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2009-10-26 09:10:12
By EBEN HARRELL Eben Harrell . Sat Oct 24, 10:10 am ET
Modern Homo sapiens is still evolving. Despite the long-held view that natural
selection has ceased to affect humans because almost everybody now lives long
enough to have children, a new study of a contemporary Massachusetts population
offers evidence of evolution still in action.
A team of scientists led by Yale University evolutionary biologist Stephen
Stearns suggests that if the natural selection of fitter traits is no longer
driven by survival, perhaps it owes to differences in women's fertility.
"Variations in reproductive success still exist among humans, and therefore
some traits related to fertility continue to be shaped by natural selection,"
Stearns says. That is, women who have more children are more likely to pass on
certain traits to their progeny. (See the top 10 scientific discoveries of
2008.)
Stearns' team examined the vital statistics of 2,238 postmenopausal women
participating in the Framingham Heart Study, which has tracked the medical
histories of some 14,000 residents of Framingham, Mass., since 1948.
Investigators searched for correlations between women's physical
characteristics - including height, weight, blood pressure and cholesterol
levels - and the number of offspring they produced. According to their
findings, it was stout, slightly plump (but not obese) women who tended to have
more children - "Women with very low body fat don't ovulate," Stearns explains
- as did women with lower blood pressure and cholesterol levels. Using a
sophisticated statistical analysis that controlled for any social or cultural
factors that could impact childbearing, researchers determined that these
characteristics were passed on genetically from mothers to daughters and
granddaughters.
If these trends were to continue with no cultural changes in the town for the
next 10 generations, by 2409 the average Framingham woman would be 2 cm (0.8
in) shorter, 1 kg (2.2 lb.) heavier, have a healthier heart, have her first
child five months earlier and enter menopause 10 months later than a woman
today, the study found. "That rate of evolution is slow but pretty similar to
what we see in other plants and animals. Humans don't seem to be any
exception," Stearns says. (See TIME's photo-essay "Happy 200th Darwin Day.")
Douglas Ewbank, a demographer at the University of Pennsylvania who undertook
the statistical analysis for the study, which was published Oct. 21 in the
Proceedings of the National Academy of Sciences (PNAS), says that because
cultural factors tend to have a much more prominent impact than natural
selection in the shaping of future generations, people tend to write off the
effect of evolution. "Those changes we predict for 2409 could be wiped out by
something as simple as a new school-lunch program. But whatever happens, it's
likely that in 2409, Framingham women will be 2 cm shorter and 1 kg heavier
than they would have been without natural selection. Evolution is a very slow
process. We don't see it if we look at our grandparents, but it's there."
Other recent genetic research has backed up that notion. One study, published
in PNAS in 2007 and led by John Hawks, an anthropologist at the University of
Wisconsin at Madison, found that some 1,800 human gene variations had become
widespread in recent generations because of their modern-day evolutionary
benefits. Among those genetic changes, discovered by examining more than 3
million DNA variants in 269 individuals: mutations that allow people to digest
milk or resist malaria and others that govern brain development. (Watch TIME's
video "Darwin and Lincoln: Birthdays and Evolution.")
But not all evolutionary changes make inherent sense. Since the Industrial
Revolution, modern humans have grown taller and stronger, so it's easy to
assume that evolution is making humans fitter. But according to anthropologist
Peter McAllister, author of Manthropology: the Science of Inadequate Modern
Man, the contemporary male has evolved, at least physically, into "the sorriest
cohort of masculine Homo sapiens to ever walk the planet." Thanks to genetic
differences, an average Neanderthal woman, McAllister notes, could have whupped
Arnold Schwarzenegger at his muscular peak in an arm-wrestling match. And
prehistoric Australian Aborigines, who typically built up great strength in
their joints and muscles through childhood and adolescence, could have easily
beat Usain Bolt in a 100-m dash.
Steve Jones, an evolutionary biologist at University College London who has
previously held that human evolution was nearing its end, says the Framingham
study is indeed an important example of how natural selection still operates
through inherited differences in reproductive ability. But Jones argues that
variation in female fertility - as measured in the Framingham study - is a much
less important factor in human evolution than differences in male fertility.
Sperm hold a much higher chance of carrying an error or mutation than an egg,
especially among older men. "While it used to be that men had many children in
older age to many different women, now men tend to have only a few children at
a younger age with one wife. The drop in the number of older fathers has had a
major effect on the rate of mutation and has at least reduced the amount of new
diversity - the raw material of evolution. Darwin's machine has not stopped,
but it surely has slowed greatly," Jones says. (See TIME's special report on
the environment.)
Despite evidence that human evolution still functions, biologists concede that
it's anyone's guess where it will take us from here. Artificial selection in
the form of genetic medicine could push natural selection into obsolescence,
but a lethal pandemic or other cataclysm could suddenly make natural selection
central to the future of the species. Whatever happens, Jones says, it is worth
remembering that Darwin's beautiful theory has suffered a long history of
abuse. The bastard science of eugenics, he says, will haunt humanity as long as
people are tempted to confuse evolution with improvement. "Uniquely in the
living world, what makes humans what we are is in our minds, in our society,
and not in our evolution," he says.