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/Nairobi.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
Available from amazon.com or University of Chicago Press.
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William H. Calvin
University of Washington
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
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.
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
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)
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.
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
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
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.
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).
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.
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
There are a lot of anatomical differences between us and all
those ancestral bones that I saw today at the museum.y
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.
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
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