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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

My specialty is the time when man was changing into man. But, like a river that twists, evades, hesitates through slow miles, and then leaps violently down over a succession of cataracts, man can be called a crisis animal. Crisis is the most powerful element in his definition.

    -Loren Eiseley, The Night Country, 1971        


You might think of the climate as a drunk:  When left alone, it sits; when forced to move, it staggers. 

   - Richard B. Alley, The Two-Mile Time Machine, 2000

To:                  Human Evolution E-Seminar
From:             William H. Calvin
51.4°N     0.1°E   
                        Darwin's Home (Downe, England)

Subject:         Catastrophic Gradualism

You signed up for this, I trust, because you read my preamble, all about seeing human evolution in the context of a bust-then-boom climate episode, with scattered groups surviving the fiery population crash.  And like a phoenix arising from the ashes, going on to great things (well, at least, us) after a sufficient number of repeat performances, just one concentration and expansion episode after another, pumping us up.

          I’m starting this little tour at Darwin’s home, sitting at a park bench under a magnificent oak tree that dates back to Darwin’s time here.  Five years after returning from the voyage of the Beagle, Charles Darwin and his young family moved from central London to a pleasant country home about 16 miles to the southeast, near the village of Downe.  He lived here forty years until his death in 1882.  No more voyages around the world, not even trips to the Continent, but Darwin had correspondents everywhere, and sometimes they showed up at his door.

          And it was here at Down House that he raised pigeons, studied earthworms, and dissected barnacles.  Here he sat, pen in hand, and wrote out his books that provided so much of our modern understanding of how nature came to be the way it is.

          But only ten years ago, a scientific pilgrimage to Darwin’s country home was remarkably difficult, unless you got directions from someone who had been here before.  Only the most detailed guidebooks had a mention of Down House, and then only in the fine print.  Get off the train from London at Bromley South or Orpington, and the taxi driver, upon learning your destination, would knowingly suggest that there were much finer country homes to visit than Down House – clearly not understanding that it was Charles Darwin that made Down House so important, not its gardens.

If I were to give an award for the single best idea anyone has ever had, I'd give it to Darwin, ahead of Newton and Einstein and everyone else.  In a single stroke, the idea of evolution by natural selection unifies the realm of life, mean­ing, and purpose with the realm of space and time, cause and effect, mechanism and physical law.

- Daniel C. Dennett,
Darwin's Dangerous Idea, (Simon & Schuster 1995

          Arrive, pay off your I-told-you-so taxi driver, and you’d find a low budget operation financed over the decades by the London surgeons, with only several rooms restored to what they were like in Darwin’s day, back before the place had been turned into a girls boarding school in the early 20th century.  There were a few rooms filled with old-fashioned museum cases laden with a dozen coats of paint, but most of the house was in sad need of repairs and unsuitable for visitors.  And this for one of the great scientists of all time, not just one of England’s greats.

          Still, it was enormously inspiring to anyone who understood the intellectual triumph of Charles Darwin, this chance to see where he had thought it all through – his study with his microscope, his chair by the living room fireplace, and his “sand walk” out back, where he went for three walks a day to digest his thoughts.  Often, one supposes, Darwin sloughed through the fine English rain, likely blowing in from the west after forming above the warm Gulf Stream.

          Most people who think a little about evolution are wedded to the basic idea of gradual improvements in efficiency – and not much concerned with the origins of what was later improved (it was just “mutations“).  Yet it was Darwin himself (a point omitted from even the modernized science exhibits at Down House) who first cautioned readers about getting fixated on efficiency, and who – at the same time – offered a route for invention.  He noted that changes in function could be “so important,” that an anatomical structure improved for one function could, in passing, serve some other function that utilized the same anatomical feature.  (Darwin’s example was the fish’s swim bladder serving as a primitive lung.)  Novelties come from those nascent secondary uses, not from a bolt out of the blue, as a cosmic ray mutates a gene.


If you haven’t seen Down House since the reopening in 1998, there’s a lot more to see, thanks to much fund-raising by the British Museum.  It is currently operated by English Heritage, which provides audio wands to guide you through the rooms.

          Next to Darwin’s study, there’s his billiard room, where cause and effect operated on a simpler, more direct, level than it does in biology.

          Across the hall is the large dining room with its bay windows; it was also the “justice room” where Darwin served as a magistrate on occasion.

