One of the central issues that philosophers have been debating over the centuries is the nature of human intelligence. We’re different from the other animals, but why? Do we have a soul? Is there some sort of duality between the corporeal flesh and the mind or spirit? Did we acquire our unique capacity for rational thought and all that goes with it as a result of some special act of creation or did it just happen as a result of evolution through natural selection?
The Rev. Paley once cited the human eye as evidence of a Creator. How could such a complex organ have evolved by itself? Dawkins and others have refuted that argument by citing the enormous time spans involved, the selective advantages conferred by even small incremental improvements in such a sensory organ, and the fact that eyes have evolved separately in various different forms of life. But what about human intelligence? Can we adduce a similar set of arguments to support an evolutionary explanation for the emergence of this singular phenomenon? So far, noone has, at least not as convincingly as the scientists have for most other life forms, and for human organs like the eye. The human mind, defined as the human brain at work, is still largely a mystery, despite much investigation and research. For the creationists, it is their last bastion, the ultimate defence of their faith in a divine creator. For the evolutionists, it is the ultimate challenge.
How close is the scientific community to achieving a coherent explanation of human intelligence? Not very. We still need to know a great deal more about how the mind works, and about the physical evolution of our remote ancestors. Nevertheless, modern science has pulled together a lot more relevant information than most of us realize. We have come halfway toward assembling the knowledge we need, and can now construct a theory that is anchored in verifiable evidence. To be sure, some of the anchor points are far apart, and the gaps can still only be filled in with speculation. Nevertheless, the overall picture that emerges is both plausible and humbling. It is not a picture of a species that was predestined to greatness; rather, it suggests that our ancestors did the right things for the wrong reasons, and lucked out.
Overview: What Happened?
Our ancestors were already social animals several million years ago, back in the early Pleistocene. Their cognitive capabilities were probably about equal to those of modern chimpanzees. These capabilities were tested during a period of unusually rapid climate shifts, that frequently imposed a selective advantage on those individuals and groups which could best adjust their behavior by learning from experience. Gradually the brains of our hominid ancestors increased in size and versatility. Lucy’s descendants segued through several intermediate stages into homo ergaster and homo erectus. Eventually, anatomically modern humans appeared on the scene, probably in Africa, possibly about 130,000 years ago as some of the DNA evidence suggests. These “Out of Africa” humans spread far and wide, and may or may not have interbred with more archaic populations they met on the way.
About 30,000 years ago there was a kind of cultural explosion and we find the first extensive evidence of humans that not only looked like us but thought like us. Cave paintings, complex tools, carved figures, and burial sites pointed to the evolution of a human mindset that was based as much on symbolic representation as on more basic kinds of intelligence. The uniquely human attribute of culture took over, and launched humankind on a new trajectory.
For the serious student of evolution, this scenario raises major questions which have not yet been fully answered. Life has, after all, been going on on our planet for a billion years. Why us, and why at this time? What were the obstacles that prevented life with our level of intelligence from evolving a long time ago? What combination of circumstances made it possible for an evolutionary process to produce, first, big-brained anatomically modern people, and, 100,000 years later, people that used their brains as we do?
What is intelligence?
Intelligence is what an animal’s brain does when it guides its behavior. The brain itself can be analyzed physically in terms of which parts do what, and historically in terms of how far back in the evolutionary tree of life the various parts and functions first appeared. Similarly, there are many different kinds of behavior, and they vary with the animal concerned, as well as appearing at different times in evolutionary history. There’s a lot of discussion and argument these days as to how to describe these elements of both brain and behavior, and if there is a clear consensus, I haven’t been able to find it. I shall, therefore, select those concepts that seem to me to provide the most useful tools for this attempt to explain how and why human intelligence evolved.
The most basic and ancient form of animal intelligence is instinct, which produces innate behavior. It’s not something the animal learns, because it’s inherited through the genes. The animal’s mental machinery is constructed in ways that tell the animal how to perform functions like walking or flying, and what to do when some kinds of situations happen (touch a hot stove, jerk back). Virtually all animals have it, including humans.
