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William H. Calvin, A Brain for All Seasons:  Human Evolution and Abrupt Climate Change (University of Chicago Press, 2002). See also

copyright ©2002 by William H. Calvin
ISBN 0-226-09201-1 (cloth)    GN21.xxx0     
Available from or University of Chicago Press.
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This 'tree' is really a pyramidal neuron of cerebral cortex.  The axon exiting at bottom goes long distances, eventually splitting up into 10,000 small branchlets to make synapses with other brain cells.
William H. Calvin

University of Washington
Seattle WA 98195-1800 USA

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To:                  Human Evolution E-Seminar
From:             William H. Calvin
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                                Layover Limbo
IQ and evolution’s package deals



One common 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.


One 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.


What 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.



Notes and References
(this chapter
corresponds to 
pages 59 to 64 of the printed book)

Copyright ©2002 by
William H. Calvin

The nonvirtual book is
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All of my books are on the web.
You can also click on a cover for the link to

Conversations with Neil's Brain:  The Neural Nature of Thought and Language (Calvin & Ojemann, 1994)

The Cerebral Code:  Thinking a Thought in the Mosaics of the Mind (1996)

How Brains Think:  Evolving Intelligence, Then and Now (1996)

Lingua ex Machina:  Reconciling Darwin and Chomsky with the Human Brain (Calvin & Bickerton, 2000)

The six out-of-print books are again available via Authors Guild reprint editions,
also available through (click on cover):

Inside the Brain

The Throwing Madonna:  Essays on the Brain

The River That Flows Uphill


The Cerebral Symphony

The Ascent of Mind

How the Shaman Stole the Moon