Re: Becoming altricial/bi

J. Moore (
Mon, 2 Oct 95 12:00:00 -0500

Pa> The most astonishing aspect of human evolution is not the development of
Pa> the brain, nor the acquisition of bipedalism. It is that we became
Pa> "secondarily altricial".

Pa> Infants of the common ancestor were undoubtedly as precocial as those of
Pa> the chimp and could, from birth, have held onto their mothers. They
Pa> would also have been able to clamber around the trees at an early stage.
Pa> However, at some point hominid babies became as helpless as newborn
Pa> rats.

<deletia> but note that the phrase "helpless as newborn rats"
is wildly exaggerated.

Pa> This amazing change had one cause: the development of the bipedal foot.

Our altricial infants -- altricial relative to other hominoids --
are not the result of bipedal feet, but are clearly the result of
a change in infant development timing. Human infants have a long
development period, relative to other hominoids (and most other
animals). It is true that this change in development timing had
the effect of allowing for longer post-natal brain expansion, but
it cannot be said that "we became altricial to allow for a long
period of brain development". Such a statement would be a
non-evolutionary view attributing prescience to evolution.

Pa> It has long been thought that we became altricial to allow for a long
Pa> period of brain development. It seems that this cannot be the case.
Pa> The long period was already there for physical development. Eventually
Pa> Nature took the opportunity to slot in large brain growth as well.
Pa> Paul.

You offer no support for your contention that "the long period was
already there for physical development". Infants having feet
adapted for easier and more frequent bipedalism would not depend
on a period of longer infant development, as you suggest. Then
you suggest that for some millions of years, this longer period of
infant development didn't include greater brain development -- why
would the brain, alone out of all other physical features, not
develop during this period of infant growth? And then suddenly
a capital-N "nature took the opportunity to slot in large brain

This notion of Nature simply halting development of one organ out
of an entire organism undergoing development requires extreme
special pleading; it is contrary to what anyone would expect from
a developing organism. Therefore this change, to infants which
underwent a longer period of post-natal development, can be fairly
reliably dated by looking at when our species first underwent
relatively massive brain expansion. This would put it during the
period from about 2.5 mya (at earliest) to 1.5 mya, the transition
from australopithecine to *Homo erectus*, much later than the
transition from common ancestor to hominid.

I've included below a bit of a post on this subject which was
posted on 17 Aug 95.

Jim Moore (

JM> The *beginning* of this process would almost certainly be some
JM> kids being born less developed; this could be due to a mutation
JM> in genes related to fetal development. This would probably
JM> precede the other features mentioned (larger and/or more
JM> flexible pelvises [pelvi!... well, maybe not] and larger-brained
JM> [at birth] kids). The reason it would seem likely that it would
JM> precede the other features is that it's something that could
JM> happen due to a fairly simple genetic "mistake", whereas the
JM> other features are almost certainly the product of a much more
JM> complex process, tightly interrelated in a co-evolutionary
JM> process with each other (one evolves a little, which allows
JM> the other to pop up, which allows the first to become more
JM> pronounced, which allows the second to do likewise, etc., etc.).
JM> This "earlier developmental stage at birth" (perhaps more
JM> accurately called "longer infant development"? [pre- and
JM> post-natal]), like many such genetic "mistakes", would be
JM> a problem (a big, probably deadly problem) under some
JM> conditions but okay under other conditions.
JM> This means it could pop up in various populations now and again,
JM> but would commonly be weeded out, even though it holds a possible
JM> benefit at a later time. This potential benefit would be the
JM> ability to give birth to a child whose brain is as large at birth
JM> as previously normal kids, yet is actually still at an "earlier"
JM> stage of development and can eventually develop further. One of
JM> the obvious disadvantages is the need for greater parental care,
JM> and especially the need for an ability to actively carry the infant
JM> most of the time (rather than let the infant just hang on a lot).
JM> You can see why such a trait might pop up in chimps, for instance,
JM> but be weeded out, while for hominids it would be less
JM> deleterious. It might have even needed the innovation of an
JM> infant carrier, such as a sling.
JM> This doesn't mean it would necessarily be a great advantage when
JM> it first arose. Larger brains, in infants or adults (which we
JM> will assume here means greater intelligence), are not necessarily
JM> an enormous benefit. We tend to think so, and we see a trend in
JM> that direction when we look back at our evolution, but at any
JM> given time, greater intelligence may or may not be a great
JM> advantage. Remember that selection, natural selection especially,
JM> during evolution doesn't drive for optimal results, but instead
JM> allows what's "good enough" to continue. Greater intelligence is
JM> like any other feature in this regard; whether or not it's an
JM> advantage at any given time depends on the physical and social
JM> world the animal lives in.
JM> This means that it wouldn't necessarily, at first, be something
JM> that was really "driven" by selection pressure, but might instead
JM> just spread through some or even much of the population because it
JM> wasn't excessively harmful. At some juncture, such as we see
JM> during the transition from australopithecines to homo, this
JM> obviously changed. The fact that the change, not only to brain
JM> sizes, but body size as well, was so rapid compared to the change
JM> that took place in the several million years before, shows that
JM> selection pressure was pretty intense. During this time, even a
JM> few females with the then rather oddball combination of
JM> child-bearing and child-rearing abilities you mention below might
JM> do disproportionately well in evolutionary terms, leaving many
JM> more descendants than others.

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