Would you trade immortality for sex? Oops. Too late. You already did.

Or your genes did anyway. Sex (in this case gene-sex, the mixing of genetic information to ensure a progeny more likely to survive catastrophic changes in the environment) and reproduction (replication of the genetic code) happen to be the same thing in humans. But for certain other organisms, e.g. bacteria, algae, and protozoids, these activities are very different things. Reproduction occurs by mitosis, or self-cloning. Gene-sex occurs separately, in meiosis, when two different cells meet and exchange some of their DNA for the DNA of their sex partner. Their DNA may be changed, but they’re still the same cell.

But somewhen, way down the evolutionary stalk, while E.coli was optimizing for lifespans in the millions of years, our genetic ancestry began to combine gene-sex with reproduction into our more familiar, err, sloppy sex, and suddenly aging and death became important, ironically, for genetic survival.

Or so goes the theory by Tom B. L. Kirkwood, building on W. D. Hamilton/Richard Dawkinsselfish gene model of evolution. Kirkwood’s theory says that among humans (and by extension any of the other sloppy sexers), nature selects genes that waste energy on nothing but getting their host to survive long enough to reproduce.

Ah, you cry, but if we were more bacterial and didn’t age, couldn’t we produce a much greater number of offspring and thereby win the generace? If it were solely up to the gene, perhaps. But the gene can only control its host organism, not the chaotic environment in which it lives. Even if we didn’t age, there would still be cellular damage to repair, not to mention earthquakes, famine, and car crashes. So the gene that diverts some of its biological energy on internally immortal hosts who die anyway from external causes will fall behind the gene that keeps its energies focused on survival-until-reproduction at around the same age. Genes therefore tend to optimize for likelihood-of-survival, which for most of the time since we walked out of the Afar Triangle, seems to have been about 20-30 years. As far as our genes are concerned, after that time, the body, or soma—in which humans are so vested—is, for the gene, disposable. (Soma in this case from somatic, not from the Vedic drink of immortality, though they share a root.) For this reason, Kirkwood’s theory became known as the Disposable Soma Theory of aging. (circa 1985)

But it’s only because gene-sex and reproduction are coupled that this is an issue. Our genes are pressured to reproduce so they can benefit from the simultaneous effect of gene-sex and thereby gain an optimally responsive rate of evolution. For our distant cousin E.coli, his genes can optimize gene-sex separately from reproduction, and so doesn’t have the same constraint. We may get the delightful benefit of sloppy sex, but getting it cost us death.

Unfortunately, as individuals we didn’t really have a choice in the matter, but it makes a fun little cocktail question.

Postscript: With this in mind, it’s hard for me to imagine choosing chastity as a sexual lifestyle. It’s like refusing to drive a car for which you’ve already been forced to forfeit your entire paycheck.

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