|A book by|
William H. Calvin
UNIVERSITY OF WASHINGTON
SEATTLE, WASHINGTON 98195-1800 USA
HOW BRAINS THINK
A Science Masters book (BasicBooks in the US; to be available in 12 translations)
copyright ©1996 by William H. Calvin
There is an updated version
of the abrupt climate change story
in my cover article for the January 1998
issue of The Atlantic Monthly.
Evolving Intelligent Animals
Answering the how questions is often our closest approach to answering a why question. Just remember that the answers to how mechanisms come in two extreme forms, which are sometimes known as proximate and ultimate causation. Even the pros sometimes get them mixed up, only to discover that theyve been arguing about two sides of the same coin, so I suspect that a few words of background are needed here.
When you ask, How does that work?, you sometimes mean how in a short-term, mechanical sense how does something work in one person, right now. But sometimes you mean how in a long-term transformational sense involving a series of animal populations that change during species evolution. The physiological mechanisms underlying intelligent behavior are the proximate how; the prehistoric mechanisms that evolved our present brains are the other kind of how. You can sometimes explain in one sense without even touching upon the other sense of how. Such a false sense of completeness is, of course, a good way to get blindsided.
Furthermore, there are different levels of explanation in both cases. Physiological how questions can be asked at a number of different levels of organization. Both consciousness and intelligence are at the high end of our mental life, but they are frequently confused with more elementary mental processes with what we use to recognize a friend or tie a shoelace. Such simpler neural mechanisms are, of course, likely to be the foundations from which our abilities to handle logic and metaphor evolved.
Evolutionary how questions also have a number of levels of explanation: just saying that a mutation did it isnt likely to be a useful answer to an evolutionary question involving whole populations. Both physiological and evolutionary answers at multiple levels are needed if we are to understand our own intelligence in any detail. They might even help us appreciate how an artificial or an exotic intelligence could evolve as opposed to creation from top-down design.
Everyone was admiring the bald eagles as our cruise ship slipped through the narrow
passage at the top end of Straits of Georgia, between Vancouver Island and the mainland of
British Columbia. In one eagle nest after another, busy parents were feeding open mouths.|
I was watching the raven, myself. It had found a clam and was trying to break open the shell to get at the innards, which were thus far successfully holding the two halves of the shell tightly together. It picked up the clam in its beak, flew several stories high, and dropped the clam on a rocky area of shoreline. This had to be repeated three times before the raven could settle down to pick his meal out of the shattered shell.
Was that instinctive behavior, or learned by observing others, or learned by trial and accidental success, or intelligently innovative? Did some ancestral raven contemplate the problem, then guess the solution? We have a difficult time seeing the intermediate steps between reacting and thinking, yet we also have an unwarranted faith that more is better that having more behavioral options is better than having fewer.
Nature is full of specialists that do one thing very well, with no frills like a character actor
who only plays one kind of role, and never a repertoire. Most animals are specialists. The
mountain gorilla, for example, processes fifty monotonous pounds of assorted greenery every
single day. The pandas diet is just as specialized.|
In terms of finding what they like to eat, neither gorilla nor panda needs to be any smarter than a horse. Their ancestors may have needed to be intelligent in a different niche but now both have retreated into a niche that doesnt require much intelligence. The same is true of the big-brained marine mammals we saw on the Alaskan cruise animals that now make their living in more or less the same way as the small-brained fish, which specialize in eating other fish.
In comparison, a chimpanzee has a varied diet: fruit, termites, leaves even a small monkey or piglet, when its lucky enough to catch one. So the chimp has to switch around a lot, and that means a lot of mental versatility. But what aids building up a wide repertoire? One can be born with many movement programs, or learn many different ones during life, or recombine existing ones in ways that cause novel behaviors to suddenly emerge. Omnivores, such as the octopus, crow, bear, and chimpanzee, have got many moves, simply because their ancestors had to switch among different food sources. They need a lot more sensory templates, too images and sounds they are in search of.
