MEN HAVEN'T CHANGED IN 270,000 YEARS
By Sean Henahan, Access Excellence
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
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
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.
More information on the WWW
Molecular Genetics Primer