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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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Africa at Night


To:                  Human Evolution E-Seminar
From:             William H. Calvin
29°N      22°E       9,100m ASL
                        Libya by moonlight
The last big step toward humans



I just happened to look out of the airplane window in the middle of the night, and saw dozens of fires scattered across the moonlit landscape of Libya.  No, they aren’t the romantic campfires of the desert tribes, scenes from Lawrence of Arabia.

     There is a lot of methane gas mixed in with crude oil, and they often burn it off rather than capturing it to export as natural gas.  Just look at one of those posters of the night side of the Earth as seen from space and you’ll soon learn where the oil fields are, from all the bright lights in thinly populated places like the shores of the Red Sea.

     A methane torch isn’t as satisfying as a nice campfire.  There might, of course, be reasons why we find campfires so pleasing, particularly campfires in a cave.  By dawn, we’ll be over southern France, overflying a lot of famous caves.


Talk about satisfying views – I’m told that you can’t drive around the river valleys in southwestern France without remarking on all the nice sites for rock shelters in the limestone walls of the valleys.  The ledges, sheltered by overhanging rock, look out on water and grass – and were likely a superb place to spot grazing herds, back in the ice ages.  I must remember to ask Gordon Orians if landscape aesthetics can distinguish between savanna with wide views and such limestone valleys with narrowed views, given that both could feature elevated viewpoints, scattered trees, grass, and edible grazing animals.

     The rock art thereabouts is what draws most visitors to the area, not paleohunting tales or the Cro-Magnon fossil site.  And it focuses us on another important development, perhaps the last big one in human biological evolution – or perhaps the first really big episode of cultural evolution.  The number of big steps is surely arbitrary, but I’d say that there have been at least four during the last five-to-seven million years, the time since we last shared a common ancestor with the chimp and bonobo.

     The first was attaining upright posture, what I discussed at Sterkfontein cave in South Africa, featuring the australopithecine skeletons.

     Then comes adapting to the limited amounts of savanna, perhaps 2-3 million years ago.  We might have had a number of changes in prolonging childhood and in social behaviors, including such things as pair-bonding.

     Third is toolmaking (and likely some improvements in other plan-ahead behaviors, maybe even protolanguage).  Toolmaking got off the ground by about 2.6 million years ago, and that’s when the brain started to enlarge beyond the size of the great apes.  Apparently it was Benjamin Franklin who coined that term “Man the Toolmaker” two centuries ago, but now it looks as if toolmaking complexity was not a long steady rise.  It is widely suspected that tools were mostly relevant because of the addition of a lot more meat to the diet at this time.  Homo changed in many ways:  a jump in brain size of almost half back before 2 million years ago, a much less heavy body (australopithecines had apelike body mass for their height), and with males closer in size to females.  Growth probably slowed back then, with a prolonged childhood.

     Finally came what the anthropologists call “behaviorally modern” humans.  The beginnings of this were found west of Lake Naivasha (page 156 ) at more than 50,000 years ago.  Since then, tools became much finer, fashioned from delicately struck stone blades, and fishing implements appear.  Sewing needles are seen by the coldest part of the ice age.  Bone and antler came to be used for sharp tools, and were even decorated with carvings.  Cave art appears, as do beads and pendants.  They probably thought a lot like we do, even though some still had brow ridges and bigger teeth.

My father used to say that, through culture, humans effectively domesticated themselves.  As we know, domestication – of plants and animals – leads to rapid evolutionary change.
- Richard E. Leakey, 1992

     Earlier than that, whatever they were thinking didn’t often show up as durable art or as more sophisticated tools (except, notably, in Africa).  As Ian Tattersall said in Becoming Human, the last 50,000 years “stands in dramatic contrast to the relative monotony of human evolution throughout the five million years that preceded it.  For prior to the Cro-Magnons, innovation was . . . sporadic at best.”

     Now if only we could measure it in the archaeology, a better set of criteria for modern human behavior would include abstract thinking, planning depth, innovation, and symbolic behavior.  I formulate it a different way, as the suite of structured thought called higher intellectual function, but they largely overlap.

The British anthropologist Kenneth Oakley suggested in 1951 that this efflorescence might have also been when fully modern language appeared on the scene, and that’s still the leading idea, as I mentioned back at the Maasai Mara.  While there were likely earlier forms of communication, “Only language,” to quote the linguist Derek Bickerton [he means, of course, syntax – not just words], “could have broken through the prison of immediate experience in which every other creature is locked, releasing us into infinite freedoms of space and time.”  (Though, some psychotherapists note, there remain a few modern people who still cannot break through the prison of immediate experience.)