The other way to accumulate novel behaviors is through social life and play, discovering new combinations. A long life span ought to help both learned and innovative behaviors accumulate, and a long life span is what even the smartest of the invertebrates, the octopus, lacks (the octopus is about as smart as a rat in some ways). Smart animals have arisen from various branches of the vertebrate tree of species ravens among the birds, marine mammals, bears, the primate line.
If specialization is most commonly the name of the game, however, then what selects for versatility? A fickle environment is one answer an answer that highlights the environmental factor in natural selection. But let me start with another major contributor to sophistication: social life itself, which involves the the sexual-selection aspect of natural selection.
Social intelligence is another aspect of intelligence: I refer not to just mimicry but to the
challenges that social life (living in groups) poses challenges that require innovative problem
solving. The British psychologist Nicholas Humphrey, for one, considers social intelligence, not
tool use, to be of primary importance in hominid evolution.|
Certainly a social life is an enormous facilitator of an expanded repertoire of actions. Some animals arent around others of their species, to partake of observational learning. Except for brief mating opportunities, adult orangutans seldom encounter one another, because their food sources are so sparse that it takes a large area to support a single adult. A mother with one offspring is about the biggest social group (except for the transient alliances formed by adolescent orangs), so theres not much opportunity for cultural transmission.
Social life, besides facilitating the spread of new techniques, is also full of interpersonal problems to be solved, like pecking orders. You may need to hide food from the view of the dominant animal, in order to keep it for yourself. You need a lot of sensory templates to avoid confusing one individual with another, and a lot of memory to keep track of your past interactions with each of your colleagues. The challenges of social life go well beyond the usual environmental challenges to survive and reproduce that the solitary orang confronts. It would therefore seem that a social life is central to the cultural accumulation of good moves though I suspect nevertheless that a sociable dog lacks the mental potential of the solitary orang.
Natural selection for social intelligence doesnt involve the usual staying-alive factors that are commonly stressed in adaptationist arguments. The advantages of social intelligence would instead manifest themselves primarily via what Darwin called sexual selection. Not all adults get a chance to pass on their genes. In harem-style mating systems, only a few males get the chance to mate, after having outsmarted or outpushed the others. In female-choice mating systems, acceptability as a social companion is likely to be important for males; for example, they need to be good at grooming, willing to share their food, and so forth. The male that can spot approaching estrus better than other males, and who can persuade the female to go off into the bushes with him for the duration of estrus, away from the other males, will stand a much better chance of passing on his genes, even in a promiscuous mating system. (And this female-choice bootstrap might improve more than just intelligence: I argue elsewhere that female choice would have been an excellent setup for improving language abilities, were a female to insist on male language ability at least as good as her own).
[S]ocial primates are required by the very nature of the system they create and maintain to be calculating beings; they must be able to calculate the consequences of their own behaviour, to calculate the likely behaviour of others, to calculate the balance of advantage and loss and all this in a context where the evidence on which their calculations are based is ephemeral, ambiguous and liable to change, not the least as a consequence of their own actions. In such a situation, `social skill goes hand in hand with intellect, and here at last the intellectual faculties required are of the highest order. The game of social plot and counter-plot cannot be played merely on the basis of accumulated knowledge.... It asks for a level of intelligence which is, I submit, unparalleled in any other sphere of living.
The most frequent environmental stress likely to drive natural selection occurs in the
temperate zones. Once a year, there is a period of a few months when plants are largely dormant.
