William H. Calvin, A Brain for All Seasons: Human Evolution and Abrupt Climate Change (University of Chicago Press, 2002). See also http://WilliamCalvin.com/BrainForAllSeasons/AMS.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
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William H. Calvin
University of Washington
assumption about brain size is that if evolution acts on discrete
abilities, that the brain enlarges bump by bump – that the visual
cortex bulges out when visual acuity comes under selection, then the
motor cortex if dexterity comes under selection, and so forth.
Something like this may be true for smell – looking at mammals
broadly, you can enlarge olfactory areas without necessarily enlarging
much else – but “enlarge one area, enlarge them all” seems a bit
closer to the truth for neocortical areas.
So if visual acuity comes under selection, so that more
finer-grained neurons are needed, requiring more space for visual
cortex, the whole neocortex may enlarge and so you may get more
auditory cortex as a side benefit.
Much of the interest in brain size comes
from a commonplace assumption, that bigger is better.
If the fourfold increase from the apes to our average brain size
(about three pints, 1350 cc) was so useful, maybe (so the assumption
goes) bigger-brained people are also smarter.
I have certainly come to be skeptical of this latter assumption,
about how people vary.
Variation between species and variation
within a species are two very different things, as it turns out.
It’s not that I doubt the data – IQ and brain volume
aren’t that hard to measure and there is some correlation between IQ
and brain size in modern humans – but I’m cautious about the
cause-and-effect presumption usually involved in interpreting the data.
Bigger feet may correlate with height, but we don’t usually
assume that having bigger feet makes us taller.
Instead we assume they’re just both consequences of some other
aspects having to do with nutrition, number of childhood diseases
suffered, and heredity.
I also know that fat (in the form of the myelin which insulates
the long wire-like parts of nerve cells) is a big part of brain volume,
and that no one has yet evaluated whether the bigger-within-our-species
brains are just fatter, much like two computers which differ only by
the larger one having thicker insulation on the wires.
We should soon be able to measure how much of brain size
variation is just slender versus padded styles in brain fat and how
much is processing power (and how much of that is in neocortex, the
only part relevant to most innovative aspects of intelligence).
I’m also skeptical because intelligence
(in the general sense of the word) simply isn’t what is so easily
measured with pencil and stopwatch.
IQ does correlate with the “quickness” which is part of our
general impression of someone’s intelligence.
Speed of decision making and how many abstract concepts you can
juggle simultaneously (major aspects of what IQ measures) are
undoubtedly important for being a modern physician but that’s surely
a peculiarity of the profession (and, more generally, of our modern
survival-of-the-fastest society), not a general trait of goodness and
human environments more generally.
So don’t conflate IQ with intelligence.
Much of the practical side of intelligence has to do with being
innovative in dealing with social and environmental challenges, and on
longer time scales than a 10-minute office visit.
Note that the groups so renowned for
practical intelligence are not the somewhat bigger-brained groups.
(Asians lead, followed by Europeans and other mixed-race groups,
but we’re talking of differences of about two percent in average
cranial capacity between Asians and Africans. This is splitting hairs.)
Consider the environmental challenges of the Australian outback
and the fickle climate in which Africans have thrived.
You see a lot of assumptions about bigger
brains having been important for colonizing Eurasia with its wintertime
challenges, but that’s simplistic.
I certainly suspect that the different environments of Eurasia
caused some variants among the African immigrants to thrive better than
others (and planning ability is often needed to get through the
winter), and I’m quite willing to assume that the somewhat bigger
brains came along with a package, but I’d really like to know what
that package is.
of the few candidates thus far is the so-called r-K spectrum of
parental investment, well studied in the animal world and surely
applicable to humans (“r” stands for the lay-them-and-leave-them
parental strategy, “K” for the opposite extreme, heavy investment
in relatively few offspring). “K”
usually involves slower growth, delayed sexual maturity, and longer
life span. Being able to
afford only one or two children because of the thirty-year expense of
putting them through higher education is often offered as the new
extreme example in “K” but r-K is mostly a biological thing, seen
in things not normally under voluntary control, such as having twins or
not – and some animals switch strategies when the climate improves to
cut corners (taking chances by having twins, weaning sooner, and so
forth). Humans are at the
far end of the animal world’s r-K spectrum but we vary, with some not
quite so extreme as others, particularly when boom times start
suggesting corner-cutting possibilities.
But how is r-K implemented mechanistically?
It’s probably a package deal, just like you can’t get power
windows on your car unless you also get leather seats.
They just aren’t customized piecemeal, as much as customers
would prefer it. With such
a package deal in biological bodies, only some aspects need to have
immediate payoffs in dealing with the environment or social life.
Just because I opted for power windows, you can’t infer that I
like my leather seats (I’d much prefer cloth, cooler in the summer
and warmer in the winter). They
were merely dragged along.
So too, some things are dragged along in biological bodies –
maybe even bigger brains, all without brain size having been one of the
important aspects under natural selection.
Genes just don’t do things piecemeal, despite our tendency to
name them as if they did; they too involve package deals.
Many aspects of r-K (including brain size) might simply flow
from slowing down the overall pace of prenatal and childhood
development, or from decreases in overall testerosterone levels –
global things, not piecemeal customization.
It’s easy to let the bigger-is-better assumption keep you from
thinking more deeply about this foggy subject, but as the view clears
we may discover that brain size was just swept along in a general flow
of other, more immediate, things.
And that intelligence is mostly an aspect of another kind of
package, those curb-cuts of secondary use, where the whole suite of
higher intellectual functions profits from improvements paid for by
more restricted uses.
would constitute a satisfying explanation of brain size?
We’d like to know what happened when brain size took its
larger steps up. We’d
like to know how the course of development changed, likely via
disturbing the regulation that held brain growth to the ape standard.
(That’s probably done by deleting or inactivating a regulatory
gene, not necessarily by adding a new “big brain gene.”)
We’d like to know what variety of natural selection paid the
price of admission. (Likely a series of them, different package deals at
different times.) We’d
like to know what curb cuts were involved, what new secondary uses were
facilitated. And where.
You can easily double that brain-size list. Try another list for intelligence, while you are at it.
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