If you write about genetics and evolution, one of the commonest questions you are likely to be asked at public events is whether human evolution has stopped. It is a surprisingly hard question to answer.
I’m tempted to give a flippant response, borrowed from the biologist Richard Dawkins: Since any human trait that increases the number of babies is likely to gain ground through natural selection, we can say with some confidence that incompetence in the use of contraceptives is probably on the rise (though only if those unintended babies themselves thrive enough to breed in turn).
More seriously, infertility treatment is almost certainly leading to an increase in some kinds of infertility. For example, a procedure called “intra-cytoplasmic sperm injection” allows men with immobile sperm to father children. This is an example of the “relaxation” of selection pressures caused by modern medicine. You can now inherit traits that previously prevented human beings from surviving to adulthood, procreating when they got there or caring for children thereafter. So the genetic diversity of the human genome is undoubtedly increasing.
Or it was until recently. Now, thanks to pre-implantation genetic diagnosis, parents can deliberately choose to implant embryos that lack certain deleterious mutations carried in their families, with the result that genes for Tay-Sachs, Huntington’s and other diseases are retreating in frequency. The old and overblown worry of the early eugenicists—that “bad” mutations were progressively accumulating in the species—is beginning to be addressed not by stopping people from breeding, but by allowing them to breed, safe in the knowledge that they won’t pass on painful conditions.
Still, recent analyses of the human genome reveal a huge number of rare—and thus probably fairly new—mutations. One study, by John Novembre of the University of California, Los Angeles, and his colleagues, looked at 202 genes in 14,002 people and found one genetic variant in somebody every 17 letters of DNA code, much more than expected. “Our results suggest there are many, many places in the genome where one individual, or a few individuals, have something different,” said Dr. Novembre.
Another team, led by Joshua Akey of the University of Washington, studied 1,351 people of European and 1,088 of African ancestry, sequencing 15,585 genes and locating more than a half million single-letter DNA variations. People of African descent had twice as many new mutations as people of European descent, or 762 versus 382. Dr. Akey blames the population explosion of the past 5,000 years for this increase. Not only does a larger population allow more variants; it also implies less severe selection against mildly disadvantageous genes.
So we’re evolving as a species toward greater individual (rather than racial) genetic diversity. But this isn’t what most people mean when they ask if evolution has stopped. Mainly they seem to mean: “Has brain size stopped increasing?” For a process that takes millions of years, any answer about a particular instant in time is close to meaningless. Nonetheless, the short answer is probably “yes.”
I say this for two reasons. First, it’s clear, from glancing around society, that clever people—who on average have slightly bigger brains—aren’t having more babies than less-clever people. Second, the fossil record strongly suggests that our brain size peaked at 1,500 cubic centimeters around 20,000 years ago and has since shrunk to 1,350 cc.
This neither worries nor surprises me. We ceased relying upon individual brain power tens of thousands of years ago. Our civilization now gets all its inventive and creative power from the linking of brains into networks. Our future depends on being clever not individually, but collectively.
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