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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
26.01552°S   27.73381°E    1,480m ASL
                        Sterkfontein Caves, South Africa
The big change in hominid diet

There are a series of hominid fossil sites in this beautiful valley outside Johannesburg, now a World Heritage Site.  The most famous is the Sterkfontein Grotto, where the “Mrs. Ples” australopithecine skull and endocast was found.  Most of the extracted fossils are now in a walk-in vault at the Wits medical school in Johannesburg, where my wife and I spent the day.

     It’s hard to say which was more impressive:  Phillip Tobias showing us the original fossil skulls and comparing them feature for feature with chimpanzee skulls, or Ronald Clarke and Kathleen Kuman showing us an ancient skeleton still in the rock at Sterkfontein’s Silberberg Grotto.

     In the vault, incredible variety and hard-earned detail.  That natural endocast of the Taung child has amazing imprints of blood vessels and the rounded hills and valleys on the brain’s surface.

     At Sterkfontein itself, first navigating a dizzying aerial skywalk over the surface excavations, ducking under the wire grids that hang over the area to provide a coordinate system.  And then scrambling into a dark cave, though several grottos, and eventually coming to a handheld spotlight – showing several figures huddled over a skeleton emerging from the hard rock.

     There is likely a complete australopithecine skeleton embedded in the rock at Sterkfontein.  It’s been there for about 3.33 million years, judging from the other animal species found nearby.  As I noted earlier, most surviving skeletons from East Africa were buried in mud, as a lake expanded and covered them up (or they drowned offshore, failed swimmers).  This kept some of the bones from being trampled into fragments by herds coming to drink.  But the Sterkfontein skeletons were instead embedded in very hard stuff, what makes stalagmites and stalactites.  They are emerging very slowly as Ron Clarke uses the miniature dental version of a jackhammer to remove the surrounding brecca grain by grain.

     The foot of Stw 573 has already told a big story.  While “Little Foot” is manifestly adapted for upright stance and stride, with a heel more capable of weight-bearing than a chimp’s, the big toe is almost as mobile as that of a chimpanzee.  So, despite Little Foot’s upright adaptations, the big toe might have been of more help in climbing trees than a modern one.  The lack of transitional forms used to be a big objection to Darwin, but with a lot of hard work, scientists are finding important transitional forms.

     The transition to upright posture is still an unsettled issue in paleoanthropology, as I mentioned when discussing the island advantage.  Upright posture is likely all tied up with the transition to gathering and hunting (as opposed to the chimp’s eat-it-on-the-spot and snatch-and-grab).  There are a lot of aspects to gathering (eggs, seeds, nuts, leaves, roots – and the selection is greatly enhanced by food preparation from soaking and pounding through to cooking).  Ditto for hunting (grabbing the defenseless young hiding in the tall grass, surrounding in the chimpanzee manner, maneuver via stampeding over a cliff, and projectile predation of many sorts).


Hunting is once again part of the overall explanation for our ancestral way of life, what with the evidence for eating a lot of grass by 2 million years ago.  Grass, not leaves or fruit.  We weren’t baking bread back then (which is the way we now ingest grass directly).  No, some other animal probably converted the grass into meat on the hoof, and the hominids got the characteristic carbon stable isotope ratio from eating the grazing animal – they ate grass indirectly, at one remove.  Had they eaten monkeys who in turn had eaten fruit, the bones would have the nongrass carbon isotope ratio instead.

     It used to be that you couldn’t mention hunting without someone undertaking to correct you.  (Phillip Tobias isn’t correcting me; he’s just demonstrating another use of a hominid femur.)  If it wasn’t Lewis Binford’s minimalist approach to the archaeological evidence back before cut marks were found overlaying carnivore tooth marks on bones in 1979, it was someone saying that gathering was far more important, that even in surviving nonagricultural tribes, meat is only a small percentage of the calories.  Ditto for the chimpanzees which hunt.

     All quite true, so far as I know, though the Inuit are obviously an exception, since they have very little to gather for so much of the year.  Indeed, gathering as more important had a certain attractive logic to it.  It seemed to follow from the economistlike view of evolution, which is a common misconception about the nature of evolutionary change.  So perhaps I should undertake to explain why everyday importance is not necessarily what you want to focus on.  This will be heresy to some, old hat to others, but here goes.


Darwin oversold gradualism to some extent, in his effort to show that there were ways other than catastrophic change for evolution to occur (recall that nice algorithmic crank that I mentioned back at Down House).  This is an Alice-in-Wonderland sort of principle (the Red Queen told Alice that you have to keep running just to stay in the same place).  Certainly “automatic gradualism” is the lesson that people most easily remember a week later, rather than Darwin’s nuanced notions of evolution.

     But there is really nothing automatic about evolutionary change, where everyday usefulness is automatically rewarded (and so you can remember a single principle rather than all those messy details).  Biological adaptations can backslide, if not backstopped by speciation, just as cultural improvements are notoriously easy to lose (recall how the Tasmanians lost fire-starting and fishing techniques, once isolated from Australia).

