|
Gene Study Says Men Have Been Tomcatting for Ages,
Not Always Successfully
Sept. 19, 2004 - New insights into ancient mating
says there is no covering up ancient sexual dalliances of men tomcatting
around and traveling far from home to do it. They also report men often
were squeezed out of mating by stronger males, which caused twice as
many women as men to pass on their genes.
Researchers at the University of Arizona in Tucson
have developed these new insights into ancient mating and migration
patterns in humans by using what they call “genetic smoking guns.”
Men and women differed in their participation in
reproduction, the researchers report. More men than women get squeezed
out of the mating game. As a result, twice as many women as men passed
their genes to the next generation.
"It is a pattern that's built up over time. The
norm through human evolution is for more women to have children than
men," said Jason Wilder, a postdoctoral fellow in UA's Arizona Research
Laboratories and lead author on the research articles. "There are men
around who aren't able to have children, because they are being
outcompeted by more successful males."
Co-author Michael Hammer, a research scientist in
UA's Arizona Research Laboratories, said, "We may think of ourselves as
a monogamous species, but we're coming from an evolutionary history
that's probably slightly polygamous. If we're shifting toward monogamy,
it's so recent it hasn't left an imprint on our genome."
Or the same reproductive behavior is continuing,
but in a culturally accepted fashion, Wilder said. "The modern version
that we generally don't find offensive is that men tend to remarry and
have more children much more often than women do."
The team's research also overturns the
long-accepted idea that, on average, women's genes traveled farther from
their birthplace than did men's. That idea was based on a common
marriage practice called patrilocality, wherein women tended to move
from their natal village to their husbands' village.
If anything, men and their genes moved farther
overall, the new research indicates.
To sort out how far men and women’s genes traveled,
the UA researchers used DNA from the Y chromosome, which is passed from
father to son. Women's lineages were traced using mitochondrial DNA,
which passes from mother to daughter.
The researchers report their findings in two
related articles, one in the online early edition of the October issue
of Nature Genetics and one in an upcoming edition of Molecular
Biology and Evolution. The research was funded by the National
Institutes of Health.
Scientists have puzzled over the fact that men’s
common ancestor, dubbed Y-chromosome Adam, seems to have lived around
100 thousand years ago, whereas women’s common ancestor, known as
mitochondrial Eve, lived almost 200 thousand years ago.
Worldwide, the DNA from the Y chromosome has much
less genetic variability than does mitochondrial DNA.
“We wanted to know what shapes the patterns of
Y-chromosome and mitochondrial DNA variation,” said Wilder. “What can we
learn about human behavior?”
To find out, Wilder, Hammer and Zahra Mobasher, a
research specialist at UA's Arizona Research Laboratories, tested
Y-chromosome DNA and mitochondrial DNA from three far-flung populations
of humans: the Khoisan of southern Africa, Mongolian Khalks and highland
Papua New Guineans. For each group, DNA from 24 or 25 people was tested.
Previously, researchers assumed equal numbers of
men and women procreated. Based on that assumption, scientists explained
the relative youth and low variability of the Y chromosome by suggesting
that a beneficial mutation on the Y had swept through the whole world.
However, the genetic patterns the UA researchers found contradicts those
ideas.
If a beneficial mutation had swept through the
males, men's common paternal ancestor would be the same age no matter
where the UA researchers looked. Instead, the age of men's common
ancestor differs between the southern African, Mongolian and Papua New
Guinean populations studied. The finding tends to rule out some global
beneficial mutation as the reason Y-chromosome DNA is less variable than
mitochondrial DNA.
"Because we don't think the pattern we see was
caused by an event that swept across the globe, we had to re-examine our
assumptions about whether equal numbers of men and women are mating,"
Wilder said.
The team thinks the genetic patterns are all about
sex.
Or lack thereof. Lots more men than women are
childless, and it has ever been thus, the researchers say.
To see whether men and women traveled equal
distances while sowing their wild oats, Wilder, Hammer and Mobasher
teamed up with research specialist Sarah Kingan and Maya Pilkington, a
doctoral candidate in UA's department of anthropology. The group tested
DNA from 389 individuals representing 10 distinct human populations from
places as far apart as the Netherlands, South Africa and Papua New
Guinea.
Previous explanations of women and men's migration
patterns had been based on a combination of genetic patterns and
knowledge about the marriage practice of patrilocality. About 70 percent
of the world's cultures practice patrilocality.
When populations are compared, patrilocality would
predict greater differences between populations in Y-chromosome DNA than
in mitochondrial DNA, which is just what earlier population genetic
research found. Patrilocality also predicts that, on average, men do not
migrate as far as women.
On the basis of the genetic work, women’s rate of
migration had been pegged as eight times higher than men’s.
The UA team didn't think local marriage practices
could govern global-level patterns of genetic variation. But variation
in the Y-chromosome is hard to study.
The researchers started looking at bits of the Y
chromosome known as the Alu family of retrotransposons.
“We found lots of Y-chromosome variations that
people didn’t know exist,” Wilder said.
The new technique revealed that Y-chromosome DNA
and mitochondrial DNA differ to the same degree among populations.
Hammer said, “That alone wouldn’t say the Y
chromosome is migrating more, but if just half as many males are getting
their genes into the next generation on average as are females, then it
implies more males are migrating to do that.”
The pattern the researchers see in our genes
doesn’t require mass migrations of people across continents. The sailors
with a girl in every port could have done it. Or it may be that village
women snuck out for trysts with tall, dark strangers. Either way, Y
chromosomes got around.
Copyright: SeniorJournal.com |
