Shadows of Forgotten Ancestors

Occasionally, the dead manage to have sex and generate offspring. When a bacterium dies, its contents are spilled into the surroundings. Its nucleic acids don’t know much about the death of the bacterium and even as they slowly fall to pieces, the fragments remain for a time functional—like the severed leg of an insect. Should such a fragment be ingested by a passing (and intact) bacterium, it may be incorporated into the resident nucleic acids. Perhaps it is used as an independent record of what undamaged instructions should say, helpful in repairing DNA altered by oxygen. Maybe this extremely rudimentary form of sex arose along with the Earth’s oxygen atmosphere.

Bizarre chimerical gene combinations happen more rarely—for example, between bacteria and fish (not only are there bacterial genes in fish today; there are also fish genes in bacteria), or baboons and cats. They seem to have been brought about by a virus attaching itself to the DNA of a host organism, reproducing with and accommodating to the host over the generations, and then shaking loose to infect another species while carrying some of the original host’s genes with it. Cats are known to have acquired a baboon virogene somewhere on the shores of the Mediterranean Sea 5 to 10 million years ago.⁵ Viruses are looking more and more as if they are peripatetic genes that cause disease only incidentally. But if genetic exchanges can occur today in such widely divergent organisms, it must be far easier for them to occur, by accident, in organisms of the same or very closely related species. Perhaps sex started out as an infection, becoming later institutionalized by the infecting and infected cells.

Two distant relatives, members of the same species, each in the process of replication, find their nucleic acid strands, one from each, laid down, cozily, alongside one another. A short segment of one very long sequence might be, say,

 … ATG AAG TCG ATC CTA …

and the corresponding segment of the other

 … TAC TTC GGG CGG AAT …

The long nucleic acid molecules both break apart at the same place in the sequence (here, just after AAG in the first molecule and TTC in the second), whereupon they recombine, each picking up a segment of the other:

 … ATG AAG GGG CGG AAT …

and

 … TAC TTC TCG ATC CTA …

Because of this genetic recombination, there are two new sequences of instructions and therefore two new organisms in the world—not exactly chimeras, since they come from the same species, but nevertheless each constituting a set of instructions that may never before have coexisted in the same being.

A gene, as we’ve said, is a sequence of perhaps thousands of As, Cs, Gs, and Ts which codes for a particular function, usually by synthesizing a particular enzyme. When DNA molecules are severed, just prior to recombination, the cut occurs at the beginning or the end of a gene, and almost never in its middle. One gene may have many functions. Important characteristics of the organism—height, say, aggressiveness, coat color, or intelligence—will generally be the consequence of many different genes acting in concert.

Because of sex, different combinations of genes can now be tried out, to compete with the more conventional varieties. A promising set of natural experiments is being performed. Instead of generations patiently waiting in line for a lucky sequence of mutations to occur—it might take a million generations for the right one, and the species might not be able to wait that long—the organism can now acquire new traits, new characteristics, new adaptations wholesale. Two or more mutations that don’t do much good by themselves, but that confer an enormous benefit when working in tandem, might be acquired from widely separated hereditary lines. The advantages (for the species, at least) seem clear, if only the costs were bearable. Genetic recombination provides a treasure trove of variability on which natural selection can act.⁶