HUMAN EVOLUTION:
Out of the Chattering Ice
A review by Robert N. Proctor*
A Brain for All Seasons Human
Evolution and Abrupt Climate Change
William H. Calvin
University of Chicago Press, Chicago, 2002. 349 pp. $25, £16.
ISBN 0-226-09201-1.
Neocatastrophism has become fashionable in
climatology and the geosciences. In the 1970s, there was the
discovery that the Mediterranean every now and again pinches off
from the Atlantic, dries up, and then catastrophically refloods
when the Straits of Gibraltar are once again breached. The 1980s
brought us the idea that meteorites could cause mass
extinctions, notably the Cretaceous-Tertiary event later traced
to the Chicxulub crater beneath the Yucatan Peninsula. The 1990s
introduced the suggestion that all the world's oceans froze,
perhaps as far down as a kilometer, for a few million years in
the Neoproterozoic, and that the melting of this snowball Earth
opened up some of the niches that made possible the Cambrian
explosion. Today, geomorphologists are realizing that the Grand
Canyon is surprisingly young: Five or six million years was the
commonly cited age only a decade ago, but the uplift of the
Colorado Plateau that led to the cutting of the canyon may
actually be only a million or so years old--a revision
creationists will no doubt try to exploit.
Historians have not yet sorted out the causes of all this
attention to calamity. It could have something to do with those
apocalyptic scenarios fretted over in the Reagan era (e.g.,
nuclear winter), or with social critics of the 1960s and 1970s
gaining professorships (Stephen Jay Gould used to call himself a
"dialectical materialist"), or even with the fact that
catastrophes make good television. One also has to reckon,
though, with the blunt fact that nature has been reasserting
itself against the blinders of gradualistic prejudices, as Gould
and others began to stress more than 20 years ago. Some
catastrophes are just plain real.
The first two-thirds of William Calvin's A Brain for All
Seasons is a creative attempt to incorporate abrupt climate
change into theories of human origins. A neurobiologist at the
University of Washington School of Medicine, Calvin builds on
the discovery, a decade or so ago, that the temperature swings
we have experienced over the past 8000 years are small compared
with fluctuations in the deeper past. Contrary to previous
assumptions of slow and stately coolings extending over
millennia, ice cores from northern Greenland and elsewhere have
shown that ice ages can begin quite suddenly, perhaps even
within a space of only a few years. Climatologists believe that
interruptions in the flow of the Gulf Stream may be one of the
causes of such abrupt coolings: If, for some reason, the warming
waters flowing from the south cannot reach higher latitudes in
the North Atlantic, glaciers will start growing, reflecting more
and more sunlight into space, causing runaway cooling.
Calvin argues that these repeated coolings had profound
consequences for human evolution. Abrupt coolings were
accompanied by prolonged droughts at lower latitudes, reducing
herbivore populations and shrinking the numbers of predators
eating those herbivores. In Africa circa 2 to 3 million years
ago, when bipedal primates first began making and using stone
tools, abrupt climate change rewarded those creatures able to
improve their hunting powers. "Whiplash" climate
fluctuations forced hominid populations through hundreds of
severe-drought bottlenecks, during which those with the better
survival skills (including bigger brains and greater
intelligence) survived to flourish when the good times began
again. Calvin claims that selection did not work on braininess
per se, but rather on the faculties responsible for things like
the neuromotor skills involved in accurate throwing, which had a
long "learning curve." Tool-using hominids had
invented a novel techno-niche not available to other predators
(like the big cats), and when proto-human brains began to expand
to capitalize on the selective advantages of accurate throwing,
other faculties were dragged along. The net result was cerebral
modernity, a flexible and capacious hominid brain "for all
seasons."
This is a variant on the "man the hunter" thesis,
but with a couple of new and intriguing twists: Calvin holds
that the improvement was not slow and steady but episodic,
according to what he calls "catastrophic gradualism."
He proposes that these warm-to-cold, boom-to-bust cycles
(limiting access to hoofed prey) augmented brain function in
tiny spurts, as ice-age oscillations ("chatterings")
and the resulting bottlenecks sharpened hunting skills. Calvin
also elaborates his theory of how chimp-like australopithecines
first began to hunt with tools: Stones or sticks, he says, may
originally have been thrown into herds gathered at water holes,
in the hope that the commotion might cause an animal or two to
be trampled. The predators eventually learned that sharper
stones worked better than dull stones and that, by battering the
edges, they could be made sharper still. Certain shapes were
then found to fly farther and hurt more than others, whence the
origin of all those enigmatic Acheulian "handaxes."
The last third of the book strays a bit, offering
nonspecialist readers an excellent overview of the geophysics
behind abrupt climate change. Calvin focuses on the forces
involved in stabilizing the Gulf Stream and North Atlantic
Current--the "Achilles heel" in much recent climate
modeling (though itself perhaps only an index of some larger
process). The flow of the book is interrupted here to a certain
extent, as human evolution is put on a back burner and our
attention is drawn to the dangers posed to modern civilization
should global warming launch us into cooling mode. Calvin gets
graphic: We might first feel the effects of drought, including
crop failure, massive fires from lightning, and dust storms
kicked up by loss of vegetation. Populations would crash, and
wars would break out over control of resources. Calvin is
clearly aware of the uncertainties in such prognostications, but
he also reminds us that if history (as preserved in the ice
cores) is any guide, we are due for a cooling. Prior to the
relatively mild climate of the past eight millennia, there were
catastrophic cooling events like the Younger Dryas (12,900 to
11,600 years ago), when North Atlantic temperatures dropped by
about 8ºC. If such a cooling were to repeat today, it would
devastate most of the world's agriculture and then some.
Readers may be disappointed to find certain topics missing
from the text: Calvin talks about bombing ice jams in
Scandinavian fjords to prevent catastrophic releases of fresh
water into the North Atlantic, and speculates on the value of
reopening the trans-Panama oceanic throughway closed off 3 to 4
million years ago, but we hear little or nothing about some of
the more obvious ways to reduce warming--and therefore the
threat of abrupt cooling--such as conservation, curbing
greenhouse gases, and a serious shift to globally responsible
fuels.
Calvin does make it clear, though, that too much focus on
global warming in itself--i.e., the heat--could obscure this
larger danger of catastrophic cooling. He aptly cites Ray
Pierrehumbert's caution that if one pulls on a sleeping dragon's
tail without knowing how much it takes to waken it, one had
better be prepared for the unexpected.
The author is in the Department of History, Weaver Building,
Pennsylvania State University, University Park, PA 16802, USA.
E-mail: rnp5@psu.edu
Volume
296, Number 5577, Issue of 28 Jun 2002, pp. 2342-2343.
Copyright
© 2002 by The American Association for the Advancement of
Science. All rights reserved.
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