Define life? That’s easy to do…or is it?

Originally published 19 January 1997

What is life?

No ques­tion in sci­ence is more fun­da­men­tal. No ques­tion is more dif­fi­cult to answer.

It is easy to rec­og­nize life when we see it, but dev­il­ish­ly hard to say what it is.

Half a cen­tu­ry ago, the great Aus­tri­an physi­cist Erwin Schrödinger tried a def­i­n­i­tion in a lit­tle book called What Is Life? He was con­vinced that life would even­tu­al­ly be account­ed for by physics and chem­istry, and his book helped inspire the bio­mol­e­c­u­lar rev­o­lu­tion, but the best he could do for a def­i­n­i­tion was “an elab­o­rate, coher­ent, mean­ing­ful design traced by the great master.”

More recent­ly, the Amer­i­can biol­o­gist Lynn Mar­gulis and co-writer Dori­on Sagan have tack­led the ques­tion again in a book of the same title: What Is Life? They pro­vide a bril­liant sum­ma­ry of what we have learned about life, but the sought-for def­i­n­i­tion remains elusive.

To answer the ques­tion accord­ing to the rules of gram­mar, we must sup­ply a noun, the name of a thing, say Mar­gulis and Sagan; but life is more like a verb. They try their hand at sev­er­al definitions:

“It is a mate­r­i­al process, sift­ing and surf­ing over mat­ter like a strange, slow wave.”

“It is the watery, mem­brane-bound encap­su­la­tion of spacetime.”

“A plan­e­tary exuberance.”

“Exis­tence’s celebration.”

All of which are love­ly metaphors, but none of which get us any clos­er to the ineluctable heart of the mystery.

What we know for sure is that life has exist­ed on Earth for near­ly 4 bil­lion years, and that all life on Earth is relat­ed by com­mon descent.

As to where the first self-repli­cat­ing organ­isms came from, no one has a clue.

Let us assume an ances­tral liv­ing cell, as sim­ple as the sim­plest bac­teri­um exist­ing today — an unnu­cle­at­ed blob of pro­to­plasm enclosed by a mem­brane. Micro­scop­i­cal­ly small. Autopoi­et­ic: that is, capa­ble of main­tain­ing itself by chem­i­cal inter­ac­tion with the environment.

A typ­i­cal bac­teri­um repro­duces every half hour. One makes two, two make four, four make eight, eight make six­teen, and so on. Start with a sin­gle bac­teri­um on the ear­ly Earth, and 20 gen­er­a­tions lat­er — 10 hours — you have a mil­lion, enough to cov­er the head of a pin.

Two days lat­er — 120 gen­er­a­tions — you will have enough bac­te­ria to fill the oceans of the world chock-a-block with life. A few hours lat­er, the entire sur­face of the Earth will be wrapped in a lay­er of bac­te­ria 10 miles thick!

Clear­ly, some­thing must be wrong with my cal­cu­la­tion; the globe is not wrapped in a thick lay­er of liv­ing slime. For unre­strict­ed repro­duc­tion, organ­isms need access to chem­i­cal nutri­ents from the envi­ron­ment. As my cal­cu­la­tion vivid­ly illus­trates, com­pe­ti­tion for resources can be fierce even for bacteria.

From the very first days of life on Earth, bac­te­ria were locked in a bat­tle to sur­vive. The corol­lary of life is death. Culling. Recy­cling. Por­tion­ing out resources. No def­i­n­i­tion of life is com­plete that does not include death.

For bil­lions of years, microbes com­pet­ed “tooth and claw” for the oppor­tu­ni­ty to repro­duce. Far more failed than suc­ceed­ed. Most branch­es on the tree of life were nipped in the bud. A few lucky lin­eages fin­gered into the future, avoid­ing the sweep­ing scythe of death, like the few stalks of grain that remain stand­ing in a har­vest­ed field.

Those few lucky lin­eages were helped by chance, by rare muta­tions that honed the chem­i­cal machin­ery of autopoiesis, that gave the organ­ism an edge in the strug­gle to survive.

In biol­o­gy text­books, the time­lines of the first 3 bil­lion years of life on Earth are most­ly blank. Pho­to­syn­the­sis. Res­pi­ra­tion. Nucle­at­ed cells. Sex­u­al repro­duc­tion. It would appear that not much hap­pened. But in fact every­thing was hap­pen­ing; life was per­fect­ing the com­plex chem­istry that sus­tains every liv­ing crea­ture on Earth today.

By the time the first mul­ti-celled organ­isms appeared about 700 mil­lion years ago — and the time­line of Earth his­to­ry becomes crowd­ed and famil­iar — most of the real work of evo­lu­tion is fin­ished. The basic chem­i­cal machin­ery of autopoiesis and repro­duc­tion is in place. Every­thing that fol­lows will be vari­a­tions on a theme.

I’ve yet to find a pop­u­lar treat­ment of the first 3 bil­lion years of life that paints a suf­fi­cient­ly vivid pic­ture of those del­i­cate lin­eages of microbes fin­ger­ing into the future, a branch­ing fam­i­ly tree replete with myr­i­ad dead-ends, inch­ing for­ward under the great over­ar­ch­ing shad­ow of death, always bear­ing the residue of the past, teas­ing self-main­te­nance from the envi­ron­ment, trans­form­ing the Earth­’s crust, atmos­phere and oceans, com­pet­ing, occa­sion­al­ly turn­ing exploita­tion into mutu­al advan­tage, per­fect­ing meta­bol­ic path­ways of aston­ish­ing complexity.

Of writ­ers who have attempt­ed to tell the sto­ry of those first 3 bil­lion years of life on Earth, Lynn Mar­gulis and Dori­on Sagan come clos­est to suc­cess. Their book is a must read for any­one who wants to tus­sle with nature’s most recal­ci­trant question.

What is life? How does one define a flame, a Mozart sym­pho­ny, or 50 tril­lion gen­er­a­tions of micro­bial evo­lu­tion? Per­haps, after all, it is not pos­si­ble to improve upon Schrödinger’s def­i­n­i­tion: “an elab­o­rate, coher­ent, mean­ing­ful design traced by the great master.”