Future Directions of Human Evolution

The most improbable feature of science fiction movies is not the faster-than-light travel or the transporter beams but a feature that audiences accept without a second thought: the people. The inhabitants of the far future are always portrayed as looking and behaving exactly like people today.

All that is certain about future evolution is that people will not remain the same as they are today. They will differ because the environments of the future will be different, and people will either have adapted to them or perished. Many futures are possible. Perhaps the gracilization of the human form will continue as societies favoring trade and cooperation extend their advantage over those more inclined to aggression. Perhaps our distant descendants will be far more intelligent, having evolved in response to the ever increasing intellectual demands of a more complex society. Perhaps they will be stockier, with shorter arms and legs, having followed the standard biological rules for adapting to cold climates as the Earth plunges back again into the inevitable next ice age. Perhaps the human lineage will resume its speciation, dividing into two or more castes inhabiting different social niches. And if self-sustaining populations are ever established on Mars or Europa, they will certainly follow independent evolutionary paths, adapting in form and social behavior to the ecological rules of their new planet.

Theorists have not yet reached agreement on the future evolution of the human species. Two of the founders of population genetics, Ronald Fisher and Sewall Wright, disagreed strongly on the conditions that favor evolutionary change. Wright believed that evolution worked best when a population was divided into small independent groups with limited gene flow between them; a genetic innovation that emerged in one group could then be allowed to spread to the others. Fisher, on the other hand, thought beneficial innovations were more likely to arise in large populations with a high degree of mixing. "Which of them is right? No one really knows," says Alan Rogers, a population geneticist at the University of Utah. As it happens, the conditions favored by Wright apply well to most of recent human history before 10,000 years ago, when the population was divided into small, distant groups spread across the globe. But the world at present, with increasing travel and migrations, seems much closer to Fisher's ideal conditions for evolutionary change. "You used to marry a lass from your local village, now it's anyone you can track down on the internet," says Mark Pagel, an evolutionary biologist at the University of

Reading in England.349

In Fisher's major work, The Genetical Theory of Natural Selection, published in 1929, he developed the argument that genes for mental ability are more frequent among the wealthy, who have fewer children, whereas the poor, who tend to be less intelligent, have more children; therefore natural selection acts against genes that promote intelligence. This aspect of Fisher's work is not much discussed, because it was used to support the disastrous eugenics policies of the early twentieth century. But in the view of some population geneticists, its theoretical argument has not been refuted: at least in developed countries, people of higher intelligence tend to have fewer children, so it would seem that their genes cannot become more common in the next generation. Others argue that the poor tend to have more children from lack of education, not any lack of intelligence. "Fisher's empirical observation is correct, that the lower orders have more babies, but that doesn't mean their genotypes are inferior," says Pagel.

Human brain size and intelligence have clearly expanded throughout most of evolution, and it would be strange if this trend should suddenly grind to a halt just as societies, and the skills needed to flourish in them, have become more complex than ever. It would be stranger still if humans, selected throughout evolution on the basis of maximum fitness, the propensity to leave as many descendants as possible, should suddenly abandon this deeply ingrained behavior. Nor is there any evidence from IQ tests to suppose that human cognitive ability is falling, as Fisher predicted. Therefore, despite the apparent correctness of Fisher's premise, that in modern societies the rich and more intelligent tend to have fewer children, his conclusion of inexorable intellectual decline seems somehow to be false.

The reason, evolutionary psychologists suggest, is that the rich are able to invest more in their children—a college education makes a big difference to future success—and thus they may leave more descendants in the long run, even if they have fewer children. The argument assumes that children who are well educated and well endowed will have children of similar quality, generation after generation, whereas at a certain level of destitution fertility will be reduced. So at some level of wealth, the better way for parents to maximize their Darwinian fitness will be to have fewer children in the expectation of leaving many more great-grandchildren.

Whether this is the case in practice is unclear. Teasing out the relationship between wealth and fertility is no easy matter and the demographic data needed to resolve the issue seem to be lacking. "This is a tricky and subtle business," writes the evolutionary psychologist Bobbi Low, "and most currently available data, gathered to answer other questions, are inadequate."350

One way in which future human evolution will differ from that of the past is that in larger populations the effect of genetic drift is much diminished. The larger the population, the longer it takes for one version of a gene to supplant all the alternative versions. Since drift is a principal mechanism for reducing the diversity that is constantly introduced by mutation, it follows that human genomes will become more diverse as neutral mutations accumulate. Too much diversity, according to theoretical calculations, could eventually make people infertile unless they mated only with people whose genomes were similar to their

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