          The now-rebuilt stairway to the upstairs leads you to a series of former bedrooms, filled with modern exhibits about Darwin’s science.

          Darwin traveled into London for scientific meetings, but mostly he kept up an enormous correspondence.  His was something like the modern “home office” style of working, that computers and communications are making possible even for scientists without inherited wealth.  Darwin’s life shows you another style of doing science, one without classes to teach or students to supervise, without grant applications to write, one where piecing together the big story operated alongside the careful dissection of barnacles, digesting it all on yet another loop around the Sand Walk, carrying a great stick which he struck loudly against the ground, making a rhythmical click as he walked along with a swinging gait.

          It’s when making your own third loop around the Sand Walk (now pebble covered in the familiar English Heritage style, though there are still some flints to be found) that you find yourself wanting to tell Charles Darwin about all that has happened in the last 130 years, about how he was right about Africa being the place where humans happened.  Then you scale back your plans to something more suitable for the time it takes to make several more loops.  I decide on abrupt climate change, since it shows how you can have catastrophic gradualism.


Explaining things via catastrophes was seen then (as now) as a form of style without substance.  It was simply too reminiscent of miracles.  Gradual explanations were to be preferred, if they could be found.  Jerks were to be avoided.

          A nice algorithmic turning of the crank was, in comparison, a thing of beauty – and Darwin found a wonderful crank via his inheritance principle, where the more successful of the current generation were the ones who generated more of the minor new variants which future generations would test against their environment.  Variations on a successful theme was the name of Darwin’s game.

          Darwin saw that the climate had changed many times - he immediately offered some geological details to support Louis Agassiz’s 1837 notion of an ice age – and he assumed that animals and plants had to change too, to keep up with the times.  The variants more in tune with the new environment would reproduce better, in turn spawning yet more variants around their gene type (many variants, of course, are worse than their parents, but they don’t reproduce very well, what with high childhood mortality).  So adult body characteristics could track the climate, thanks to some novelties proving to be heritable.

          Efficiency improvements do, of course, result in the long run in a “lean mean machine,” where many features not used for a long time are stripped out as excess fat.  Until recent years, economists loved this view of things, with all its improving efficiency – until it became so apparent that it didn’t explain an innovation, only its subsequent improvement.  And in an economy dominated by market capture, where the first to market may overshadow a better late arrival, innovation is becoming much more important than efficiency.  It’s the “survival of the fastest.”


The natural assumption, surely valid in some cases, is that climate will change slowly enough for little improvements to track climate over the generations – say, more and more upright posture as the blister-like uplift of the East African highlands helped convert forests into open woodlands and then savannas.  What I’d want to tell Darwin is that, just a decade ago, the ice cores revealed that there have also been very abrupt climate changes every few thousand years (on average; most are somewhat irregular exaggerations of an otherwise minor 1,500-year climate rhythm).

          These jumps are superimposed on the better-known gradual trends arising from variation in the earth’s orbit.  They are so large and so quick that a single generation gets caught, forced to innovate on the spot – innovate behaviorally, that is, since there is no time for anything in the gradual adaptations line.

          And this provides a way around the lean-mean-machine implications of traditional Darwinism.  Continuing to carry around a lot of useful-in-a-pinch abilities is a good thing when, about once in every hundred human generations, the climate goes mad for a while.  The variants that became lean mean machines didn’t survive very well in the crunch.

          Climate catastrophes are often mixed up with evolution­ary jumps (imagined macromutations and the like).  But when the climate catastrophes repeat so often, then a little one-percent change each time can jack us up, producing major changes in body and behavior in only a million years or so.  Darwin, I like to think, would have been intrigued by this “catastrophic gradualism“ insight.



Neither glossary items (starting at page 301 ) nor the endnotes (starting at
page 317 )
are denoted in the text, to avoid superscript clutter.
Consult them early and often.


Imagine a world without Darwin.  Imagine a world in which Charles Darwin and Alfred Russel Wallace had not transformed our understanding of living things.  What . . . would become baffling and puzzling . . . , in urgent need of explanation?  The answer is: practically everything about living things. . . .

- Helena Cronin, The Ant and the Peacock, (Cambridge University Press 1992



Notes and References (this chapter corresponds to pages 11 to 19 of the printed book)

Copyright ©2002 by
William H. Calvin

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