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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.
Webbed Reprint Collection
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
0.39804°S    36.08224°E    1,800m ASL
                        Lake Nakuru
Where droughts cause a boom time


I wouldn’t have spent so much time on handaxes if it were not for how central predation strategies are, if you want to understand why Homo erectus could have thrived at the water’s edge.  Now let me turn to how climate change attracts herds down to the lakeshore.

     Here at Lake Nakuru, as at the other Kenyan national parks, one is amazed at how many different species of mammal and bird can be seen in any ten-minute period.  Besides the variety, sheer numbers are also striking.  It isn’t like riding the elevated train around the San Diego Zoo’s Wild Animal Park.

     What’s so different about the real thing (besides one animal eating another, something frowned on at the best zoos) is the savanna setting itself.  Acacia trees are everywhere in East Africa, each with a characteristic branching pattern rather like that of the apical dendrites of the pyramidal neurons of our neocortex.  Maybe I ought to start a movement within neuroscience to rename them “acacia neurons.”

     Another difference in the Kenyan game parks is the relative lack of people inside.  Outside, there are an amazing number of people but without the infrastructure to match.  If you stop your car at a deserted pullout with a nice desert view, an entire family of Africans will appear within a few minutes, emerging from somewhere in the seemingly empty landscape.  It certainly makes you cautious about saying that an empty landscape is truly uninhabited.  At least in Kenya, the people are there, hidden somewhere in whatever shade offers.  Their ability to make a living in such sparse environments is quite an accomplishment.  But they live not far from the edge of famine, as any worsening of rainfall results in more people than food.

     Which reminds me of what would happen if the Kenyan government became even more ineffectual than at present.  Would-be anarchists and the minimal-government types really ought to try living for a few years in a country where the government is paralyzed.

     A truly humane society would have no more people than can be fed in the worst years of climate fluctuation.  But many tribal leaders, and some leaders of whole countries (say, Israel at various times, worried about the high Palestinian birthrates), tend to think that an increasing population gives them an edge over neighbors.


Lakes like Nakuru show you a lot about climate fluctuations.  The lake level itself varies a good deal, and thus the lake’s size.  The shoreline is shrinking right now, as it is in a number of large lakes in Kenya, and so you see the mud flat drying out.  Grass starts growing there, supported by the water table, and many species arrive to graze on the new shoots, even if they have other water supplies.  Fly over Chobe National Park in Botswana and you will see a series of bull’s-eye targets:  shining water in the middle, an intermediate ring of wet mud and grass, and then a dry ring on the outside.  These ponds have animal trails connecting them, looking like a network with many nodes stretching across the landscape.  Some of this drying up is seasonal (Amboseli, the ephemeral lake on Kenya’s southeastern border, is famous for it) and some is longer-term climate change.


     Furthermore, in dry times, the margin of a large lake ought to concentrate populations quite a bit.  In good times they can spread out, but in a drought they all have to stick close to the rivers or a lake margin.  So lake margins are refugia, places where a species can continue to find most of the elements of its niche – food, protection from most predators (recall those bush airstrip landscape esthetics favoring open views), tolerable levels of parasites, nesting sites and other elements needed for reproduction.  And for a savanna-specialized animal like our hominid ancestors a million years ago, a drought would concentrate the game nicely.  The drought even produces more savanna locally, thanks to that ex-mud-flat rim turning grassy.

     What was a crisis for many species might not have been such a crisis for our ancestors, if they usually lived along waterways and lakes.  A drought would force the game to come to them.  After hominids spread away from waterways, a drought would have been a problem – but before that range expansion, a drought might have been a boom time.



A single dry year may kill off stock, reduce grazing land, and devastate crops, but improved rainfall the next year will mitigate the impact.  However, a succession of arid years may have a cumulative effect on cattle and humans, to the point that an unusually severe drought can deliver a knockout blow to already weakened communities.  Sahelian dry cycles can persist for up to fifteen years, as can periods of higher rainfall.  The latter lulls everyone into a false sense of security.  Cattle herds grow, fields are planted ever farther north into normally arid land, contributing to the disaster if a long dry period arrives without warning.  If anything is “normal” in the frontier lands, it is the certainty that severe drought returns.  The ancient Sahelian cattle herders planned their lives accordingly.

- Brian Fagan, Floods, Famines, and Emperors, 1999


Notes and References
(this chapter
corresponds to 
pages 146 to 149 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