My latest Mind and Matter column in the Wall
Street Journal.
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.