Re: [asa] Two questions... (bottlenecking)

Several questions on the thread...
>> If you go back far enough, you should have a single ancestral pair for all modern humans.
>1. Would that be true of all phyla that require sexual reproduction?<

That would be expected of any sexually reproducing taxon (with minor
caveats about lateral gene transfer, hybridization, etc.) It ties in
also to the question about mitochondrial Eve, Y chromosome Adam, etc.

Not all individuals successfully reproduce, and those that do do not
have equal numbers of offspring. Over time, some lineages eventually
die out and others come to dominate, whether due to selective
pressures or just random variation [i.e., probabilistic, not outside
God's control.]
For mitochondria in most animals, mammal Y chromosomes, chloroplasts
in plants, etc., the DNA in question is [almost] always inherited from
the parent of one sex. For example, mitochondria are usually
inherited just from the mother. I have two brothers and no sisters,
so anything distinctive in my mother's mitochondria will not make it
to the next generation. Y chromosomes and, in certain cultures,
surnames, are inherited paternally. Although J. S. Bach had about 20
kids, no direct male descendants are living today-all his sons that
had kids either had all daughters or else their sons had no sons or
their sons had no sons. Jews of the high priestly lineage (typically
indicated by the surname Cohen) have a distinctive Y chromosome
sequence reflecting their all descending from Aaron, although various
subsequent events could mean that the actual last shared male ancestor
was after Aaron. Because of greater attention being paid to royal
lineages, because he had a lot of kids (some legitimate), and because
he lived well over a thousand years ago, Charlemagne is documentable
as an ancestor of a good chunk of the people with any western European
ancestry. Pitcairn Island has needed some new people. The Bounty
mutineers picked up Polynesian wives and settled a deserted island.
By now, everbody is close relatives of everybody else, and to avoid
inbreeding there's a need to look offshore for spouses.

Additionally, evolutionarily, for a taxon to become distinctive it
needs to share similar genetic material, which would ultimately
reflect some level of close relationship.

Thirdly, over geologic time there have been occasional disasters that
produced very small populations.

All of these mean that, if you take a group of eukaryotes (bacteria
swap around DNA so much that a single lineage is hard to pin down) and
trace back far enough, you would expect to eventually reach a point
where a single pair was the ancestor of that whole group. However, it
is possible that similar pairs existed at the same time; their lineage
was just one that didn't make it through (at least to the group of
interest).

>2. Why is a pair of biological human bodies related to the origin of
a pair of human minds (souls)? I mean, don't some people believe that
the human physiology was a sufficient substrate to support the mind
(and/or soul) but that the actual infusion of the soul was a separate
event in history? I mean, could not there have been a large
pre-existent population of homo sapiens into which God breathed life
(where life here means not bio but zoe)?<

That's certainly one possibility. There are several possibilities
here and little evidence to go on:
What exactly distinguishes Adam and Eve and other true humans, in the
spiritual sense (I think this is what you mean by zoe), from
non-humans? Is the process gradual or abrupt? To what extent does it
involve intervention-style action versus unfolding of a frontloaded
design? These relate to long-standing questions of how do babies
aquire their status as image bearers of God, the place of animals in
the overall scheme of things, where Cain's wife comes from, etc. and
are not uniquely brought up by evolutionary issues.

Are there physical differences between Adam and Eve versus all prior
hominids or not? Do all humans descend physically from Adam and Eve,
or might the status as fully human been transferred laterally? For
example, if Adam and Eve were selected out of an existing population
that was physically human, they could have been representatives (just
as Jesus can represent all believing humans). The status of everybody
else, whether contemporary or subsequent, is determined by what they
did. Another alternative, described in Perspectives on an Evolving
Creation, suggests that there were many Adam and Eve pairs-every group
of humans chose to defy God at some point.

Did God make a physical body for humans via evolution and then insert
a spirit at the appropriate moment, or did He design evolution such
that an organism that reached a certain cognitive level would have a
spirit? If something like the latter, was this gradual or abrupt?

>This theory would be congruent with a view that human minds are of natural origin, and not supernatural at all.<

Drawing the line between natural and supernatural is rather
problematic here. Biblically, humans are not just physical bodies.
However, the emphasis on bodily resurrection suggests a very close
connection between our physical bodies and our spiritual component-a
more monistic than dualistic view. As noted above, adding full human
spirituality to an evolved body could conceivably be achieved more
intervention-style or through design built into the laws of nature.

> Everything comes from something else. If you say there is a single ancestral pair from which all humans came from, what would that entail? The change of 1 gene? The change of 100's of genes? What could be the biological change where you say the parent is nonhuman but the child is human?<

It might not entail anything unique genetically. They might have had
identical twins, but they just happen to be the ones to which all
modern lineages trace back.