Many animals can also guide their behavior through a learning process that takes place after birth. (Once you’ve touched that hot stove, you try not to touch it again). Learned behavior gets superimposed on innate behavior, like when a dog gets housebroken, and substantially increases the animal’s ability to adapt to special circumstances.
There are different kinds of learned behavior. Social learning is when the animal learns from observing the behavior of other members of its own species or group. Mama deer tenses when she smells a hunter, and Bambi follows suit. Social learning supplements behavior learned directly, allowing for an accumulation of “what works” instructions over generations. It can only happen if the species is of a social nature, at a minimum involving postpartum bonding between mother and offspring.
A phenomenon often called “imprinting” occurs early in life for animals capable of social learning. The hatchling duckling bonds to the first mama-sized moving object it sees. The village priest instills his faith in the small child. (But I am getting ahead of myself).
Please note that all these forms of intelligence are interrelated and work with each other. It is essentially the mix or proportion of these different qualities that distinguish the behaviors of different species of animals from each other. Humans have them all, in substantial measure, plus a crucially important additional learning capacity which the professionals call “imitation.” More on that later.
The First Hurdle: Big Brains:
Why did our relatively small-brained ancestors start on a trajectory toward ever-larger brains in the first place? The evolutionist knows, from a large number of studies and from verifiable experiments, that brains are expensive. They consume resources that might otherwise be used for physical survival and reproduction. The bigger the brain (proportional to the size of the animal) the greater the cost. Therefore, big brains can only be expected to evolve when they confer some significant advantage in the context of the immediate environment. There must have been powerful factors at work to keep brains evolving uphill, so to speak, against such a steep gradient.
The most plausible explanation I know lies in a correlation between increases in the brain sizes of mammals generally, and climatic fluctuations. The climatic history as we now understand it is one of recurrent climatic change that has become gradually more intense during the last tens of millions of years, and has climaxed in an extraordinarily “noisy” period during the Pleistocene (roughly the last 2.5 million years). Particularly during the Pleistocene, there were abrupt, intense climatic changes throughout many extended periods. Tropical as well as higher latitudes were affected by swings in temperature and rainfall, causing recurrent floods, droughts, windstorms and the like. By comparison, the climate during the last 10,000 years (the Holocene) has been relatively stable.
For roughly the same period of the last tens of millions of years, there has been a general trend towards increased size of mammalian brains (as a proportion to total body mass). The brains of some animals haven’t increased, while others have; there is an increase in the spread between the bigger and the smaller. The rate of increase in mammalian encephalization peaked during the Pleistocene, when it was many times greater than the previous average.
This correlation between increasing variability in climate and increasing mammalian brain size supports the common sense impression that learned behavior confers a selective advantage when the environment is changing rapidly enough so there isn’t time for the animal’s genetic material to adjust through the classic evolutionary procedure of variation and natural selection.
What was actually going on during that long and literally tempestuous incubation period that witnessed the emergence of anatomically modern humans? We can begin several hundreds of thousands of years ago, with a thinly scattered population of archaic humans, big-brained by comparison with their own ancestors, but not yet physically the same as modern people. Their capacity for thinking in symbols must have been rudimentary, compared to ours; language must have played a much smaller role, if any, than later on. For one thing, they may not have had fully evolved speech organs. They probably acquired the skills they needed to survive mainly through learned behavior, including social learning. These are forms of intelligence we can observe even now in chimps, dolphins, and a few other species. A young man would learn to knap flint or start a fire through watching an elder, and then doing the same. It would not have been possible at that time for the two men to sit around the camp fire while the older one described the process to the younger one, using words instead of physical example. Likewise with a girl, learning from watching older women what to gather by way of edible roots and berries.
During times of extreme environmental challenges, this kind of society would favor the survival of those individuals who had the sharpest cognitive skills then available in the human repertoire. People who were capable of putting their varied experiences together to find new solutions to current problems would be at a premium. So would those individuals who were most adept at maintaining group equilibrium in times of crisis. Assume, for example, a shift within a decade or two from a warmer to a much colder climate. You don’t have to have a written manual to teach you how to keep warm, but you do need someone who can figure out a new use for all those animal skins lying around. And once you’ve started preparing these skins and tailoring them, social learning kicks in, and within a very few generations, you have a group of proto-people well equipped clothing-wise to withstand the rigors of the new environment.
