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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     
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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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To:                  Human Evolution E-Seminar
From:             William H. Calvin
1.28680°S   36.81486°E    1,680m ASL
                        Nairobi, Kenya
Creating new species from old ones

Let me also recommend Ian Tattersall’s Becoming Human, which I’ve been reading on the plane and in the more comfortable Nairobi watering-hole settings given by the GPS coordinates (with five decimal places of accuracy in latitude and longitude, you can almost locate my exact table).  As Phillip Tobias pointed out to me in Johannesburg, Tattersall likes to subdivide things more than most, being on one end of the splitter-to-lumper spectrum.

     I too think it probable that there have been a lot more hominid species than are usually recognized, simply because speciation speeds up evolution by preventing backsliding, just like that rock sill when the tide goes out.  Because human evolution has been unusually rapid, it pays to look for fast tracks amidst the more general possible progressions, and additional speciations might be part of the fastest track.


For all our talk about evolution, we find it surprisingly hard to define a species.  Alfred Russel Wallace, in answering some of Darwin’s critics who were confused about what a species is, emphasized that variability wasn’t unlimited, that donkeys and horses each vary only within a certain range, that there isn’t a continuum between them.  Donkeys and horses may try to interbreed, but conception may not occur, or spontaneous abortions may prevent most live births.  Those offspring that grow up may, like mules, be sterile, so that they don’t contribute to maintaining some new middle ground between the two species.

     But paleontologists cannot measure reproductive continuity like the zoologists can.  They’re stuck with, “Well, it looks different than this other one.”  Much confusion arises from these fundamentally different connotations of the word species.  The fossil record only sees the anatomical differences, and so that’s what we mostly talk about, when naming things.  We constantly argue at cross purposes.

     Of course, we know full well that some species have enormous regional variety but are still able to interbreed.  Paleontologists digging up a pet cemetery would define dozens of domestic dog species that way, a great overestimate given how well they still interbreed, yielding short-haired mongrels.  Then there are populations that look identical but cannot interbreed with much success (they have some behaviors that keep them apart, or perhaps chromosomal peculiarities).  They are two different species but the paleontologists would count only one.

     Many people working on chromosomes or DNA sequencing tend to talk about speciation in terms of major rearrangements, say the consolidation of the 24 pairs of chimpanzee chromosomes into our 23 pairs that occurred somewhere along the line.  Yes, a prehuman male with 24 chromosome pairs might not be very effective at impregnating a more modern female with 23 pairs – but, like the focus on “mutations,” this view of speciation vastly oversimplifies a lot of more relevant evolutionary and population biology.

     It is often said that “attractiveness” would  be important, that the behaviorally-modern people invading Europe might not have found Neandertals suitable mates.  An exchange that I overheard at a physical anthropology meeting bears on this.  Genetic model maker:  “Even if only two percent of modern males attempted to mate with Neandertal women. . . . ”  Famous paleoanthropologist, interrupting:  “Two percent?  More men than that will try to mate with a sheep!  And they’re not even closely related. . . .”

     More effective as a behavioral isolating mechanism is a shift in the breeding season of a month or two (a behavioral change, perhaps with no anatomical correlate) between two regional populations of a species.  That will keep two populations from effectively interbreeding when they meet again.  The tassel-eared squirrels on the North Rim of the Grand Canyon, for example, breed in June, three months later than their brethren on the somewhat lower South Rim, where the snow melts in March.  Should an individual from the north meet one of the opposite sex from the south on the hiker’s footbridge across the Colorado River, they might not be interested in one another at the same time.  Penned up together for a year or two, they might well hybridize, but the shift is pretty effective in maintaining their other differences (the North Rim squirrels have a skunk-like stripe) from dilution.

     Accumulating some physical differences is much easier in a small population.  The local environment can really “select for” those variants that fit its peculiarities.  Similarly, sexual selection’s peculiarities like peacock tails can also get going most easily in small situations.  But when the climate improves, and some immigrants arrive from a larger population elsewhere, this dilutes whatever adaptations that might have been achieved locally (one definition of “progress“).  “Genes for cooperation” might have increased to involve half of the small population but then the percentage backslides with the dilution.  The locals become more average.


So you have to distinguish between the evolution of physical differences in a regional population (“adaptation”) and the reproductively isolating mechanisms which occasionally preserve those physical differences (“speciation“) from the usual back-and-forth mixing.  Adaptation is the crank, speciation is the ratchet which prevents adaptation from backing up.  Adaptation usually doesn’t produce speciation, but sometimes they coincide.  It’s much like trying to cross a two-way street, waiting for gaps in the traffic.  A gap in one lane doesn’t help much, so you await simultaneous gaps before crossing.

