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/Libya.htm.
ISBN 0-226-09201-1 (cloth) GN21.xxx0
Available from amazon.com or University of Chicago Press.
Webbed Reprint Collection|
William H. Calvin
University of Washington
Africa at Night
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
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
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
The first was attaining upright posture, what I discussed at
Sterkfontein cave in South Africa, featuring the australopithecine
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.
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
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.)
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.
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
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.
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
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
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
[The following three paragraphs have been corrected from the first
[The following three paragraphs have been corrected from the first printing.]
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
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
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
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.
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.
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
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.
On to the NEXT CHAPTER
Copyright ©2002 by
Book's Table of Contents
Calvin Home Page
All of my books are on the web. The six out-of-print books
are again available via Authors Guild reprint editions,
All of my books are on the web.
The six out-of-print books
are again available via Authors Guild reprint editions,