The emergence of the anatomically human brain can thus be seen as the logical product of an evolutionary process. We don’t know whether it happened more than once, over a period of generations, or whether there was something like a Garden of Eden someplace in Africa where the modern brain and speech organs were born just once, and then spread out. It doesn’t really matter as far as our central argument is concerned. The fact is that a modern brain did evolve, along with speech organs that together created the phenomenon of the anatomically modern human being.
There’s an important point here, crucial to our argument: these new physical features did not evolve because they enabled people to talk and think the way we do. They evolved because, and only because, they enabled people to do the jobs at hand a little better than the brains that had gone before. The latent possibilities of language and abstract thought were just that at first, latent, and it took about a hundred thousand more years, with a lot of interbreeding and strife and commotion along the way, before those new and uniquely important potentials were realized.
The Catch 22 Problem:
As we have noted, brains are expensive.The human brain is significantly larger, proportional to body size, than those of our remote ancestors, including homo erectus and homo ergaster. Bigger brains consume more of the body’s total resources, leaving less for the basic purposes of surviving and reproducing. They also, in the case of our ancestors, produced at least one additional disadvantage: The skull of the infant became too big to pass through the mother’s pelvis, unless birth came at an earlier stage in the development of the offspring. A chimp baby is a lot closer in time to being a viable adult when it is born than its human equivalent. It leaves the nest in a couple of years, after weaning, but its human cousin requires not only nursing and coddling for a comparable period, but care and feeding and protection for years thereafter. So we have two major items on the debit side of the ledger: all that blood carrying food and oxygen to a big brain, resources that could otherwise stoke muscles and glands more directly engaged in surviving and reproducing; and all that expense of childrearing, with the vulnerability to predation and other mortal problems that go with it. What lies on the credit side in this evolutionary equation? There must be something or we wouldn’t be here.
That something can only have been the increasing ability to cope with environmental challenges. The new, fully modern human brain didn’t have to think the same way we do to have significant cognitive advantages over the archaic ones. There must have been immediate payoffs, before the breakthrough to fully symbolic thinking, or our particular line of humans would have been bred out by natural selection long before 30,000 years ago. It is interesting to speculate what those advantages might have been.
The fossil record doesn’t do us much good here, but we can infer that the nuisance of protracted childrearing had a collateral benefit in causing a greater investment by the male parent and a rapidly evolving sense of family as a subunit of a kin-related tribe. A more complex division of labor within the tribe, a new capacity to organize several tribes into cooperative enterprises like mammoth hunts; these and other facets of exploiting the possibilities of the new neural circuits and speech organs all must have grown organically and synergistically during that long cold winter before our ancestors developed their capacity for symbolic thinking and language as we know it.
Still, it was a dicy period, with a fancy new investment with heavy carrying costs persisting for thousands of generations before it finally paid off, big time. There may well have been periods where the flame flickered and almost went out. Perhaps the fact that our kind of intelligence has only happened once during the very long period life has existed on our planet can be explained in this way–it was a very long chance, and our ancestors succeeded against the odds.
The Second Hurdle: Symbolic Thought
The scholars are reasonably agreed that perhaps 30,000 years ago human cognitive abilities took a massive leap forward. The evidence is the appearance about that time of cave paintings, burial sites, new tool technologies, and other evidence suggesting that people then had begun thinking about the same way we do now.
Although there’s plenty of evidence telling us this transition did happen, and where and when it happened, there is virtually no evidence as to why or how it happened. For one thing, thoughts don’t fossilize very well. I must therefore rely on informed speculation.
We have to pause here, and expand our previous classification of various types of behavior. It’s time to discuss the variation of learned behavior that is technically known as imitation. This is not something one learns by observing someone else’s physical behavior. It’s something someone learns because someone else told him about it. Or because he heard it on the radio, or saw a certain pattern in a dress on sale in the bazaar. In other words, imitation involves acquisition of knowledge about a pattern or technique, or some symbolic representation, rather than about something happening in the physical world. It involves the acquisition, retention, and onward transmission of mental constructs, or “memes”.
