RE: [asa] (fall) biological evolution and a literal Adam- logically inconsistent?

From: Dehler, Bernie <bernie.dehler@intel.com>
Date: Wed Sep 03 2008 - 17:22:52 EDT

" What the mitochondrial "Eve", Y chromosome "Adam", etc. point to is the most recent common ancestor of all known modern versions."

How can one identify a "most recent common ancestor?" Every mother has a mother. Where is a stopping point? How is it identified?

...Bernie

-----Original Message-----
From: asa-owner@lists.calvin.edu [mailto:asa-owner@lists.calvin.edu] On Behalf Of George Cooper
Sent: Wednesday, September 03, 2008 1:57 PM
To: asa@calvin.edu
Subject: FW: [asa] (fall) biological evolution and a literal Adam- logically inconsistent?

Thanks David for this info. I haven't had a chance to digest it, but hope
to do so soon. I limit my questions on this but I would like to ask you a
few that are on my mind...

Are mutation rates part of the age estimation?

Is the Y-chromosome clock less accurate than the MtDNA clock?

George Coope

-----Original Message-----
From: asa-owner@lists.calvin.edu [mailto:asa-owner@lists.calvin.edu] On
Behalf Of David Campbell
Sent: Tuesday, September 02, 2008 5:28 PM
To: asa@calvin.edu
Subject: Re: [asa] (fall) biological evolution and a literal Adam- logically
inconsistent?

A couple of posts raised issues of molecular clocks and what the
mitochondrial Eve, etc. mean:

> It is my understanding that MtDNA will not point likely point to an
ultimate
> first mother (ie Eve) but only to an "Eve" down a given MtDNA trail.
> Confidence in this dating system seems fairly high. However, I get the
> impression that the Y-chromosome method of dating is much less reliable.
I
> am especially weak in micorbiology, so, hopefully, someone here will
address
> this issue.

> Another way around it is to find a mitochondrial Eve or a Y chromosome
Adam.
> I guess you can trace back mitochondrial DNA through women, back to "one"
> about 150,000 years ago. The issue is that a similar process with men and
> the Y chromosome only goes back about 50,000 years. So unless your
> biological Adam and Eve lived 100,000 years apart, you end up with a bit
of
> a problem.

What the mitochondrial "Eve", Y chromosome "Adam", etc. point to is
the most recent common ancestor of all known modern versions. This
will almost certainly be later than the actual time of origin of the
group of interest. For example, suppose for the purpose of
illustration that all living humans descend from Noah's family. All Y
chromosomes would derive from Noah, makign the earliest possible date
of divergence later than Adam. For the mitochondria, we don't have
any evidence on the ancestry of Mrs. Shem, Ham, and Japeth, but if it
traces back to a single female ancestor more recent than Eve then the
divergence would be somewhat later. Furthermore, things can be lost
later, too. If one of Noah's sons had only sons, (or if all his
daughters had only sons, etc.), then that wife's mitochondrial lineage
would be lost.

Extrapolation of dates based on the degree of molecular difference can
involve numerous assumptions, many of them incorrect. This is
typically especially bad in
the case of studies of molecular sequences in organisms. Dan Graur
and William Martin (Reading the entrails of chickens: molecular
timescales of evolution and the illusion of precision. TRENDS in
Genetics Vol.20 No.2:80-86.) showed that more realistic assesment of
the uncertainty in some popular molecular clocks do not allow you to
draw more specific conclusions than that the organisms probably
diverged from each other sometime between the Big Bang and now.

There's a better data set for detailed studies on humans, and the
extrapolation is not nearly as extreme, but still the uncertainty is
probably much larger than typically reported. It also depends on
whether a simplistic or a credible evolutionary model is used for
calibration. Thus, I'm doubtful that one can confidently assert that
Y chromosome Adam is younger than mitochondrial Eve based on the
molecular data. However, such a result would be expected a priori as
the higher variability in male than female fecundity (polygamy, etc.
allowing for a higher maximum and conversely increasing the
probability of other males having no reproduction) would tend to make
male lineages more vulnerable to loss, increasing the chance of
resetting the clock. Males also tend to disperse more, promoting
homogenization of the male genetic lineage (e.g., the number of
populations with no European component to the Y chromosome lineage is
rather small).

The presence of multiple MHC alleles in both humans and apes does not
require a continuous human population containing each of those
alleles, because the MHC genes are under strogn selective pressure to
vary rapidly. Convergent arrival at the same allele is quite
possible, not to mention the multiple paralogous alleles in any one
individual.

For Adam and Eve to be ancestral to all living humans, they could be
either the last common ancestors of all living people or else
ancestors of that ancestor. (The picture gets much more confused if
you allow some genetic component from pre-adamites-i.e., Adam and Eve
are in everyone's family tree but not the unique heads. Being _an_
ancestor of everyone alive today is possible rather more recently than
being _the_ ancestral pair). To be representatives, they would only
need to be early enough to allow for whatever you make of the
genealogies, plus time indicators based on any archaeologically
detectable features that you think are uniquely associated with being
spiritually human.

--
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 Wed Sep 3 17:23:22 2008

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