Re: 'Out of Africa' still only a theory?!?!?
Bearcat (bcat@netcom.com)
Sat, 30 Sep 1995 17:44:53 GMT
henry harpending (hxh5@psu.edu) wrote:
: The real import of the mtDNA ("Eve") evidence is that it suggests we only had
: several thousand ancestors for a long time and that the recovery from this
: pinch started during the last interglacial. The arguments about the mutation
: rate really aren't very significant, since if the time of the coalescent
: ("Eve") were a million years rather than 250,000 the suggested ancestral
: population would be perhaps 20,000 rather than 5000. An older Eve would NOT
: support regional continuity unless she were a whole lot older. The chimp
: Eve is maybe several million years old (depending on one's idea of the mtDNA
: transition/transversion ratio) implying that chimps did not go through such a
: bottleneck.
this is indeed compelling evidence for a single source for a.m.h.
ancestors.
: The "african origin" is supported by the greater african diversity in genetic
: systems with high mutation rates (metrics, mtDNA, repeats) but NOT in low
: mutation systems like nuclear markers. This implies that ancestors of
: africans were successful first, but I don't see any evidence that this
: happened in Africa which is pretty empty of archaeological evidence from 75
: kya to about 25 kya or so.
At the American Assoc. of Physical Anthropologists 1994
annual conference in Denver, Yale researchers presented
findings based not upon mitochondrial DNA but on DNA found in the cell's
nucleus. This study proved Africa to be, indeed, the birthplace of
modern man.
Sarah Tishkoff and Ken Kidd of Yale chose nuclear DNA, which is inherited
from both parents, in order to account for the possibility of men and
women becoming separated, for instance, after a hunt, and thus reflecting
population movement more accurately. Their study revealed a three-way
population division: Sub-Saharan Africans, northeastern Africans, and
non-Africans. South of the Sahara, there is *tremendous* genetic
variation. In northeast Africa, among Ethiopian Jews and Egyptians there
is less variation, and outside of Africa, there is less variation still.
This is because, Tishkoff and Kidd assert, the small population that
left Africa altogether did so in the very recent past, compared to the
thousands of generations we *all* lived in Africa. "The migration
was recent enough so that mutations and recombination haven't yet
reintroduced variability to the pattern. It's very consistent with a
recent spread of modern humans out of Africa - it's hard to imagine
anything else that could explain the data," says Kidd.
: The small number of human acestors has been known to population geneticists
: for years, but anthropologists never paid any attention until the Eve fracas
: was picked up by journalists. A nice discussion of recent evidence is a
: column by Ann Gibbons in SCIENCE 6 January 1995 called "The mystery of
: humanity's missing mutations".
: Back to your point: if there were only a few thousand ancestors then such a
: small number could not have been spread all over the globe. This is IMHO the
: real genetic evidence for the african origin hypothesis. The topology of the
: mtDNA tree, on the other hand, is bogus and unknowable I think. That the
: root often seems to divide humanity into africans and non-africans is not
: anything more than a reflection of the higher mtDNA diversity in Africa.
actually, along with the above, the most emphatic point made by
cavalli-sforza et al in _history and geography of human genes_ is
that our origins were in sub-saharan africa. the entire study rests
not on mtDNA, but on specific genetic markers culled over a period
of a decade or more from aboriginal populations the world over
(avoiding populations that have migrated since 1492).
so that's all of the genetic research of which i'm aware, all pointing
to a single africa origin for mankind: mtDNA studies, of which there
have been many attempting to overcome limitations present in the
methodology of the original study which goes back 15 years or so by
now; nuclear DNA studies; and finally, this very conservative and
solid treatise published by cavalli-sforza which uses specific
genetic markers.
as for the polygenesis theory, it boggles my mind that poeple
continue to champion it. the genetic similarity of humans (as
revealed quite tellingly in the "genetic bottleneck" research)
was again driven home in the recent research on the y chromosome.
it came as a complete shock to the researchers that all of the
y chromosome samples from widely distant populations used in
the investigation were not just similar, but genetically
identical.
- bearcat
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