Re: African Eve
dmitry pruss (pruss@helix.nih.gov)
Mon, 9 May 1994 22:29:23 GMT
In article <2qbl69$nq8@deathstar.cris.com> Kreegah@deathstar.cris.com (Patrick H. Adkins) writes:
>
>From: pruss@helix.nih.gov (dmitry pruss)
>Subject: Re: African Eve
>
>DP> The problem with mit DNA analysis in H.sapiens was that it produced
> too counterituitive results - and wasn't paralleled by anything
> similar on other modern (recently evolved) species.
>
>[interesting material (thanks!) deleted]
>
>One of the things I don't understand about this approach is _whose_ mit DNA
>are we tracing back to? For instance, if we have an "Eve" (individual with a
>mutation that makes her the first of what will become H. sapiens), why should
>we presume that the mit DNA originated with her--instead of with her non-
>mutated mother, grandmother, or some still more distant ancestor, perhaps
>even an earlier species.
It definitely originated from the earlier ancestors and before that, from
the earlier species.
The answer is, with earlier species/ population had widely diversified mit
DNAs themselves. Only the tiniest portion of this past genetic diversity
ended up inherited by our human ancestors - and the diversity was then
gradually restored in the course of hundereds of generations, just by
slow naturally occuring accumulation of mutations.
The task (one of) of the mit DNA mapping was tp get a quantitative measure
of our mitochondrial DNA iversity - and to estimate how much time would it
take to create this level of divercity from a genetically
homogenous ancestral population.
Could the answer (200 Kyr) be different from the actual age of H.sapiens?
Yes, in two ways :
-if we underestimate our genetic diversity because many genotypes are
unaccounted for, or just lost to extinction. In fact recent extinction can
hardly change the estimate: to affect the figure, we should loose the most
divergent branches of human evolution, i.e. either eliminate whole ancient
races (which unlikely took place) or eliminate the precusors of races-to-be
(i.e. to have massive human extinctions early on).
The latter possibility is described in other words as 'bottlenecking'. If
there was (were) bottlenecks in human evolution, then the species is older
than its mitochondrial age predicts - but all the present-day races evolved
from a common roots recently in human history. More precisely, their common
root was about 200Kyr ago...
-if the first human populations already had a good deal of diversity. Then,
the species is younger than predicted - with an important limitation: its
pre-sapiens ancestors were already closely related to on another - i.e. the
modern humans evolved from an evolutionarily isolated subspecies of their
ancestral species (with the 200Kyr bottleneck attributed to a predecessor
species rather than to H.sapiens).
Neither complication is compatible with a multicentric hypothesis...
BTW, there was a separate study not long ago - reported in Science - about
mapping somatic DNA divesity in humans. (This is inherited from both
parents). There's no clock rate estimate for such things, so the datings
are relative. Yet the results that emerge are very interesting.
Skipping the details (read Science!), the study detects two past
bottlenecks in our intraspecies evolution: the earlier one, which excluded
Ethiopians from the rest of Africans, and more recent one, on the way from
Ethiopians to *all other* humans outside Africa.
D
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