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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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 From the Greeks to the nineteenth century there was a great controversy over the question whether changes in the world are due to chance or necessity.  It was Darwin who found a brilliant solution to this old conundrum: they are due to both.  In the production of variation chance dominates, while selection itself operates largely by necessity.  Yet Darwin's choice of the term “selection” was unfortunate, because it suggests that there is some agent in nature who deliberately selects.  Actually the “selected” individuals are simply those who remain alive after all the less well adapted or less fortunate individuals have been removed from the population.

- Ernst Mayr, This is Biology, 1997


To:                  Human Evolution E-Seminar
From:             William H. Calvin
51.47794°N     0.29089°W     11m ASL
                        Evolution House, Kew Gardens

Subject:         The Darwinian Quality Bootstrap


Back in London, I’ve been wandering around the Royal Botanical Gardens at Kew and its new Evolution House.  It’s a good place to think about all of those things that I didn’t have time to tell my imaginary companion on the Sand Walk.

          And, of course, try to make clear where the unresolved questions about human origins still lie.  I’ve heard lots of them discussed at the various paleoanthropology, archaeology, primatology, and linguistics meetings that I’ve attended in the last several years.  In the course of this e-seminar, I’ll try to clarify this boundary between the known and the dimly seen.  


What, someone asked by email, did Darwin really discover?  It probably isn’t what you always thought.

          It wasn’t evolution per se.  There had been an active public discussion of evolution since before Darwin was born (his grandfather Erasmus even wrote poems on the subject).

          It wasn’t adaptations to fit the environment, as the religious philosophers had already seized on that idea as suggesting design from on high.

          Nor was it “survival of the fittest.”  That idea had been floated by Empedocles 2,500 years ago in ancient Greece, long before Herbert Spencer, in the wake of Darwin, invented the phrase we now use.

          It certainly wasn’t the basic biological and geological facts that Darwin discovered, although during his voyage around the world, and after discovering natural selection, Darwin did add quite a bit in the factual line.

          What Darwin contributed was an idea, a way of making various disconnected pieces of the overall puzzle fit together, something like trying to solve a jigsaw puzzle without a picture for a model.  He imagined the picture.  

You do not recognize answers to questions that have never arisen.

- Alan Walker & Pat Shipman, 1996


          It wasn’t, however, the idea of descent from a common ancestor.  Diderot, Lamarck, and Erasmus Darwin had all speculated on that subject two generations earlier.  And there were trees of descent around to serve as examples, given how by 1816 the linguists were claiming that most European languages had descended from the same Indo-European root language.

          By 1837 Darwin had concluded that nature was always in the process of becoming something else, though again there had been other attempts like Lamarck’s along this line.  Darwin just looked at the biological facts in a different way than his predecessors and contemporaries, not forcing them to fit the usual stories about how things had come about.  Fitting facts to an idea is a primary way in which progress is made in science, but a fit in one aspect has often blinded scientists to more overarching explanations.

          But even that wasn’t his main contribution.  Charles Darwin had an idea that supplied a mechanism, something to turn the crank that transformed one thing into another.

 Basically, Charles Darwin (in 1838 and, independently, Alfred Russel Wallace in 1858) had a good idea about the process of evolution, how one thing could turn into another without an intelligent designer supervising.  Out of all the variation thrown up with each generation (even children of the same two parents can be quite unlike one another), some variants fit the present environment better.  And so, in conditions where only a few offspring manage to reach adulthood (both Wallace and Darwin got that insight from Malthus and his emphasis on biological overproduction), there is a tendency for the environment to affect which variants get their genes into the next generation.

          Many are called, few are chosen by the hidden hand of what Darwin labeled “natural selection.”  The name comes from the contrast to animal breeding, so-called “artificial selection.”  It is, as Ernst Mayr noted, an unfortunate term, as it suggests an agent doing the natural selecting.

          As Thomas Huxley said, when reading Darwin’s book manuscript before its publication in 1859, “How stupid not to have thought of it before.”  Two and a half millennia of very smart philosophers trying to solve the problem, and then the answer turns out to be so simple.  Like the Necker cube and similar perceptual phenomena, there are often several ways to look at the same facts, just as there are two equally valid roots of a quadratic equation, both of which give satisfaction.  And “seeing” the alternative form can be difficult when your culture guides you to see the usual explanation.  But the alternate form may lead you to a more coherent solution, one that also explains a much bigger jigsaw puzzle.

