Interesting graphix at link.
/-------------------- advertisement -----------------------\
Explore more of Starbucks at Starbucks.com.
http://www.starbucks.com/default.asp?ci=1015
\----------------------------------------------------------/
Why Humans and Their Fur Parted Ways
August 19, 2003
By NICHOLAS WADE
One of the most distinctive evolutionary changes as humans
parted company from their fellow apes was their loss of
body hair. But why and when human body hair disappeared,
together with the matter of when people first started to
wear clothes, are questions that have long lain beyond the
reach of archaeology and paleontology.
Ingenious solutions to both issues have now been proposed,
independently, by two research groups analyzing changes in
DNA. The result, if the dates are accurate, is something of
an embarrassment. It implies we were naked for more than a
million years before we started wearing clothes.
Dr. Alan R. Rogers, an evolutionary geneticist at the
University of Utah, has figured out when humans lost their
hair by an indirect method depending on the gene that
determines skin color. Dr. Mark Stone- king of the Max
Planck Institute for Evolutionary Anthropology in Leipzig,
Germany, believes he has established when humans first wore
clothes. His method too is indirect: it involves dating the
evolution of the human body louse, which infests only
clothes.
Meanwhile a third group of researchers, resurrecting a
suggestion of Darwin, has come up with a novel explanation
of why humans lost their body hair in the first place.
Mammals need body hair to keep warm, and lose it only for
special evolutionary reasons. Whales and walruses shed
their hair to improve speed in their new medium, the sea.
Elephants and rhinoceroses have specially thick skins and
are too bulky to lose much heat on cold nights. But why did
humans, the only hairless primates, lose their body hair?
One theory holds that the hominid line went through a
semi-aquatic phase - witness the slight webbing on our
hands. A better suggestion is that loss of body hair helped
our distant ancestors keep cool when they first ventured
beyond the forest's shade and across the hot African
savannah. But loss of hair is not an unmixed blessing in
regulating body temperature because the naked skin absorbs
more energy in the heat of the day and loses more in the
cold of the night.
Dr. Mark Pagel of the University of Reading in England and
Dr. Walter Bodmer of the John Radcliffe Hospital in Oxford
have proposed a different solution to the mystery and their
idea, if true, goes far toward explaining contemporary
attitudes about hirsuteness. Humans lost their body hair,
they say, to free themselves of external parasites that
infest fur - blood-sucking lice, fleas and ticks and the
diseases they spread.
Once hairlessness had evolved through natural selection,
Dr. Pagel and Dr. Bodmer suggest, it then became subject to
sexual selection, the development of features in one sex
that appeal to the other. Among the newly furless humans,
bare skin would have served, like the peacock's tail, as a
signal of fitness. The pains women take to keep their
bodies free of hair - joined now by some men - may be no
mere fashion statement but the latest echo of an ancient
instinct. Dr. Pagel's and Dr. Bodmer's article appeared in
a recent issue of The Proceedings of the Royal Society.
Dr. Pagel said he had noticed recently that advertisements
for women's clothing often included a model showing a large
expanse of bare back. "We have thought of showing off skin
as a secondary sexual characteristic but maybe it's simpler
than that - just a billboard for healthy skin," he said.
The message - "No fleas, lice or ticks on me!" - is
presumably concealed from the conscious mind of both sender
and receiver.
There are several puzzles for the new theory to explain.
One is why, if loss of body hair deprived parasites of a
refuge, evolution allowed pubic hair to be retained. Dr.
Pagel and Dr. Bodmer suggest that these humid regions,
dense with sweat glands, serve as launching pads for
pheromones, airborne hormones known to convey sexual
signals in other mammals though not yet identified in
humans.
Another conundrum is why women have less body hair than
men. Though both sexes may prefer less hair in the other,
the pressure of sexual selection in this case may be
greater on women, whether because men have had greater
powers of choice or an more intense interest in physical
attributes. "Common use of depilatory agents testifies to
the continuing attractions of hairlessness, especially in
human females," the two researchers write.
Dr. David L. Reed, a louse expert at the University of
Utah, said the idea that humans might have lost their body
hair as a defense against parasites was a "fascinating
concept." Body lice spread three diseases - typhus,
relapsing fever and trench fever - and have killed millions
of people in time of war, he said.
But others could take more convincing. "There are all kinds
of notions as to the advantage of hair loss, but they are
all just-so stories," said Dr. Ian Tattersall, a
paleoanthropologist at the American Museum of Natural
History in New York.
Causes aside, when did humans first lose their body hair?
Dr. Rogers, of the University of Utah, saw a way to get a
fix on the date after reading an article about a gene that
helps determine skin color. The gene, called MC1R,
specifies a protein that serves as a switch between the two
kinds of pigment made by human cells. Eumelanin, which
protects against the ultraviolet rays of the sun, is
brown-black; pheomelanin, which is not protective, is a
red-yellow color.
Three years ago Dr. Rosalind Harding of Oxford University
and others made a worldwide study of the MC1R gene by
extracting it from blood samples and analyzing the sequence
of DNA units in the gene. They found that the protein made
by the gene is invariant in African populations, but
outside of Africa the gene, and its protein, tended to vary
a lot.
Dr. Harding concluded that the gene was kept under tight
constraint in Africa, presumably because any change in its
protein increased vulnerability to the sun's ultraviolet
light, and was fatal to its owner. But outside Africa, in
northern Asia and Europe, the gene was free to accept
mutations, the constant natural changes in DNA, and
produced skin colors that were not dark.
Reading Dr. Harding's article recently as part of a
different project, Dr. Rogers wondered why all Africans had
acquired the same version of the gene. Chimpanzees, Dr.
Harding had noted, have many different forms of the gene,
as presumably did the common ancestor of chimps and people.
