannot self-sustain without plants. Plants are alive, but they cannot fix nitrogen and require bacteria to do so. So are we to say that autotrophic nitrogen-fixing bacteria the only living species on earth? So maybe the autonomy criterion be limited to self-reproduction rather than metabolism? But those are hard to separate: without sugar, there is no DNA. Without essential amino acids, which most heretrophs acquire by consuming other organisms, there are no proteins to effect reproduction.
despite the difficulties, my definition would be (seven words, unfortunately):
Life is autonomous self-reproduction with variations.
Fuzzy? You betcha. That’s life.
PS: as you can see from the article’s Pubmed page, it generated a flurry of comments. Those make for a great read too. Enjoy.
Trifonov EN (2011). Vocabulary of definitions of life suggests a definition. Journal of biomolecular structure & dynamics, 29 (2), 259-66 PMID: 21875147