       A few years later, Darwin realized that he needed to add an “inheritance principle,” to emphasize that the variations of the next generation were preferentially done from the more successful of the current generation (the individuals better suited to surviving the environment or finding mates).  This means, of course, that the new variations were not just at random, but were centered around the currently-successful model.  In other words, they were little jumps from a mobile starting place, variations on a theme, not big jumps where the starting place becomes irrelevant because the jump carries so far.  (Warning:  Except for the pros, half of the people who write about evolution, whether pro or con, may be confused about this important short-distance randomness aspect.)

          Many variations, of course, are not as good as the parents – nature appears not to worry about this waste, to our distress – but a few variants are even better than their parents.  And so, with passing generations, there is a chance for drift to occur towards the better solutions to environmental and mate-finding challenges.  Perfection you don’t get, but occasionally you do get something that, locally, could be called “progress“ – that ill-defined something that makes us so impressed by the Darwinian process.  Nature can be seen to pull itself up by its own bootstraps, amidst a huge waste in variations that go nowhere.


You can summarize Darwin’s bootstrapping process in various ways, from our modern perspective.  A century ago, Alfred Russel Wallace emphasized variation, selection, and inheritance.  It reminds me of a three-legged stool:  evolution takes all of them to stand up.

          But there are some hidden biological assumptions in that three-part summary and, when trying to make the list a little more abstract to encompass non-biological possibilities, I wound up listing six ingredients that are essential (in the sense that if you’re missing any one of them, you’re not likely to see much progress):

1.    There’s a pattern of some sort (a string of DNA bases called a gene is the most familiar such pattern, though a cultural meme – ideas, tunes – may also do nicely).

2.    Copies can be made of this pattern (indeed the minimal pattern that can be semi-faithfully copied tends to define the pattern of interest).

3.    Variations occur, typically from copying errors or superpositions, more rarely from a point mutation in an original pattern.

4.    A population of one variant competes with a population of another variant for occupation of a space (bluegrass competing against crabgrass for space in my backyard is an example of a copying competition).

5.    There is a multifaceted environment that makes one pattern’s population able to occupy a higher fraction of the space than the other (for grass, it’s how often you water it, trim it, fertilize it, freeze it, and walk on it).  This is the “natural selection“ aspect for which Darwin named his theory, but it’s only one of six essential ingredients.

6.    And finally, the next round of variations is centered on the patterns that proved somewhat more successful in the prior copying competition.

Try leaving one of these out, and your quality improvement lasts only for the current generation – or it wanders aimlessly, only weakly directed by natural selection.

          Many processes loosely called “Darwinian“ have only a few of these essentials, as in the selective survival of some neural connections in the brain during development (a third of cortical connections are edited out during childhood).  Yes, there is natural selection producing a useful pattern – but there are no copies, no populations competing, and there is no inheritance principle to promote “progress“ over the generations.  Half a loaf is better than none, but this is one of these committees that doesn’t “get up and fly” unless all the members are present.

          And it flies even faster with a few optional members.  There are some things that, while they aren’t essential in the same way, affect the rate at which evolutionary change can occur.  There are at least five things that speed up evolution.

          First is speciation, where a population becomes resistant to successful breeding with its parent population and thus preserves its new adaptations from being diluted by unimproved immigrants.  The crank now has a ratchet.

          Then there is sex (systematic means of creating variety by shuffling and recombination – Don’t leave variations to chance!).

          Splitting a population up into islands (that temporarily promote inbreeding and limit competition from outsiders) can do wonders.

          Another prominent speedup is when you have empty niches to refill (where competition is temporarily suspended and the resources so rich that even oddities get a chance to grow up and reproduce).

          Climate fluctuations, whatever they may do via culling, also promote island formation and empty niches quite vigorously on occasion, and so may temporarily speed up the pace of evolution.

          Some optional elements slow down evolution:  “grooves” develop, ruts from which variations cannot effectively escape without causing fatal errors in development.  And the milder variations simply backslide, so the species average doesn’t drift much.  Similar stabilization is perhaps what has happened with “living fossil” species that remain largely unchanged for extremely long periods.

          You’ll notice that I didn’t even mention changes in the rate of mutations.  Since sex and gene shuffling were invented, mutation rate may have fallen pretty far down the list of important factors controlling the pace of evolution, even though mutations are the usual beginner’s example.  Species shifts more often involve changes in the relative proportion of existing gene versions (gene frequencies).  It’s the committee’s composition that counts; sometimes all it takes is removing one member to break a deadlock or open up new paths.



Notes and References (this chapter corresponds to pages 20 to 26 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