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/Nakuru.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
Available from amazon.com or University of Chicago Press.
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William H. Calvin
University of Washington
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
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
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.
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.
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.
Floods, Famines, and Emperors, 1999
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