The strong signal from the behavioral record, then, is that our acquisition of the human capacity was a recent, and emergent, happening.  Much as paleoanthropologists like to think of our evolution as a linear affair, a gradual progress from primitiveness to perfection, this received wisdom is clearly in error.  We are not simply the inevitable result of a remorseless process of fine-tuning over the eons, any more than we are the summit of creation.
- Ian Tattersall, 1998

     I suspect that this cultural efflorescence followed the step up to the modern suite of structured mental abilities that I call the higher intellectual functions, of which syntax is the prime example.  It’s not clear how long the higher intellectual functions have been around, which is why people talk of anatomically-modern Homo sapiens and then behaviorally-modern Homo sapiens.  Judging from bones and DNA, our species is now thought to go back perhaps 150,000-200,000 years, and Africa’s Middle Stone Age starts at about 250,000 years ago.

     The most spectacular evidence for lively mental abilities comes from cave paintings.  The Chauvet cave in France is dated to 33,000 to 38,000 years ago and an Italian cave may be slightly older yet.  The cave paintings speak, as Richard Leakey says, “of a mental world we readily recognize as our own.”  It, like the scattered use of diagrams and bone, could go back earlier.  Because Chauvet has fully representational paintings, not primitive stuff, art may go back much earlier in other undiscovered places.  As the archaeologists like to say, “Absence of evidence is not evidence of absence” and it is surely an appropriate caution where ephemeral art forms are concerned.

     The dichotomy (if there is one; it might disappear as the archaeology gets better) between anatomically and behaviorally modern Homo sapiens is opposite of the situation we usually face.  In general, it is best not to identify innovations with the species that obviously practiced them.  Big innovations (say, upright posture) are likely invented earlier by the predecessor species.  Behavior invents, bones follow (a less elegant way of saying form follows function).  In this view, the bony transitions are merely solidifying progress already achieved by behavioral flexibility and learning abilities, likely including cultural transmission.  But in the case that the cave art illustrates, the bony changes could be 100,000 years earlier.  This is behavioral innovation without (as yet) a corresponding change in body proportions and other things that paleoanthropologists focus upon.


The last big step up may turn out to have something to do with augmenting “primary process” in our mental lives.  The notion is that it got an addition called “secondary process.”  The distinction was one of Freud’s enduring insights, though much modified by now.

     Primary process connotations include simple perception and a sense of timelessness.  You also easily conflate ideas, and may not be able to keep track of the circumstances where you learned something (what the memory experts call “source monitoring”).  Primary process stuff is illogical and not particularly symbolic. You consequently engage in a lot of displacement behaviors like kicking the dog, when frustrated by something else.  While one occasionally sees an adult who largely functions in primary process, it’s more characteristic of the young child who hasn’t yet learned the differences in point of view between itself and its mother.  There is a lot of automatic, immediate evaluation of people, objects, and events in one's life, occurring without intention or awareness, driven by the immediacy of here-and-now needs and having strong effects on one’s decisions and behavior.

     Secondary process is layered atop primary process, and is much more symbolic.  Language is used to relate experiences to someone with, commonly, a different viewpoint.  Secondary stuff involves symbolic representations of an experience, not merely the experience itself.  It is capable of being logical, even occasionally achieves it.  You can have extensive shared reference, the intersubjectivity which makes institutions possible.  Piaget’s developmental stages show you another way of looking at what might have also been evolutionary stages.


Now let me tackle what happens in the Upper Paleolithic (in Europe; in Africa, the Middle and Later Stone Ages), and why.  The “50,000 year problem” is that some modern mental abilities become apparent in art and toolmaking between 50,000 and 35,000 years ago, with the stone-tool improvements visible much earlier in Africa’s Middle Stone Age.  While there’s no big change in gross anatomy, that doesn’t rule out some brain and behavioral change caused by a changed mix of genes – such as improvements in language and planning abilities.

     Before then, did anatomically-modern humans just talk silently to themselves, like the animals in Gary Larson cartoons?  Most people assume they did.  They also believe that our pet cats and dogs think, and certainly apes.  So let me tell you what I always say to such statements about the great apes:  If they could talk to themselves with the complexity we seem to assume, then they could think complex thoughts.  And if they could think complex thoughts, they’d be able to plan ahead and do other things clearly advantageous to themselves.  We’d see the evidence for complex thought in their behavior, even if they didn’t talk about it.

     But after you’ve learned enough animal behavior to also know what they don’t do, you begin to wonder.  If chimps had complex thought, for example, they’d be the terror of Africa.  Instead of their ganglike hit-and-run raids on an isolated neighbor, chimps would make war on whole groups of neighbors using stockpiling of supplies and staged, coordinated attacks.  We do not see that.  No one sees much evidence of logical planning in the chimps, not the kind of planning where two or three novel stages need to be worked out in advance of acting.  No such evidence in chimps, and certainly not in Gary Larson’s talking snakes.