     More important, to my mind, is that many adaptations and inventions don’t have growth curves.  If you invent a digging stick to expand the range of gathering possibilities, there isn’t too much you can do to improve it into a shovel until a lot of ancillary improvements occur in the creation of sharp tools of the sort needed to make other tools.  To re-double your payoff in terms of calories gathered via the digging stick takes a lot of further invention.  If you invent soaking to help remove the bitter taste from plant toxins, it is again hard to double and redouble your payoff until you invent boiling.  Efficiency improvements are often difficult.  The long stasis in rock toolmaking doesn’t surprise me a bit.

     The same is true of most aspects of hunting.  But one aspect of hunting, projectile predation, has an extraordinarily long growth curve because the distance achieved with accuracy can be redoubled so many times.  No matter how many times you’ve improved your throwing distance, doubling again has additional payoffs in terms of how many days each week that your family gets high-calorie, low-toxicity meat.  Furthermore, there may not be much to gather in certain commonplace climate transitions, and getting through the crunch and its aftermath may turn out to be far more important than everyday efficiencies.

There are strong signals of a massive improvement in dietary quality between Australopithecus habilis and Homo erectus.  These include the 60 percent increase in female body mass, a large reduction in tooth and jaw size, and… a substantial reduction in the size of the gut.  Additionally, this period [about 1.88 million years ago] witnessed a continuing increase in brain volume beyond the size of the living African apes.  Homo seems to have eaten better than Australopithecus.  Why is that?
Richard Wrangham, 2001

     Hominid hunting (beyond the chimp and baboon opportunistic style) is now thought to go back several million years.  You can tell a lot from how much chewing had to be done.  Gorillas have to chew fifty pounds of plant foods every day, and their skulls have a lot of extra space to attach the muscles, all those ridgelines down the middle and around the back.  But in the Homo skulls, all that anchoring space starts to disappear, as if chewing was no longer such a problem.  The cross-section of the jaw muscle can be gauged from the space inside the zygomatic arch that goes from face to ear – I can fit six fingers into the muscle’s space on my erectus skull cast, but barely two into the same space on a modern sapiens skull.  Notice too the decreasing size of the teeth.

     So, as was obvious even back in the gathering-is-politically-correct days, there has been a big change in diet since the time of the common ancestor with the chimpanzees and bonobos.  While cooking – which has made life safe for vegetarians, because of a major expansion in what plants can become food – is part of that story, it may arrive late in the story, long after the major bony changes are seen (although there is one important proposal that would place it back at 1.88 million years when Homo erectus arrives on the scene).  Gorillas are vegetarians and it has trapped them in a limited ecological niche that requires fifty pounds of rough food every day, with gut length and jaw muscles to match.

     One food source, occasionally exploited by chimpanzees in the more arid areas, is underground:  the underground storage organs such as tubers and other roots.  They occur only infrequently in rain forests but are found more often in woodland on the fringes of forests, where herbs benefit from storing water and nutrients during dry periods.  Most animals can’t eat them, as they are rich in toxins and hard to dig up.  Mole-rats and pigs specialize in them, and their bones are found in association even with 4 million-year-old hominid fossils.  Pigs are particularly interesting because their molar teeth (whose size and enamel thickness tell you about diet) have a lot of similarities to those of the australopiths.

     At some point (the problem is when, early or late), our ancestors learned how to exploit such roots as a fallback food when their preferred foods were in short supply.  To extract them often requires digging and chopping.  They are improved by pounding and soaking, which involves a degree of food preparation not usually seen in the other apes.  Cooking is even better.  One formulation has it that eating grazing animals goes back to 2.5 million years (there are butchery marks on the bones of large mammals, back about then), and that cooking (or some other equivalently major improvement in food quality) starts about 1.9 million years ago.


Growth curves are where there is a payoff to repeated improvements.  If some brain enlargement is good (we usually assume), perhaps a little more is even better.  Curves sometimes turn over, of course, why self-medication based on more-is-better can be dangerous.  Physicians since Hippocrates have been warning about the trouble that the more-is-better metaphor gets us into.

     But upward growth curves are especially relevant when you have something like abrupt climate change that can pump you up the curve.  When there isn’t much to gather, hunting temporarily becomes rather important, even if providing less than 10 percent of calories in other times – and one aspect of hunting, accurate throwing, has a nice long growth curve.


Should civilized man ever reach these distant lands, and bring moral, intellectual, and physical light into the recesses of these virgin forests, we may be sure that he will so disturb the nicely-balanced relations of organic and inorganic nature as to cause the disappearance, and finally the extinction, of these very beings whose wonderful structure and beauty he alone is fitted to appreciate and enjoy.  This consideration must surely tell us that all living things were not made for man.

                  - Alfred Russel Wallace, The Malay Archipelago, 1869

Notes and References
(this chapter
corresponds to 
pages 102 to 110 of the printed book)

Copyright ©2002 by
William H. Calvin

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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