Eons of sex

Originally published 15 December 1986

How could I resist a book with a chap­ter called “Three Bil­lion Years of Sex?”

But wait. Before you rush out to buy it, you should know that this book is about micro­bial sex, those zil­lions of tiny cou­plings between bac­te­ria that kept life going on Earth for 3 bil­lion years before such things as clams and starfish arrived on the scene to liv­en things up.

The book is Ori­gins of Sex by Boston Uni­ver­si­ty biol­o­gist Lynn Mar­gulis and writer Dori­on Sagan, pub­lished ear­li­er this year by Yale Uni­ver­si­ty Press. It’s a mar­velous con­coc­tion, full of good sci­ence, good prose, and provoca­tive speculation.

Mar­gulis is an evo­lu­tion­ary biol­o­gist who can be relied upon for uncon­ven­tion­al but insight­ful views about how it all hap­pened. She is a great fan of bac­te­ria. Some­times you get the feel­ing that Mar­gulis believes crea­tures like you and I are just huge crowds of bac­te­ria that man­aged to get them­selves orga­nized into walk­ing, talk­ing macro­scop­ic clumps. In Mar­gulis’ view, every­thing real­ly inter­est­ing was invent­ed by microbes: indi­vid­u­al­i­ty, coop­er­a­tion, metab­o­lism, motil­i­ty, pho­to­syn­the­sis, res­pi­ra­tion, and now, in this book, the best thing of all — sex.

Reproduction and sex

Mar­gulis and Sagan make a clear dis­tinc­tion between sex and repro­duc­tion. Repro­duc­tion appeared with the very first organ­isms. One thing shared by all liv­ing cells is a con­tin­u­ous repli­ca­tion of DNA, the mol­e­cules that con­tain the genet­ic infor­ma­tion. Per­haps, Mar­gulis and Sagan believe, organ­isms repro­duce pre­cise­ly because they nev­er stop mak­ing DNA and oth­er large mol­e­cules (RNA and pro­teins). When a bac­teri­um gets too big it tends to divide. Each off­spring of the split takes along a com­plete copy of the orig­i­nal genes. Two indi­vid­u­als have replaced one. That’s repro­duc­tion, and it appears to be a bio­log­i­cal imperative.

Sex, on the oth­er hand, is gravy. Sex is icing on the cake. Life, repro­duc­tion, and evo­lu­tion could have hap­pened with­out it. We could all divide like amoe­bas, spread spores like fun­gi, or bud like tree cut­tings. It is only because of acci­dents of evo­lu­tion that sex appeared at all.

Mar­gulis and Sagan define sex as any process that brings rogeth­er in a sin­gle indi­vid­ual genes from at least two sources. We know what that means for human beings, but the authors of this book have some­thing much broad­er in mind. When a virus invades a cell and inserts its genes into the DNA of its host, that’s a sex act. When sev­er­al hun­dred slime mold cells come togeth­er to pro­duce one Acra­sia slug, that’s sex. When two para­me­cia con­ju­gate, exchang­ing DNA through their cell walls, that’s sex. One won­ders at what lev­el in the lad­der of life this promis­cu­ous mix­ing start­ed being fun.

And why did this busi­ness of gene swap­ping begin at all? Mar­gulis and Sagan take the uncon­ven­tion­al view that sex evolved as a DNA repair mech­a­nism, in response to threats from ultra­vi­o­let light.

Started with photosynthesis

The sto­ry goes some­thing like this. Ear­ly in Earth­’s his­to­ry cer­tain bac­te­ria evolved the abil­i­ty to make sug­ar from sun­light and water, a process called pho­to­syn­the­sis. At that time there was no free oxy­gen in the atmos­phere and, because ozone is a form of oxy­gen, no ozone lay­er to screen out ultra­vi­o­let light from the sun. Bac­te­ria need­ed sun­light for pho­to­syn­the­sis, but ultra­vi­o­let rays had the ten­den­cy to dam­age the del­i­cate genet­ic mate­r­i­al of the cells. And so evolved a bat­tery of mech­a­nisms for ultra­vi­o­let pro­tec­tion, includ­ing mol­e­cules called enzymes that repaired dam­age to DNA integri­ty. These enzymes became the basis for sex­u­al­i­ty. They were the chem­i­cal equip­ment that enabled cells to cut, splice, share, and repair DNA.

Oxy­gen is a by-prod­uct of pho­to­syn­the­sis. After mil­lions of years of bac­te­ria mak­ing food from sun­light, the lev­el of oxy­gen in the atmos­phere increased, and final­ly ozone blocked the dead­ly ultra­vi­o­let radi­a­tion from the sun. Still, organ­isms retained the DNA cut-and-splice tech­niques for var­i­ous rea­sons, not least of which was their con­tin­u­ing use­ful­ness for pre­serv­ing the integri­ty of the genes in the face of oth­er threats and poisons.

Mar­gulis and Sagan trace the evo­lu­tion of sex across 3 bil­lion years. The sto­ry they tell is close­ly tied to Mar­gulis’ con­tro­ver­sial the­o­ry of cell evo­lu­tion by sym­bio­sis. Accord­ing to Mar­gulis, the many-com­part­ment­ed “mod­ern” cell — with a nucle­us for DNA, chloro­plas­ts for pho­to­syn­the­sis, mito­chon­dria for res­pi­ra­tion, and whip­like appendages for mobil­i­ty — evolved by a com­ing-togeth­er for mutu­al ben­e­fit of small­er, spe­cial­ized organ­isms. Through all of this con­struc­tive coop­er­a­tion, new ways of exchang­ing and shar­ing DNA were incor­po­rat­ed into the stream of life. The appa­ra­tus of sex was stretched and polished.

Mar­gulis has always insist­ed that we have much to learn about our­selves by study­ing microbes. In this book, she extends that idea to sex­u­al­i­ty. The views expressed by Mar­gulis and Sagan will annoy some read­ers and dri­ve some con­ven­tion­al biol­o­gists into fren­zies of dis­dain. I’m no biol­o­gist, but to me the sto­ry told in Ori­gins of Sex has the ring of truth.