Where to recognize the first humans (meaning spritually accountable)
is not necessarily the same as where the single ancestral pair is. If
the first humans included more than a single pair (seems likely but
not absolutely provable, given the problem of identifying the first
humans). Under an "insert the soul"-type model, there might be no
physical or genetic difference. If having spirituality reflects the
arrival at a threshold level of mental capacity, then presumably there
is some degree of genetics underlying it, although environment plays a
role as well. The genetic difference might be quite subtle, however.
The only case in which a line can easily be drawn between parent
species and new species is when there is an instantaneous barrier to
interbreeding. For example, in Glenn Morton's scenario, the
chromosome fusion that distinguishes humans from apes plays a key
role. Chromosome fusion (or other chromosome rearrangement) makes
interbreeding difficult. Not directly applicable to the human case,
but hybridization in which the hybrid can reproduce (asexually and/or
sexually with other hybrids) but not with either parent is the easiest
way to clearly make new species abruptly. It's common in the wild and
in lab. However, only in hindsight can you tell that a given change
is key to something new rather than an individual aberration. (This
is also a problem with all the "how come we don't see new stuff
evolving today?" type arguments. We probably are, but we can't know
until we wait millions of years or so to see what happens next.)

> In addition, I'm thinking of a presentation I heard on "ring species." A animal (such as a seagull) can have many changes as you follow the geography, with the head of the line being so different from the tail that they are called different species because they can't interbreed. However, all the in-between varieties are interbreeding along the way. If you look at just the head and tail, you'd say they are different species, but if you look at the line you can't tell where one species begins and another ends. Apply that to human biological evolution.<

I don't know of evidence for a geographic ring, but there is something
similar going through time in humans. Homo habilis is fairly
different from modern humans, but when you put in a full set of
intermediate fossils, it's hard to draw firm lines.

> All this makes it impossible for me to accept a single biological pair for all humans. <

They key problem is what all are humans?

> Also the thing I don't understand about mitochondrial eve- what about her mom? What was so different about her mom... didn't she have mitochondria or how was it so different that there's some sort of break in which you can call her daughter the eve but not the mom?<

Here's an example. Suppose, for purpose of the example, that all
modern humans descend from Noah. Noah would be the Y-chromosome Adam
in this scenario-some male descendants of each of the sons have sons,
and so forth. Noah's Y chromosome might not be any different from his
father's, but Noah is the most recent common ancestor. Automatically,
there is a single Y chromosome ancestor in every generation before
that. Noah's sons' wives would provide three mitochondrial
lineages-Mrs. Noah's mitochondria die out with the death of her sons.
The three wives share a common maternal ancestor at some point, if you
go back far enough. If they were sisters, their mother would be
mitochondrial Eve. On the other hand, if Shem had sons but no
daughters, then one of the three mitochondrial lineages would be lost.
 If only one of the three couples had daughters, then that wife would
be the mitochondrial Eve.

Thus, being the mitochondrial Eve or Y chromosome Adam does not
guarentee being the spouse of the other, nor does it guarentee that no
other individuals lived at the time. It does tend to suggest a
relatively small population (either at the time when they lived or in
some sort of later bottleneck-i.e., only the descendants of a certain
individual happen to make it through).

>I can see what you are getting at. The problem though, is that in order for them to be the first humans in a Biblical sense they were not just one pair out of many pairs. They were the original pair. Now, biologically they could have been one pair out of many. But they must have possessed some other unique characteristics and abilities not possessed by the other pairs. Otherwise they were not the Adam and Eve of Christianity...instead someone else was because there were humans that came before them.

The idea that our Adam and our Eve, as a pair, are just one of many
many pairs of Adams and Eves in a population of humans is not
congruent with Christianity. Its outside the bounds of Christianity
as I understand Christianity. Why do I say that? Because the one
of many pairs idea indicates that the original man and original woman
perhap were not even mated. Instead a pair of their descendants were
mated (and became what we think of as Adam and Eve). But if this were
the case, the originals were not created in any personal sense. And
our Adam and Eve are imposters because they are mere grandchildren of
the originals. The whole Biblical story is therefore terribly wrong.
This is all rather troubling.

Therefore, pardon my ignorance, I don't see the benefit of considering
whether the entire human race today was descended from one pair if it
was not T.H.E. original pair of all humans that ever
existed. Not if the idea of worrying about that is to prop up Christianity.<

This gets back to the questions of what it means to be human and how
spirituality extends to others.

I think that Biblically, the key feature distinguishing humans is
having a spiritual nature with moral responsibility. The essentials
about humanity seem to me to be:
all humans today have the priviledge and responsibility of being
morally accountable
by nature, we are in a state of rebellion against God-we harm
ourselves, others, and our environment
we can't fix this situation ourselves

Exact details of how we got that way, though interesting, are not essential.

It is possible that God selected (by whatever method) an initial pair
to be spiritually and physically ancestral to all true humans.
However, genetic data strongly suggests [despite lousy chronological
calibration] that they would have to be relatively far back in the
hominid lineage-probably a good deal more apelike than we are.

It is possible that God selected a single pair out of a physically
more developed population to be representatives of humanity. Despite
not being made sinful, they accurately represented the inclination in
each of us and chose to sin. At that point, either all contemporary
physical humans get a spiritual but sinful human nature, or else
there's a more gradual spread by some means (contact, intermarriage,
divine fiat at certain times, etc.)

More figurative interpretations would include multiple Adam and Eve
scenarios playing out in each population of physical humans existing
at a particular time, or seeing the account in its entirety as
figurative of the spiritual rebelliousness of all of us. I prefer a
less figurative approach but the more figurative ones appear coherent.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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