Re: [asa] Dawkins on the fossil record

From: David Campbell <>
Date: Tue Dec 15 2009 - 13:26:25 EST

Quick review on mitochondrial Eve, Y Adam, etc.:

Mitochondrial DNA is, with rare exceptions (e.g., most bivalves)
inherited entirely from the mother. Likewise, the Y chromosome is
inherited paternally. The rest of our DNA generally undergoes
crossing over, in which the DNA we inherit from each of our parents
gets mixed before it is transmitted to our offspring. Thus, the
ancestry of the mitochondrial DNA and of the Y chromosome is a simple
pattern of branching and mutation. If you come up with an estimated
mutation rate (very popular, usually statistically indefensible), you
can estimate how long ago two sequences diverged. Other DNA, however,
gets inherited as a mosaic, and so is much trickier to trace,
especially as crossing over in various forms can take place within a
DNA region.

The most recent person to be _an_ ancestor of every living human was
estimated to be only a few thousand years ago, but I am suspicious
that the modeling did not adequately take into account geographic and
ethnic isolation. The error bars on the estimated date for the
genetic "Adam" and "Eve" are large, but they seem not to be at the
same time. Both would be likely to reflect a time of relatively small
population, but nothing about them indicates that they were the sole
individual of their time-just that, in the course of things, no one
else living at the time has a continuous sequence of descendants of
that same gender. Surnames are a handy analogue. If children always
receive their father's surname, then surnames in families that have no
sons will be lost. If these strictly-inherited surnames never
changed, eventually everyone would have the same surname, given enough
time and random variation in whether someone has sons or daughters.
This would happen faster if the population was relatively small, or if
some males tended to have a lot more sons than did others, or if there
was some sort of pressure favoring certain names.

Now, if you go back far enough, you will come to a single pair that is
ancestral to all modern humans. However, "far enough" is almost
certainly rather more than 100,000 years, and quite possibly at an ape
grade of evolution. Again there is nothing to indicate that there
weren't co-existing individuals who were practically the same who
merely ended up as cousins rather than direct ancestors.

Thus, it seems that having an Adam and Eve of some sort within the
past 10,000 years or so requires them to not be the sole physical
direct ancestors of all modern humans.

Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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Received on Tue Dec 15 13:26:52 2009

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