Re: [asa] Dawkins on the fossil record

From: David Campbell <>
Date: Tue Dec 15 2009 - 17:05:18 EST

> This is great! David is a much elder biologist than is Dennis. It should be
> fun to watch!

Except for my focus on paleobiology, I'm not so sure I can be credited
with elderness. I'm not sure that I have more biological background
nor am I certain which of us is older (I'm about to catch up with Jack
Benny). In fact, if I recall correctly, I believe Dennis is closer to
the field in question than I am, as I deal most closely with mollusks
and mainly work at the species level and above.

If you trace a biological lineage back far enough, it will converge on
a single pair/individual unless there's been very distant lateral
transfer. (Even then, the shared fundamental genetic mechanisms
suggest that all known organisms have a single common ancestor
somewhere rather before the eubacterial-archaean-eukaryote split). In
fact, there is a point where every individual in the ancestral
population is either an ancestor of every extant individual or else
has no living descendants. But this may be very far back. For
example, every direct maternal ancestor of mitochondrial Eve also is
ancestral to every extant human mitochondrial lineage. Likewise,
every direct paternal ancestor of Y chromosome Adam is ancestral to
every extant human Y chromosome lineage. These two lineages, if
traced back far enough, would eventually converge to a single couple
in the course of population dynamics extrapolated backwards. However,
in the case of humans, that single pair may quite easily pre-date the
human-chimp-gorilla splits, and there probably would have been nothing
biologically distinctive about them versus their neighbors or parents,
though it would be hard to absolutely rule out the possibility.

I think that Dennis has followed much more of the human population
genetic work than I have. I would note the caveat that the MHC region
is strongly affected by selection for divergence, making convergence
between human and chimp sequences very likely, but by now there should
be some very large data sets to work with.

The differences with what I am saying are that I am thinking in much
broader phylogenetic and chronological terms and that I am thinking
more "can this be unequivocally ruled out" than "is it very likely".
It's probably impossible genetically to get a single pair as the sole
ancestors of all modern humans unless that single pair is
significantly more apelike than modern humans. Thus, an "Adam and Eve
as physical ancestor of all modern humans" model is very difficult to
maintain. However, if one is willing to accept Adam and Eve as rather
furrier and smaller brained than us, one could produce a scenario that
is not absolutely ruled out by the genetic data. I'm not sure that
one can genetically rule out Glen's idea of Adam and Eve being at an
australopithecine grade and living a bit over 5 million years ago, for
example; the difficulty is more on the genealogy end.

> "but how could we reconcile his [Dick's] historical Adam theory with all
> humanity tracing back to a single couple amid a larger population if that
> was the case? Does that mean we can detect a "humanity" or an "imago dei"
> gene and assess who has it and who doesn't and in what concentrations?

Actually, I think that Dick places Adam and Eve as ancestors only of
the Semitic people. In general, I think that models that envision
Adam and Eve as representatives selected out of an existing population
usually envision the imago dei condition to be essentially spiritual
rather than genetic (though genetics are involved in producing the
mental capacities that are related to the imago dei).

> "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." - David
> This is what I asked Dennis before, though he didn't respond, preferring
> instead ad hominems on my person. If I understand David properly (due to the
> lack of agreement of verb tense, i.e. unclear language), he is saying that
> there *could* have been an 'Adam' and 'Eve' who were the ancestors
> of/"ancestral to *all modern humans.*" Otherwise, shouldn't the mathematics
> of the converse (impossibility of single pair human ancestry) have been
> obvious long ago (i.e. well before the invent of 'genetics' and 'genomics'?

Not immediately spotting the verb issue, but I would be quite
unsurprised if I didn't write clearly.

Mathematically, single pair human ancestry is a possibility as long as
the single pair lived long enough ago to have the requisite number of
descendants. This involves assumptions about the number of children
per parent and how many survive, as well as a decision on who in the
archaeological/paleontological record count as modern humans. E.g.,
it's fairly simple to work out how many generations it would take to
generate 6.5 billion people today if Adam and Eve and each of their
descendants had on average 10 sons, regarding any extinct lineages
such as Neanderthals as beyond the pale. However, long-known
genealogical data would indicate that some modern people can be traced
back quite a ways, raising problems for such a simplistic model. What
genetic data give us is extensive genealogical data that help us
constrain the possibilities. Now these data must be interpreted using
models of patterns of genetic change, and using a wrong model can give
significantly inaccurate results; on the other hand, we have a decent
idea of the general patterns likely to apply to hominids, and we can
try various models to confirm how well they match the observed

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 17:05:25 2009

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