Eating grass (which stays nutritious even when dormant) is one strategy for getting through the
winter. Another, which is much more demanding of versatile neural mechanisms, involves
eating animals that eat grass. The extant wild apes all live very close to the equator; while they
may have to cope with a dry season, its nothing like winters withdrawal of resources.|
Climate change is the next most common recurring stress, seen even in the tropics: annual weather patterns shift into a new mode. Multiyear droughts are a familiar example, but sometimes they last for centuries or even millennia. In some cases, there are state-dependent modes of climate. We saw an example in Glacier Bay, just west of Juneau. When explorers passed the mouth of Glacier Bay two hundred years ago, they reported that it was full of ice. Now Glacier Bay is open to the sea once more, as the glaciers have retreated nearly a hundred kilometers. A series of large glaciers remain in the side valleys, and our ship maneuvered to within a respectful distance of one of these walls of ice; large blocks of it were breaking off and falling into the ocean, even as we watched.
In discussing the local glaciers with a geologist on board, I learned that some were advancing (those are the ones we were taken to see) but that others were in retreat. Advance and retreat at the same time, even in the same valley, and sharing the same climate? Whats going on here, I asked?
Its as if a glacier can get stuck in advance mode for centuries or millennia, even if the climate cools in the meantime. For example, melt water from a few hot summers can get underneath the glacier and erode away the craggy connections to the bedrock. And so the glacier, even if the melting were to stop, can slide downhill faster. That in turn causes the ice to fracture rather than flow when going over bumps, and so more vertical cracks open up. Any meltwater ponds on the surface can then drain down to the bedrock, further greasing the skids and accelerating the movement. The tall mountain of ice starts to collapse by spreading sideways. Eventually you may see glacial surges, of the mile-a-month variety but in Glacier Bay, the ice pushes into the ocean, which erodes it away in giant chunks, that in turn may float away to warmer climates to melt.
Later in the trip we saw Hubbard Glacier, a cliff of ice five kilometers long and taller than our ship. Great blocks of ice, loosened by the waves, would periodically crash into the sea. Off to the right side of Yukatat Bay, we could see back up Russell Fjord. Only a decade earlier, the entrance to that fjord was blocked by a surge in Hubbard Glacier. The glaciers advance was faster than the waves could chip it away, so it crept past the mouth of the fjord and dammed it up. Water started rising behind the ice dam, threatening the trapped sea mammals as the salt water became increasingly diluted with the fresh meltwater. When the lake level got up to about two stories above sea level, the ice dam broke.
We know all about glacial surges in Washington State because they blocked the Columbia River at least fifty-nine times about 13,000 years ago; each time the ice dam broke, a wall of water went racing across the middle of Washington State, carving the terrain into scab lands as it surged to the sea. (Perhaps the ground-shaking roar warned anyone who was trying to catch salmon in the river valleys, to run for the hills.)
Serious as that was, damming up a fjord may have had even more serious consequences. They are often cut off by glacial surges, just as mountain valleys are temporarily dammed by the rubble that avalanches deposit. But dammed-up fjords serve as natural reservoirs for fresh water and, when the ice dam finally breaks, enormous quantities of fresh water flood into the adjacent oceans, a half-years worth in only a half-days time. It layers over the ocean surface and only later mixes with the salt water. Unfortunately, that freshening of the surface layer could have major consequences in the case of Greenlands fjords: it is potentially the mechanism that shuts down the North Atlantic Current that warms Europe for a few centuries, a subject to which I will shortly return.
I tell you all this to point out that there is an enormous asymmetry between the buildup of ice and its subsequent meltdown; this is not at all like the exchange of energy involved with freezing and melting a tray of ice cubes. Buildup mode keeps any cracks filled with new snow and minimizes the summertime greasing of the skids. Melting mode is like a house of cards collapsing in slow motion.
Modes of operation are familiar to us from cool-fan-heat modes of air-conditioning systems. Not only do glaciers have modes, but so do ocean currents and continental climates, perhaps even triggered, in some cases, by glacial surges far away. Sometimes annual temperature and rainfall switch back and forth so rapidly that they have major implications for evolutionary processes, giving versatile animals, like the raven, a real advantage over their lean-mean-machine competitors. Thats what this chapter is really about: how the evolutionary crank is turned to yield our kind of versatility wide repertoires and good guessing get a special kind of boost from a series of climatic instabilities.
