Just ran accross this brief summary article at:
www.gene.com/ac/WN/index.html
The article is a summary of a Science article from 1995. A little dated
and I don't know if anyone discussed it last year but it seems somewhat
relative to recent discussion and Glenn I am particulary interested with
what you do with this type of information (not that I can see that it
doesnt' fit into your model).
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MEN HAVEN'T CHANGED IN 270,000 YEARS
By Sean Henahan, Access Excellence
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NEW HAVEN, CT- A survey of genetic information contained in
the Y chromosomes of modern day men suggests that Homo
sapiens descended from a single group of male ancestors, and
that this occurred more recently than previously believed.
Researchers at Yale University evaluated samples of DNA from
38 men from all over the world. Surprisingly. no sequence
variations were found in a 729-base pair intron near a gene
thought to be involved in sperm or testes development. In
contrast, the corresponding sequence in gorillas,
chimpanzees, and orangutans did show significant
polymorphisms.
Based on the assumption that this lack of sequence variation
is attributable to ancestry and not other causes, the
researchers estimate the divergence occurred about 270,000
years ago. Previous estimates have put mankind's origins much
further back in time.
This discovery adds to growing evidence that modern humans
share the same basic genetic makeup, and that population
differences represent relatively minor variations in the
overall spectrum of human diversity, said Robert L. Dorit, an
assistant professor of biology at Yale University. Dr. Dorit
collaborated with researchers from the University of Chicago
and Harvard University on the project.
"If we all descended from a recent common ancestor, and if
the history of human populations is a history of movement and
gene flow, then the differences between us, as socially
striking as we may wish to make them, are largely irrelevant
from a biologist's standpoint," Dorit said.
The new findings appear to corroborate the timeline of the
controversial "African Eve" hypothesis, first proposed in
1987. That hypothesis holds that all humans are descended
from female ancestors who lived in Africa about 200,000
years. Those studies focused on purported regular, clock-like
mutations of genes located in the mitochondria,
energy-producing structures outside the cell nucleus that are
present in the egg and thus inherited only from mothers.
The Yale study focused on the even slower clock-like
mutations in the Y chromosome, the male sex chromosome passed
from father to son. The researchers selected the Y chromosome
because it contains the only genetic material besides
mitochondrial DNA that is inherited from just one parent. The
genetic material on all other chromosomes is inherited from
both parents and is recombined, thus making it more difficult
to reconstruct an evolutionary history.
The new research also appears to rules out an opposing theory
that modern humans simultaneously evolved in different
regions of the Old World from an earlier human ancestor -
Homo erectus - who migrated out of Africa perhaps 1 million
years ago. The lack of genetic variation found in the Yale
study makes it highly unlikely that independent Y chromosome
lineages have been evolving for a million or more years along
separate paths, Professor Dorit said.
"The lack of genetic variation in the Y chromosome region we
examined also makes it impossible for us to reconstruct the
geographic location of the last common ancestor," Dorit
noted. "The African Eve hypothesis, on the other hand, is
based on a mutation rate in mitochondria that is at least 10
times faster than the mutation rate in the Y chromosome.
Therefore, the greater number of mutations found in the
mitochondria of native Africans indicates a longer history
and a probable African origin for modern humans. We hope to
be able to confirm an African origin by looking at another
segment of the Y chromosome that is mutating slightly faster
than our original segment, which could reveal subtle regional
genetic differences," he said.
Dr. Dorit and colleagues compared the same sequence of 800
base pairs of nucleotides in humans to that of chimpanzees,
gorillas and orangutans so as to establish the mutation rate
in the Y chromosome. Humans are believed to have shared a
common ancestor with chimpanzees and gorillas about 5 million
years ago, and with orangutans about 14 million years ago.
"The idea was to get a snapshot of this part of the Y
chromosome in a worldwide sample of humans that would help us
establish some kind of evolutionary tree connecting human
populations. We were very surprised to find no genetic
differences in humans, although we found mutations as
expected when we studied the primates. This probably means we
are a very young species," noted Dorit.
It is still possible to hypothesize that Homo sapiens is
actually much older than 270,000 years, emphasized Dr. Dorit.
For example, it is possible the "genetic slate" was wiped
clean recently by a beneficial mutation that caused a rapid
sweep of that individual's genes throughout the population by
natural selection. Another possibility is a significant
reduction in the population of Homo sapiens, which population
geneticists call a "bottleneck,". Such an event would
significantly reduce genetic diversity, he said.
"However, evidence now is coming in from other parts of the
human genome besides the mitochondria pointing to a recent
origin for modern humans. In addition, molecular clocks
appear to be ticking at various speeds in different genes
that together can help us measure human evolution with
greater accuracy. We must remember, however, that we are
reconstructing the history of molecules here. While that
history is not independent of the history of the organisms in
which they are found, molecules have agendas of their own,"
Dorit said.
For more information on this study please refer to: Science,
5/26/95, Dorit et al.
Joel Duff