Bill Jenkins
CHICAGO TRIBUNE, Friday, May 26, 1995
HEADLINE: We're only 270,000 yeals old, new reseasch shows
By Peter Gorner
TRIBUNE STAFF WRITER
Modern humanity may be much younger than earlier be lieved, no more than
270,000 years old, according to new genetic evidence. And in terms of their Y
chromosomes at least, all men are brothers.
They descended from a small group of male ancestors-Adams, if you
will-whose sex-determining genes have been passed on relatively intact. In
fact, human beings may be much more closely related genetically than are
bands of other primates, such as chimpanzees or gorillas.
The finding adds to the mountain of evidence that all humans share the same
basic genetic
blueprint, and that population differences and racial characteristics, though
they appear large, are really superficial variations. "The differences
between us- as socially striking as we may wish to make them-are largely
irrelevant from a biologist's standpoint," said Dr. Robert L. Dorit, Yale
assistant professor of biology. He conducted the research with Hiroshi Akashi
of the University of Chicago and Nobel laureate Walter Gilbert of Harvard
University. Their findings are being published Friday in Science, the
journal of the American Association for the Advancement of Sci ence. In
efforts to probe the human genome for biological clocks that reveal our past,
the researchers performed molecular analysis on a specific portion of the
sex-deter mining Y chromosomes of 38 men from around the world.
Genetic mutations occur at fairly predictable rates over time, and the
scientists found the worldwide samples were identical except for two
mutations, indicating that the men all were related to progenitors who trod
the Earth about 270,000 years ago.
The mutation rate in the Y chromosome was established by comparing human
DNA with that of related apes-chimpanzees, gorillas and orangutans. Humans
are believed to have shared a common ancestor with chimps and gorillas about
5 million years ago, and with orangutans about 14 mil lion years ago.
The fact that so few variations were found in the human chromosomes,
compared with the relatively large number in the other primates, "probably
means we are a very young species," Dorit said.
Previous estimates for the origin of modern man (Homo sapiens) ranged from
100,000 to a million years ago. But if that were so, the researchers said,
they would have expected to see more variations in the 38 chromosome samples
they studied.
The "Adam" theory roughly corroborates the timeline of the controversial
"Eve" hypothesis of the 1980s, which holds that all humans are descended from
female ancestors who lived in Africa about 200,000 years ago.
Although an animal species may start off with a broad mixture of different
genes, scientists believe that it is relatively easy for genes from a related
group of individuals to become dominant in that species over time.
Evolutionists agree that hominids-early human ancestors-originally arose in
Africa about 4 million years ago and spread from there into the rest of the
world.
But no one knows where, when or how often the hominids evolved into early
humans. Now DNA evidence is yielding clues.
Aside from the Y chromosome which can be inherited only from males, the
genetic material on all other chromosomes in cell nuclei is inherited from
both parents and gets recombined during development, so it is more difficult
to construct evolutionary history.
Other genes, however, are found outside the cell nucleus in tiny
energy-producing structures called mitochondria. The mitochondria pass
undiluted from a mother to all her offspring.
Because of regular, clocklike mutations of mitochondrial genes obtained
from various geographical populations, scientists believe the genes form a
separate line of inheritance that can be traced back to our mothers and to
all their mothers before them.
In recent years, the Eve hypothesis has been challenged because of alleged
statistical problems. But the Y-chromosome study, which suggests possible
male counter parts for the African Eve, focused on much slower clocklike
mutations in the Y, that chromosome contains the only other genetic material
besides mitochondrial DNA that is inherited from just one parent.
The study challenges an opposing theory that modern humans simultaneously
evolved in different regions of the Old World from an earlier human
ancestor-Homo erectus-who migrated from Africa perhaps a million years ago.
Dorit said the lack of genetic variation found by his team makes it
unlikely that independent Y-chromosome lineages could have been evolving for
a million or more years along separate paths.
He pointed out that the lack of variation meant that scientists could not
reconstruct the geographic location of the last common ancestor. But that's
where Eve comes in.
The Eve hypothesis is based on a gene mutation rate in mitochondria that is
at least 10 times faster than the rate in the Y chromosome.
"Therefore, the greater number of mutations found in the mitochondria of
native Africans indicates a longer history and a probably African origin for
modern humans," Dorit said.