There is an updated version
of the abrupt climate change story
in my cover article for the January 1998
issue of The Atlantic Monthly.
Paleoclimatologists have discovered that many parts of the earth suffer fairly abrupt climate changes. Decade-long droughts are one example, and we now know something of the thirty-year cycle by which the Sahara expands and contracts. The El Niño cycle, averaging about six years, now appears to have major effects on North American rainfall.
There have also been dozens of episodes in which forests have disappeared over several decades because of drastic drops in temperature and rainfall. In another abrupt change, the warm rains suddenly return a few centuries later although the last time that Europe reverted to a Siberian-style climate, more than a thousand years passed before it before switched back.
In the 1980s, when confirming evidence of these abrupt climate changes was discovered, we thought they were a peculiarity of the ice ages (ice sheets have come and gone during the last 2.5 million years, the major meltoffs occurring about every 100,000 years). None of the abrupt cooling episodes have occurred in the last 10,000 years.
But it turns out that its only our present interglaciation that has been free of them, so far. The warm times after the last major meltoff 130,000 years ago were turbulent in comparison to the present interglaciation; that earlier 10,000-year warm period was punctuated with two abrupt cold episodes. One lasted 70 years, the other 750 years. During them, the German pine forests were replaced with scrubs and herbs now characteristic of central Siberia.
We have thus far been spared such civilization-threatening episodes. Climatically, we have been living in unusually stable times.
A climate flip-flop that eliminated fruit trees would be a disaster for regional populations of
many monkey species. While it would hurt the more omnivorous as well, they could make do
with other foods, and their offspring might enjoy the population boom that follows the crunch,
when few competitors remain.|
Such boom times temporarily have enough resources so that most offspring can survive to reproductive age, and this is true even of the odd variants thrown up by the gene shuffles that produce sperm and ova. In ordinary times, such oddities die in childhood. But in a boom time, they face little competition; its as if the usual competitive rules had been suspended temporarily. When the next crunch comes, some odd variants may have better abilities to make do with whatever resources are left. The traditional theme extracted from the darwinian process is the survival of the fittest but here we see that it is the rebound from hard times that promotes the creative aspects of evolution.
Though Africa was cooling and drying as upright posture was becoming established in hominids about four million years ago, brain size didnt change much. So far, theres not much evidence that brains enlarged during the climate changes in Africa between 3.0 and 2.6 million years ago a period in which many new species of African mammals appeared. This isnt the place for an extended discussion of all the factors involved in human evolution, but it is important to note that hominid brain size begins to increase between 2.5 million to 2.0 million years ago and continues for an amazing four-fold expansion in cerebral cortex over the apes. This is the period of the ice ages and, while Africa wasnt a major site of glaciers, the continent probably experienced major fluctuations in climate as the ocean currents rearranged themselves. An ice age is not confined to the Northern Hemisphere; the glaciers in the Andes change at the same time.
The first major episodes of floating ice in the Atlantic occurred at 2.51 and 2.37 million years ago, with the winter ice pack reaching to British latitudes. Ice sheets in Antarctica, Greenland, northern Europe, and North America have been with us ever since, though melting off occasionally whats called an interglaciation (were currently in one, which started about 10,000 years ago). There is a stately rhythm of ice advance and retreat, associated with changes in the earths axial tilt and its orbit around the sun.
The season of the earths closest approach to the sun varies (perihelion is currently in the first week of January); it drifts around the calendar, returning to January in 19,000 to 26,000 years, depending on where the other planets are. The configurations of the other planets approximately repeat about every 400,000 years, though they come close about every 100,000 years. Their gravitational pull causes the shape of the earths orbit to vary from near-circular to ellipsoidal (in July, were currently about 3 percent farther away from the sun, and so receive 7 percent less heat). Moreover, the tilt of the earths axis varies between 22.0° and 24.6°, a cycle taking 41,000 years (the last maximum tilt was 9,500 years ago; its currently 23.4° and declining). The three rhythms combine to contribute to a really major meltoff about every 100,000 years, typically when tilt is maximal and perihelion is also in June; that creates particularly hot summers in the high northern latitudes where most ice sheets are situated.