     Since the great apes don’t much plan for tomorrow, I’m willing for the moment to consider that complexity of thought may not be present in them.  And maybe this same viewpoint ought to be applied to our ancestors, too, at least considering the possibility that complex thought is not much older than 50,000 years.  Before then, Homo sapiens would have been unapelike in many respects.  They would have had the long childhood with lots of culture to learn; they might well have had protolanguage, with its vocabulary and short sentences.  Their feelings might have been much like ours.

     But without structured thought, with its tendency to spin scenarios about the future and concoct explanations for the past, they might have lived largely in the present.  Their logical abilities might have been limited to what we see in apes, and their sentences might have rarely been chained together for a larger effect.  They might have lacked that horizon-expanding addiction to stories that we have, or our tendency to spend years trying to fulfill an idea.


Maybe new genes appeared on the scene 50,000 years ago.  Maybe not, too – we’re always happy to credit some new gene, but it’s even more likely to be the loss of an allele than the addition of one.  It’s the committee of genes that changes, the committee that regulates early brain development trajectories and staging, from whence comes the behavioral propensities.  The Y chromosome data suggests a major branching about 50,000 years ago, as does the mitochrondrial DNA dating, raising the possibility of some late genetic change affecting higher intellectual function.

     One line of Y chromosome evidence suggests that there was a migration from central Asia into Europe between 50,000 and 35,000 years ago.  It may have brought with it what the archaeologists call the Aurignacian, an advanced culture known for its sophisticated rock-art paintings and finely crafted tools of antler, bone, and ivory.  Maternal mtDNA allows some similar inferences about migrations.  While neither mtDNA nor the Y chromosome may have carried the behavioral genes that made the difference, they do suggest when some founder effects might have marked the transition from anatomically-modern to behaviorally-modern Homo sapiens.

     It’s still “early days” on judging bottleneck dates, with different genes giving different results.  One is reminded of the most famous “young date” of all, when Lord Kelvin calculated that the earth couldn’t possibly be as old as Darwin and the geologists said (his calculations were fine but based on the assumption that the earth wasn’t heating itself – no one yet knew about radioactive decay and the energy it releases, or how poorly the deep rocks conduct heat).

     We too are likely missing something that will change our calculations.  That’s just science.  But let me take that 50,000 year date seriously for a moment, as there is an interesting window of opportunity about then for moving large numbers of people out of sub-Saharan Africa up north to the shores of the Mediterranean.

  [The following three paragraphs have been corrected from the first printing.]

The Sahara  is ordinarily a great barrier to any journey that requires food and water to be found along the way.  Especially during the cooler-and-drier parts of the ice ages, essentially no rain was thought to fall there.  As I mentioned earlier (at page 63) when flying over the Sahara and Sahel, there are pluvial periods when the monsoon rains extended many degrees farther north into the Sahara and grass became widespread.  Major lakes and many archaeological sites were found throughout the Sahara, dated to the most recent one.

          Various factors contribute to the extent, but what dominates is the month of closest approach to the sun (now in January).  When this is in the northern summer (every 19,000 to 26,000 years or so), about ten percent more sunlight is delivered.  The greater afternoon heating of the Sahara (also the Arabian peninsula and points east) means more moist ocean air is attracted into the interior by the rising hot air.

          There was a pluvial back during the prior warm period about 130,000 years ago, and another one reported sometime between 50,000 and 30,000 years ago.  During that interval, there were a dozen times when the worldwide climate kicked out of cool-and-dry into warm-and-wet within several decades time.  And, of course, a dozen collapses back into cooler-and-drier that likely affected the entire Sahara.

     In the Sahel cycle, cattle can be grazed farther north for some years (as in the 1950s) but soon a drought (as in the early 1970s) pushes human populations back south – not north or east, because the Sahara itself remains such a resource barrier.  During a period where even the Sahara has grasslands and lakes, one can imagine a window of opportunity for a sub-Saharan hunting population spreading all around the Sahara to exploit the boom-time population of grazing animals, only to be pushed out when the next flip abruptly ended the Sahara’s temporary resource boom.

     The Sahara can thus be seen as a pump, drawing populations in from the Sahel and then abruptly pushing them out a few centuries later – and not merely back into the Sahel, but with some of them fleeing the profound drought along the Mediterranean shores.  A half-dozen quick strokes on this pump may have occurred starting about 56,000 years ago, with a more prolonged interstadial-length warming of a few thousand years centered around 48,000 years ago.  A series of pulsed pluvials is far more effective than a single 9,000-year-long pluvial in spreading hunters and herders from Africa into the grasslands of Asia.


