Re: Date for Last Common Ancestor?
John Hawks (hawksj@umich.edu)
Tue, 13 Aug 1996 13:20:58 -0400
Jane Andrews wrote:
>
> . . . I
> remember reading a paper which idicated that the proposed "eve" could have
> come from an ancestoral population of up to 10 000 individuals. Her
> mitochondrial sequence has been preserved by random genetic drift and all
> the others lost by chance, stochastic processes. This means that there
> is no need to invoke a huge bottle neck reducing, the effective
> female population to one, in order to explain the apparent single female
> ancestor. . . .
The idea of the population of 10,000 comes from the original mtEve paper:
Cann, Stoneking and Wilson, 1989. Mitochondrial DNA and human evolution.
Nature 325:31-6, Jan 1 '87.
The authors make it clear that there was no time in the past during which
the lineage leading to living humans consisted of only one couple. They
assert that the individual that carried the unique mtDNA source of all living
people was a member of a group of individuals, all of which are ancestors of
living people.
However, the authors viewed this population is a small, bottleneck population.
The reason for this bottleneck is the comparatively recent date that Cann et al.
associate with their mitochondrial ancestor. If the date had been much more
ancient (like 1Ma or more), they would perhaps have needed no special
explanation--a million years is a long time for random processes to knock out
all but one million-year-old lineage and its descendent lineages. It is
easily apparent that living humans must all share a single mitochondrial
ancestor at some point (take for instance, the original mitochondrial
organism). Cann et al. didn't have to prove that there was a single mitochondrial
ancestor. What they had to do was explain why their date (ca. 200ka) was so
young.
There are at least two potential explanations. One is that the mitochondrial
variant apparently shared by living humans conferred some sort of selective
advantage, leading to its replacement of other human mtDNA variants. Cann et al.
rejected this explanation, not without good reason. At the time, few
mitochondrial diseases had been described, and researchers were finding out
that the mtDNA molecule had had most of its functions abrogated by the nuclear
DNA. Cann et al. therefore assumed that the majority of mtDNA mutations were
neutral.
The other explanation is that there was a *large* bottleneck at about the
time that the mitochondrial ancestor lived. This is the explanation that
Cann et al. chose, because it made demographic sense, and because a group
of archaeologists and paleoanthropologists were suggesting a theory that
would fit well with just such a bottleneck. Out of Africa was born.
This is the explanation of the "population" that you had heard about. I'm
sure I needn't point out that many people have disagreed with the bottleneck
explanation, and many others have disputed the 200ka date. More discussion
can follow if anyone is interested.
John Hawks
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