Ice core data of Dansgaard et al Nature 1993. Younger Dryas shown in red.
Note the two episodes during the warm period 130,000 years ago.
The Greenland ice cores go back only one-tenth of the 2.5 million years of the recent ice ages; only ice from the last 250,000 years remains in Greenland, because the antepenultimate meltoff exposed all the bedrock. But the cores do record the last two major meltoffs the one that began 130,000 years ago and the most recent one, which began 15,000 years ago and was complete about 8,000 years ago. Most importantly, one can see annual tree rings in the more recent millennia and count the years, sample their oxygen isotopes and thereby deduce the sea surface temperature at the time the water evaporated in the mid Atlantic before falling as snow in Greenland.
The paleoclimatologists now can see dozens of abrupt events in the last 130,000 years, superimposed upon the glacial slowness and even occurring during warm periods. Big glacial surges could be one factor, as I discuss in The Ascent of Mind simply because a lot of fresh water floating on the ocean surface before mixing can probably cause major changes in the ocean current that imports a lot of heat into the North Atlantic Ocean and helps keep Europe warm in the winter. Thats why I worry about a glacial surge producing an enormous freshwater reservoir in Greenlands fjords: it could all be released in a day when an ice dam is finally breached. The last time that I flew over that extensive fjord system on the east coast of Greenland at 70° north latitude, I was appalled to see fjords that looked like reservoirs though open to the ocean, they had the bathtub ring appearance of drawn-down reservoirs. There was an ice-free area extending above the present shoreline, and everywhere it appeared to be the same height. That suggests an enormous freshwater lake formed, sometime since the last ice age, that uniformly trimmed the ice sheet.
Another cold flip would be devastating to agriculture in Europe, and to the half billion people it supports and the effects of the Younger Dryas were seen worldwide, even in Australia and southern California. While another one would threaten civilizations, the cold flips of the past probably played an important role in evolving humans from our apelike ancestors, simply because they happened so rapidly.
Versatility is not always a virtue, and more of it is not always better. As frequent airline travelers know, passengers who have only carry-on bags can get all the available taxicabs, while those burdened by three suitcases await their checked luggage. On the other hand, if the weather is so unpredictable and extreme that everyone has to travel with clothing ranging from swimsuits to Arctic parkas, the jack-of-all-trades has an advantage over the master of one. And so it is with behavioral versatility that allows a species to instantly switch from square to round holes.
Versatility might well require a bigger brain. But you need some pretty good reasons to balance
out the disadvantages of a big brain. As the linguist Steven Pinker noted:
How fast things change is important for any incremental-accumulations model of intelligence,
whether it involves a bigger brain or merely a rearranged one. In any one climate, a specialist can
eventually evolve that outperforms the overburdened generalist; however, anatomical adaptations occur
much more slowly than the frequent climatic changes of the ice ages, making it hard for adaptations to
track the climate. Indeed, the abrupt transitions can occur within the lifetime of a single individual,
who either has the reserve abilities needed to survive the crunch, or doesnt.|
This sudden-death-overtime argument applies to many omnivores, not just to our ancestors. But there arent any other examples around of fourfold brain enlargements in the last several million years, so an erratic climate by itself isnt a sure-fire way of getting a swelled head. Something else was also going on, and the abrupt climate change probably exaggerated its importance, and kept those lean-mean-machine competitors from outcompeting the jack-of-all-trades types that evolved.
Everyone has a favorite theory for what this something else was. (Nick Humphrey would pick social intelligence as the driver, for example). My candidate is accurate throwing for hunting, handy for getting through the winter via eating animals that eat grass. But most people would pick language. Especially syntax.
You are reading HOW BRAINS THINK.|
The paperback US edition