If our modern life of the mind is less than 50,000 years old, however, it does raise some interesting questions.  The efficiency notions surrounding evolution often lead to generalities about how evolution produces “well-tested” parts.  (This, despite all of the evidence from medicine about how poorly “designed” a lot of things are, such as the all-important female reproductive tract with its leaky seal between ovary and Fallopian tube that allows ectopic pregnancies and endometriosis.)

     But when biological innovations are still young, then all bets are surely off because evolution simply hasn’t had much time to get the bugs out.  We don’t expect reading to work well because it has only been around for 5,000 years and then only in a few percent of the population (until recent centuries).  Five thousand years is only fifty centuries and, at four generations per century, that’s a mere 200 generations.

Human beings are animals.  They are sometimes monsters, sometimes magnificent, but always animals. They may prefer to think of themselves as fallen angels, but in reality they are risen apes.
- Desmond Morris, 1967, The Naked Ape: A Zoologist's Study of the Human Animal

     Well, 50,000 years is only 2,000 generations and that sure isn’t much time, not when comparing to that hundred-fold greater span of 5-6 million years since the common ancestor with the chimps and bonobos.  While structured thinking may be one of the aspects of human uniqueness, I suspect that evolution hasn’t yet tested it very well – that it is chunky and perhaps dangerous, both to ourselves and the other inhabitants of our planet.  Perhaps evolution didn't have time to get the bugs out before distributing it worldwide (but let me save that topic for another E-seminar, when I visit the cave art we’re flying over about now).

     It also isn’t clear that we’re good enough yet to craft a science and technology that will head off a big climate catastrophe.  We are, at least, able to perceive the problem now.  And we have the computers to help us evaluate the do-something scenarios.


Sometime not long after the efflorescence, humans floated to Greater Australia (at low sea level, New Guinea is connected) and, somewhat later, maybe even to the Americas.  There are sites in South America dating back to 20,000 to 30,000 years ago, and more recent ones in eastern North America.  Given that getting across the oceans or the Arctic required a lot of planning ahead, the journey may have been made possible by the improved suite of higher intellectual functions.

     The really definitive spread into the Americas, the one that may have overridden any surviving earlier settlements, occurred as the last ice age was coming to a close.  It is thought that some hunters arrived in Alaska about 15,000 years ago during that abrupt warming episode known as the Bølling-Allerød, where the Northern Hemisphere suddenly warmed up to almost modern levels (and in only a few decades, too).  Since it took much longer for sea level rise to follow, it would have been a good time to pursue those elephants we call mammoths across the exposed Bering Strait, and discover an even bigger supply.

     But the continental ice sheets probably kept them from spreading south through Canada until an ice-free corridor opened up 13,000 years ago from Alaska to Montana.  There were two major ice sheets, the one over the Rockies and the one centered on Hudson Bay which had spread west into the foothills of the Rockies.  As things warmed up, the latter ice sheet retreated and some grass started to grow in the gap.  This provided food for various Alaskan species, allowing them to gradually migrate southward in the corridor along the eastern foothills of the Rockies.

     The Alaskan brown bear, the grizzly, made it down south about then.  So did some big game hunters, about 13,000 years ago. Though they weren’t the first, they discovered an even more plentiful supply of mammoths, which were then living all over the Americas.  It took less than a thousand years for those hunters to spread all through North and South America.  That may sound fast, but it’s only eight miles per year – and a hunter is likely to wander as far in a single day.  A thousand years was also about the time that it took to kill off all the mammoths (and other megafauna) in the Americas, leaving the hunters with an urgent need to diversify their diet.  Note that an initial population of even a hundred people could have grown to 10 million in that same thousand years, were there a growth rate of only one percent per year (Kenya’s present population growth is three times as fast, for a doubling time of only 23 years).

     Exactly who these early Americans were is a subject of debate, as their skull shapes are sometimes more like those of people from Europe and Africa than they are like those of the northern Asians living closer to the Bering Strait.  The present-day Native Americans are mostly from later waves of immigration from Asia, with the Arctic-adapted Inuit (“Eskimo”) of Canada and Greenland being the most recent.

     And that’s where I’m heading now, to take a look at what happens in the oceans near Greenland’s eastern and southwestern coastlines, the processes that make worldwide climate flip so abruptly from warm-and-wet into cool-dry-dusty-windy.



Notes and References
(this chapter
corresponds to 
pages 192 to 204 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