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/Kew.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
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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.
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
It wasn’t adaptations to fit the environment, as the religious
philosophers had already seized on that idea as suggesting design from
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.
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.
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
“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.
for the pros, half of the people who write about evolution, whether pro
or con, may be confused about this important short-distance randomness
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.
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):
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).
Copies can be made of this pattern (indeed the minimal pattern
that can be semi-faithfully copied tends to define the pattern of
Variations occur, typically from copying errors or
superpositions, more rarely from a point mutation in an original
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
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.
And finally, the next round of variations is centered on the
patterns that proved somewhat more successful in the prior copying
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
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
Splitting a population up into islands (that temporarily
promote inbreeding and limit competition from outsiders) can do
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
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
Notes and References (this chapter corresponds to pages 20 to 26 of the printed book)
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