As soon as the ancestral human population in Africa started
losing its fur, Dr. Rogers surmised, people would have
needed dark skin as a protection against sunlight. Anyone
who had a version of the MC1R gene that produced darker
skin would have had a survival advantage, and in a few
generations this version of the gene would have made a
clean sweep through the population.
There may have been several clean sweeps, each one
producing a more effective version of the MC1R gene. Dr.
Rogers saw a way to put a date on at least the most recent
sweep. Some of the DNA units in a gene can be changed
without changing the amino acid units in the protein the
gene specifies. The MC1R genes Dr. Harding had analyzed in
African populations had several of these silent mutations.
Since the silent mutations accumulate in a random but
steady fashion, they serve as a molecular clock, one that
started ticking at the time of the last sweep of the MC1R
gene through the ancestral human population.
From the number of silent mutations in African versions of
the MC1R gene, Dr. Rogers and two colleagues, Dr. David
Iltis and Dr. Stephen Wooding, calculate that the last
sweep probably occurred 1.2 million years ago, when the
human population consisted of a mere 14,000 breeding
individuals. In other words, humans have been hairless at
least since this time, and maybe for much longer. Their
article is to appear in a future issue of Current
Anthropology.
The estimated minimum date for human hairlessness seems to
fall in reasonably well with the schedule of other major
adaptations that turned an ordinary ape into the weirdest
of all primates. Hominids first started occupying areas
with few shade trees some 1.7 million years ago. This is
also the time when long limbs and an external nose
appeared. Both are assumed to be adaptations to help
dissipate heat, said Dr. Richard Klein, an archaeologist at
Stanford University. Loss of hair and dark skin could well
have emerged at the same time, so Dr. Rogers' argument was
"completely plausible," he said.
From 1.6 million years ago the world was in the grip of the
Pleistocene ice age, which ended only 10,000 years ago.
Even in Africa, nights could have been cold for fur-less
primates. But Dr. Ropers noted that people lived without
clothes until recently in chilly places like Tasmania and
Tierra del Fuego.
Chimpanzees have pale skin and are born with pale faces
that tan as they grow older. So the prototype hominid too
probably had fair skin under dark hair, said Dr. Nina
Jablonski, an expert on the evolution of skin color at the
California Academy of Sciences. "It was only later that we
lost our hair and at the same time evolved an evenly dark
pigmentation," she said.
Remarkable as it may seem that genetic analysis can reach
back and date an event deep in human history, there is a
second approach to determining when people lost their body
hair, or at least started to wear clothes. It has to do
with lice. Humans have the distinction of being host to
three different kinds: the head louse, the body louse and
the pubic louse. The body louse, unlike all other kinds
that infect mammals, clings to clothing, not hair. It
presumably evolved from the head louse after humans lost
their body hair and started wearing clothes.
Dr. Stoneking, together with Dr. Ralf Kittler and Dr.
Manfred Kayser, report in today's issue of Current Biology
that they compared the DNA of human head and body lice from
around the world, as well as chimpanzee lice as a point of
evolutionary comparison. From study of the DNA differences,
they find that the human body louse indeed evolved from the
louse, as expected, but that this event took place
surprisingly recently, sometime between 42,000 and 72,000
years ago. Humans must have been wearing clothes at least
since this time.
Modern humans left Africa about 50,000 years ago. Dr.
Stoneking and his colleagues say the invention of clothing
may have been a factor in the successful spread of humans
around the world, especially in the cooler climates of the
north.
Dr. Stoneking said in an interview that clothing could also
have been part of the suite of sophisticated behaviors,
such as advanced tools, trade and art, that appear in the
archaeological record some 50,000 years ago, just before
humans migrated from Africa.
The head louse would probably have colonized clothing quite
soon after the niche became available - within thousands
and tens of thousands of years, Dr. Stoneking said. So body
lice were probably not in existence when humans and
Neanderthals diverged some 250,000 or more years ago. This
implies that the common ancestor of humans and Neanderthals
did not wear clothes and therefore probably Neanderthals
didn't either.
But Dr. Klein, the Stanford archeologist, said he thought
Neanderthals and other archaic humans must have produced
clothing of some kind in order to live in temperate
latitudes like Europe and the Far East. Perhaps the body
lice don't show that, he suggested, because early clothes
were too loose fitting or made of the wrong material.
Dr. Stoneking said he got the idea for his louse project
after one of his children came home with a note about a
louse infestation in school. The note assured parents that
lice could only live a few hours when away from the human
body, implying to Dr. Stoneking that their evolution must
closely mirror the spread of humans around the world.
The compilers of Genesis write that as soon as Adam and Eve
realized they were naked, they sewed themselves aprons made
of leaves from the fig tree, and that the Creator himself
made them more durable skin coats before evicting them. But
if Dr. Rogers and Dr. Stoneking are correct, humans were
naked for a million years before they noticed their state
of undress and called for the tailor.
http://www.nytimes.com/2003/08/19/science/19HAIR.html?ex=1062322105&ei=1&en=b11fdb12561d9e03
---------------------------------
Get Home Delivery of The New York Times Newspaper. Imagine
reading The New York Times any time & anywhere you like!
Leisurely catch up on events & expand your horizons. Enjoy
now for 50% off Home Delivery! Click here:
http://www.nytimes.com/ads/nytcirc/index.html
HOW TO ADVERTISE
---------------------------------
For information on advertising in e-mail newsletters
or other creative advertising opportunities with The
New York Times on the Web, please contact
[EMAIL PROTECTED] or visit our online media
kit at http://www.nytimes.com/adinfo
For general information about NYTimes.com, write to
[EMAIL PROTECTED]
Copyright 2003 The New York Times Company
--------------------------------------------
This service is hosted on the Infocom network
http://www.infocom.co.ug
