Map of life expectancy at birth from Global Education Project.

Sunday, November 20, 2005

Sex and Death

They are probably our greatest preoccupations, and they have something else in common. They are both what I will call meta-evolutionary attributes of living organisms. They are essentially universal not because they favor the survival and reproduction of individuals, but because they favor evolution itself.

Without the ability to share and reshuffle combinations of genes among individuals, a population could scarcely evolve. If a favorable mutation happened to arise in an individual of a purely parthenogentic organism, it would remain with the descendants of that organism only, and no others. It could never be paired with other mutations that might further enhance its favorability. Indeed, it might not even be favorable unless it happened to be matched with some other characteristic that the mother organism lacks, but may be present in other individuals. Prokaryotes -- cells without a nucleus -- can exchange genes in processes not directly related to reproduction, but the eukaryotes evolved the strategy of sexual reproduction and they all use it. Even single-celled protozoans that usually reproduce by simple cell division sometimes create and then fuse haploid cells, i.e. reproduce sexually.

An individual could reproduce more efficiently and certainly, and pass on 100% of its genes, by asexual reproduction. If the idea of "survival of the fittest" meant that evolution rewards success at passing on 100% of an individual's genes, we wouldn't have sex. But that's not what it means. Evolution selects lines of descent over the long term. When we speak of extinct species, we really conflate two very different concepts. Some species indeed have been wiped out without leaving descendants, but in the case of others, their descendants are simply so different from the supposedly "extinct" ancestors that we call them by a different name. But in fact, these extinct species may have been highly successful in evolutionary terms.

So, the dinosaurs, as a class, are not extinct. They are all around us. But they are small, have feathers, and fly. They don't resemble plodding 60 ton grazers, or 10 meter long killing machines with six inch teeth. Even so, the dinosaurs live among us.

Evolution has made us mortal for related reasons. Once organisms have passed on their genes, their continued existence can create problems for their progeny. Ancestors compete with progeny for resources, most fundamentally, and even lock up resources needed by their progeny's (and their own) food sources. Animal carcasses return organic nitrogen to the soil, and make plant growth possible. An immortal species could not evolve, because it could produce few generations before it would completely exhaust its ecological resources. So death may be a misfortune when it comes untimely, but it is programmed by evolution into every sexually reproducing species. In the case of reproduction by division, the very concept of death seems not to apply -- there are now two "daughter" cells, but where is the mother? So we can say that since sex is necessary for multicellular organisms, death is also necessary. Death follows from sex, inevitably.

In the case of humans, because we are social organisms, each of us may have value to our progeny and our cousins' progeny beyond our mere ability to reproduce and nurture our own young. Non-reproducing individuals can be stores of wisdom, have skills and do work which is valuable to the group. So, while salmon die as soon as they spawn, humans are endowed with lifespans that extend well past their reproductive stage. Nevertheless, evolution has struck a balance, at approximately a factor of two. So evolution has also produced a paradox. The individual is programmed to survive, at least long enough to complete its reproductive potential. In us, that programming does not switch off when we are done reproducing. It makes us egocentric, and it makes us fear death. But those impulses have no chance of prevailing.

The fortunate thing for people is that, as social beings, we also have legacies that go far beyond our genes. What we do in life makes a difference for those who follow. We are even remembered, for a generation or two by a relatively small number of people in most cases, or for thousands of years and by billions, in rare cases. But in any event, we continue to matter after our deaths. Can that realization alone be enough to overcome the fear of death, for most people?

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