     It is when adaptation and speciation coincide that life undergoes sustainable change (though most new species, like most other small populations, promptly die out the next time the climate fluctuates).  While isolation can allow regional physical differences to accumulate, it is reproductive isolation (“speciation”) that protects these features from dilution, much as a ratchet prevents backsliding.

     Even if they had viable offspring, there is a chance that such hybrids were sterile (in mammals, male hybrids such as mules are almost always sterile).  But the question of successful interbreeding mostly hinges on the spontaneous abortion rate – nearly a quarter to a third of human conceptions are presently lost, mostly before a woman suspects she is pregnant (and this doesn’t count the similar numbers of induced abortions).  Drinking the wrong tap water, or too much caffeine, more than doubles the hidden abortions (fivefold in some studies).

     Human pregnancy failure rates are surprisingly high, compared to domestic animals, enough so that it might have the same effect as a speciation, at least for awhile.  If interbreeding attempts between the local populations – say, Neandertals – and the latest African emigrants had an even higher failure rate, few offspring would have been born.  This raises the possibility that our ancestors might have easily “speciated” in the sense that immigrants to a locally adapted subpopulation – the ones that usually dilute out whatever “progress“ has been locally achieved –  might not have been able to interbreed effectively because of high local spontaneous abortion rates, just from drinking the wrong water or eating too much of the local plant toxins.  This quasi-speciation  would have the effect of saving valuable local adaptations from the dilution effects of mating with immigrants.


Once two populations become reproductively isolated (“the species has split into two”), then they tend to compete with one another.  Yes, they might ignore one another like two ships passing in the night, just a wave in passing, or they might even cooperate in some matters as several hyrax species do, but if they utilize much the same resources (food, nesting places) and have similar predators and parasites, then one of the species is likely to fare somewhat better than the other.

     That’s all that “competition” really means – though, of course, competition between populations can also include the bloodier forms (as closely related chimpanzees proved at Gombe, even without speciation separating the neighboring groups).

What speciation achieves, then, is the shifting of… the genetic and morphological centers of gravity of the parent and daughter species . . . Each species is now free to accumulate more variation and hence more potential species difference.  Their descendant lineages will thus ratchet away from each other in a variety of directions. . . .
- Ian Tattersall,
   Becoming Human, 1998

     Note that a species (an inbreeding population, now) competes with other species within an ecosystem, rather like an individual competes with other individuals within a species.  The species gets started, thrives, maybe splits off new versions, and likely dies out at some point.  Natural selection may operate on how individuals thrive and die, but it can also shape which species thrive.

     While selection may mostly operate on individual genes within the genome, there is a sense in which the performance of the entire committee of genes is also under selection, as some committees do better than others.  Indeed, it is this collection which serves as the source of new variation, what continues to look for better fits with the environment.  This committee aspect is most obvious at the cell level:  the genes within either work together or die together as the cell membrane ruptures.

     Darwin’s inheritance principle operates at the species level as well, although it doesn’t have quite the same “hang together or hang separately” aspect that the cell level has.  Still, it is the species which encompasses the full range of ancestral “good tricks” that can be used to survive and thrive in a new climate regime.  Having the right alleles (different versions of a given gene, as when the A1 allele of the D2 dopamine receptor produces 30 percent fewer receptors than the more common A2 allele) available somewhere in the population can make one species do better than another when the climate perturbs things.


There is a statue of Louis Leakey outside the research building at the National Museum of Kenya, which takes a little work to locate, as it is located back behind the left side of the museum.  Louis is shown seated, contemplating a handaxe.  I like to think that he looks a little puzzled by this enigmatic tool.

     There are a lot of anatomical differences between us and all those ancestral bones that I saw today at the museum.  As Tattersall notes, each little change probably had to be insulated against backsliding, using yet another instance of reproductive isolation.  In other words, a new (physiological) species.  That ratcheting makes it likely that there have been a lot more physiological species (say, dozens) than the half-dozen ancestral anatomical species we usually argue over.

     I can say this so simply, but there has been a century of argument about species and the pace of evolution where nothing was simple.  The debate isn’t finished yet.


Evolution is an enchanted loom of shuttling DNA codes, whose evanescent patterns, as they dance their partners through geological deep time, weave a massive database of ancestral wisdom, a digitally coded description of ancestral worlds and what it took to survive in them.

- Richard Dawkins, Climbing Mount Improbable, 1996


Notes and References
(this chapter
corresponds to 
pages 117 to 123 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