Contrary to conventional wisdom, species other than our own are remarkably deficient in any capability for true imitation. There are a few exceptions but they are mostly trivial. I’m told that in one experiment, a monkey learned to use a stick to retrieve apples that fell just outside its cage. Even though the other monkeys could see what was going on, none of them could replicate his techniques and get the much prized apples for themselves. So much for “monkey see, monkey do.” In this case, common usage is almost the opposite of the technical definition.
Imitation, in the technical sense, is primarily a human form of learned behavior. A capacity for it must have preceded and laid the groundwork for language and symbolic representation in the development of human intelligence. We postulate that it began as a byproduct of social relations within tribal groups facing severe environmental challenges. Selective advantage would have been conferred on individuals with talents that facilitated both better solutions to problems of survival, and social equilibrium within the group. Whatever the exact process, a nascent capacity for thinking conceptually could have been sufficiently useful to offset the biological disadvantage of the big brain it required. Learning by imitation became a major stepping stone to symbolic representation, and particularly language, as media through which the cognitive capabilities of our ancestors could be vastly expanded.
The development of language and other forms of symbolic representation probably culminated at the time of the Mesolithic breakthrough, some 30,000 years ago, in a process akin to the feedback that occurs in a poorly designed sound system.  The result was what we now recognize as human culture. This extraordinary phenomenon that we now take so much for granted changed everything for human society, and eventually most of the rest of the planet. (See my earlier essay, “What Am I?”).
Culture proved extraordinarily effective, not only in helping groups survive environmental rigors, but as a means of regulating interpersonal relations within groups. Equally important, it eventually allowed for the enlargement of tribal-sized groups into much larger societies.Think of the religious beliefs, the moral and ethical structures, the traditions of friendship or hostility between different tribal groups, the material technology, and much more of what we regard as distinctively human behavior. The infant and young child picks up some of this through direct observation, but as the child grows older, more and more of it is acquired through imitation. Until the age of three, the chimp baby is not very far behind the human, but after that age the chimp slows down while the human’s learning curve really takes off. Higher education, and all that goes with it are all simply advanced forms of learning through imitation.
We humans have done a lot in a brief moment of geologic time to render the whole evolutionary process enormously more complicated. We are a new kind of social animal, thanks to culture. Culture has been the thread that has held human societies together, with social learning the glue. Learned behavior by contrast, increasingly assisted by imitation (of extra-cultural memes) has been the yeast that has kept us innovative and adaptive.
This analysis may raise more questions than it answers, but it does provide a possible explanation of how, when, and why human intelligence started. In my opinion the theory is plausible enough to put the burden of proof on the creationists and the Cartesian dualists to prove that human intelligence remains beyond the ken of science, and could only have come about at the intervention of some higher force. I believe no such explanation is needed. We got here on our own. And it wasn’t inevitable. We were just damned lucky.
Carl Coon, 1/23/01
1/ Available data allow us to chart past climatic variation back over over very long periods. Unsurprisingly, the data for more recent periods are more precise than for earlier ones. For the past 90,000 years, we can fine-tune our data down to intervals of a decade. See “Climate, Culture and the Evolution of Cognition” by Peter J. Richerson of UC/Davis, and Robert Boyd of UCLA. The paper has been published in The Evolution of Cognition, ed, Celia Heyes and Ludwig Huber, MIT, 2000.
2/ We may well be witnessing a similar phenomenon in our own lifetimes: the explosion in human access to information driven by the computer and carried by the internet.
I am indebted to Dr. Peter J. Richerson of UC/Davis for comment and counsel on an earlier version of this essay, as well as for the climatological argument (see footnote #1).However, the speculation is my own; don’t blame it on him.
I have also drawn on Dr. Susan Blackmore’s recent book, The Meme Machine, for much of the theoretical discussion of the phenomenon of imitation.