Child development and pelvises, was Re: AAT Questions...
J. Moore (j#d#.moore@canrem.com)
Thu, 17 Aug 95 10:26:00 -0500
I haven't seen Ann's post yet, so I'm working from Harry's reply.
(Sorry about the rather lengthy quoting I'm doing here.)
He> ann.nunn@sstar.com wrote:
: NE>You say "Humans have children that are less developed than our
: NE>relatives because our brains are so much bigger than our closest
: NE>relatives", but the actual fact is quite different, to whit:
: NE>"Humans can have brains are so much bigger than our closest relatives
: NE>because our have children are born less developed than our relatives".
: NE>You see, the difference in wording is fairly subtle, but the
: NE>difference in meaning is enormous. Your wording says the cause
: NE>of the change is something that the change itself allows; this
: NE>would mean that the cause is something from the future, which you
: NE>can easily see is impossible.
: NE>Jim Moore (j#d#.moore@canrem.com)
Ann> Let me see if I can state this the way I understand, albeit not in
Ann> technical language:
Ann> The female in the group with the slightly larger head and the slightly
Ann> larger brain is presumably slightly more intelligent and thus more
Ann> successful in rearing offspring, and she passes those genetic traits
Ann> on
Ann> to her numerous daughters and granddaughters, and to her male progeny
Ann> also. The problem is that the greatgranddaughters have a problem
Ann> birthing the babies with the increasingly bigger heads.
At this point you don't have it quite correct, just on a couple of
points. Essentially you're still doing what the original poster
(Troy?) did, which is putting the big headed kids at the beginning
of the process. The *beginning* of this process would almost
certainly be some kids being born less developed; this could be
due to a mutation in genes related to fetal development. This
would probably precede the other features mentioned (larger and/or
more flexible pelvises [pelvi!... well, maybe not] and
larger-brained [at birth] kids). The reason it would seem likely
that it would precede the other features is that it's something
that could happen due to a fairly simple genetic "mistake",
whereas the other features are almost certainly the product of a
much more complex process, tightly interrelated in a
co-evolutionary process with each other (one evolves a little,
which allows the other to pop up, which allows the first to become
more pronounced, which allows the second to do likewise, etc.,
etc.).
This "earlier developmental stage at birth" (perhaps more
accurately called "longer infant development"? [pre- and
post-natal]), like many such genetic "mistakes", would be
a problem (a big, probably deadly problem) under some
conditions but okay under other conditions.
This means it could pop up in various populations now and again,
but would commonly be weeded out, even though it holds a possible
benefit at a later time. This potential benefit would be the
ability to give birth to a child whose brain is as large at birth
as previously normal kids, yet is actually still at an "earlier"
stage of development and can eventually develop further. One of
the obvious disadvantages is the need for greater parental care,
and especially the need for an ability to actively carry the infant
most of the time (rather than let the infant just hang on a lot).
You can see why such a trait might pop up in chimps, for instance,
but be weeded out, while for hominids it would be less
deleterious. It might have even needed the innovation of an
infant carrier, such as a sling.
This doesn't mean it would necessarily be a great advantage when
it first arose. Larger brains, in infants or adults (which we
will assume here means greater intelligence), are not necessarily
an enormous benefit. We tend to think so, and we see a trend in
that direction when we look back at our evolution, but at any
given time, greater intelligence may or may not be a great
advantage. Remember that selection, natural selection especially,
during evolution doesn't drive for optimal results, but instead
allows what's "good enough" to continue. Greater intelligence is
like any other feature in this regard; whether or not it's an
advantage at any given time depends on the physical and social
world the animal lives in.
This means that it wouldn't necessarily, at first, be something
that was really "driven" by selection pressure, but might instead
just spread through some or even much of the population because it
wasn't excessively harmful. At some juncture, such as we see
during the transition from australopithecines to homo, this
obviously changed. The fact that the change, not only to brain
sizes, but body size as well, was so rapid compared to the change
that took place in the several million years before, shows that
selection pressure was pretty intense. During this time, even a
few females with the then rather oddball combination of
child-bearing and child-rearing abilities you mention below might
do disproportionately well in evolutionary terms, leaving many
more descendants than others.
Ann> The female in
Ann> that generation with the slightly larger and more flexible pelvis will
Ann> be more successful, as will the female who tends to deliver just
Ann> slightly more premature or immature infants (but has the intelligence
Ann> to
Ann> care for them well). So there are (at least) three tendencies
Ann> cooperating in this particular process of natural selection, and the
Ann> female with the tendency for the best combination of all three will be
Ann> the most successful in producing and rearing viable progeny. Larger
Ann> brain cases, wider and more flexible pelvic girdles, and immature
Ann> infants all advance together as evolutionary trends.
Ann> Does this make sense to y'all?
He> Correct, which means the three characteristics are correlated. No more
<deleted some>
He> Harry Erwin
Jim Moore (j#d#.moore@canrem.com)
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