Homo Mysterious: Evolutionary Puzzles of Human Nature (44 page)

There is, however, a problem. Fossil evidence now shows quite conclusively that our ancestors began using tools
before
they evolved especially large brains, although this finding isn’t necessarily fatal to the hypothesized connection between tools and intelligence: Even if we started using tools as average-brained apes, once the “discovery” was made, the payoff associated with their manufacture and adroit use could still have generated a positive feedback loop that rapidly selected for high IQ.

A bigger problem, however, is that there are many other arenas wherein intelligence and evolutionary fitness seem likely to have been joined, making it difficult (perhaps impossible) to identify an evolutionary prime mover. Take language, which—like tool making—is often trumpeted as a defining human characteristic. As with tool making, there are animal examples that approach the human situation: for example, the famous “dance of the bees,” by which these insects convey remarkably accurate information about food sources; the use of predator-specific alarm calls by certain primates (different vocalizations for aerial predator, ground predator, and snakes); and the presence of distinct vocal dialects passed along via learning from adult birds to nestlings. Nonetheless, and again as with tool making, there seems little doubt that the difference between human language and that of nonhuman animals is so great that it may well be qualitatively discontinuous.
³
Symbolic thought——a key component of thought generally—depends crucially upon language, such that the two may not even be separable.

In any event, whether a difference of kind or merely of degree, the extraordinary elaboration of human language—with its complex syntax and use of symbolic verbal structures—could very well have conferred a huge reproductive advantage upon those early hominids able to make good use of it. Language may even be our most important “tool.” With language, we can plan ahead, explain the past, convey information, strategize with others, etc. It is easy to envisage scenarios by which those who had greater linguistic mastery also enjoyed greater evolutionary fitness. They would
have been able to share information and to learn more efficiently, as well as to teach, to plan and strategize, and to coordinate their actions with a level of specificity and precision that simply is unavailable to creatures essentially limited to playing charades. As with the invention, deployment, and modification of tools, once language skills began to emerge, their existence could well have generated a positive feedback loop: In the newly established linguistic environment, there would presumably have been yet more payoff to those able to employ language effectively.

Even so, the precise evolutionary course whereby language skills evolved, and we along with them, remains obscure:

To understand why humans are so intelligent, we need to understand how our ancestors remodeled the ape symbolic repertoire and enhanced it by inventing syntax. Wild chimpanzees use about three dozen different vocalizations to convey about three dozen different meanings. They may repeat a sound to intensify its meaning, but they don’t string together three sounds to add a new word to their vocabulary.

We humans also use about three dozen vocalizations, called phonemes. Yet only their combinations have content: we string together meaningless sounds to make meaningful words. No one has yet explained how our ancestors got over the hump of replacing “one sound/one meaning” with a sequential combinatorial system of meaningless phonemes, but it’s probably one of the most important advances that happened during ape-to-human evolution.
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Traditionally, when trying to identify the evolutionary basis of human intelligence, researchers have focused on such payoffs as enhanced success in obtaining food, avoiding or defeating predators, surviving despite a challenging environment, and so forth. But there is also a darker possibility.

Perhaps the most threatening and important environmental challenge faced by our ancestors was … other people. We are now and presumably have always been group-living creatures, and it is entirely possible that intergroup competition (primitive warfare, possibly including cannibalism) exerted powerful selective pressures during the evolution of prehuman hominids. It is
discouraging to contemplate the prospect that our crowning quality—our large brains and highly effective minds—may have evolved in the service of killing others of our own species or, at the least, keeping them from killing us.
⁵
But it is distressingly easy to posit ways in which this could have happened.

Other studies also point to a significant role for competition, although not necessarily revolving around primitive warfare as such. Thus, after examining 175 hominid craniums from 1.9 million to 10,000 years ago, one research team concluded that social competition—as inferred from population density—was key to the threefold increase in hominid brain size since
Homo habilis
.
⁶

Other routes to human braininess have also been proposed. An interesting one connects human brain evolution to fluctuations in the earth’s climate, and has been particularly championed by biologist/author William H. Calvin. In his book
A Brain for all Seasons
,
⁷
Calvin argues that we evolved intelligence as a way of coping with a rapidly changing physical environment. Earth’s climate began fluctuating significantly about 10 million years ago, one consequence of which appears to have been the Ice Ages, starting around 2.5 million years ago. Another result is that as the earth became drier and cooler, the tropical forests that had previously covered much of northern and eastern Africa began to retreat, transitioning first into savannah and grassland, and in some parts, to desert. Animals that had been adapted to arboreal life had to adapt; included among these were our ancestors, who evolved bipedalism, perhaps as a way of standing up among the grasses and thus freeing their hands for gathering food and for using weapons and other tools.

It is conceivable, as well, that a little-appreciated aspect of savannah life contributed in its own independent way to the evolution of human intelligence: Seeing potential prey as well as predators from a distance could have selected for an ability to anticipate events and plan ahead. Forest-dwelling creatures, whose encounters are more immediate and thus unplanned, did not likely experience such visually-based opportunities. At the same time, adopting a ground-dwelling lifestyle would probably have made our ancestors more vulnerable to predation by big cats, hyenas, and so forth, which in itself would have put an evolutionary premium on those early hominids best able to cope with these threats.
And if nothing else, intelligence would seem to fit the bill as a multipurpose “coping mechanism.”

Calvin argues that intelligence can be defined as effective adaptation to the environment, and that the early hominid environment required a whole lot of prompt adaptation:

Although Africa was cooling and drying as upright posture was becoming established 4 million years ago, brain size didn’t change much. The fourfold expansion of the hominid brain did not start until the ice ages began, 2.5 million years ago. Ice cores from Greenland show frequent abrupt cooling episodes superimposed on the more stately rhythms of ice advance and retreat. Whole forests disappeared within several decades because of drastic drops in temperature and rainfall. The warm rains returned with equal suddenness several centuries later. The evolution of anatomical adaptations in the hominids could not have kept pace with these abrupt climate changes, which would have occurred within the lifetime of single individuals. But these environmental fluctuations could have promoted the incremental accumulation of new mental abilities that conferred greater behavioral flexibility.

There is a growing consensus among atmospheric scientists that the earth experienced a series of dramatic, even catastrophic heating and cooling within the last few million years. The mechanism is complex, involving interruptions of the thermohaline circulation of the North Atlantic as a result of increased freshening of its water, which in turn can be a consequence of brief but intense periods of global warming. The pattern, then, would entail heating (presumably nonanthropogenic) followed by dramatic and possibly even catastrophic
cooling
. The argument goes that such “whiplash” climate cycles drove selection for large brains insofar as they conferred the ability to adapt to rapidly changing environments. If, for example, rapid cooling produced lots of grassland, this would have resulted in a relative abundance of large grazing animals, which, in turn, would have selected for the now-familiar “man the hunter” lifestyle.

In a very real sense, human culture—defined broadly, to include everything that we transmit nongenetically—is our most important biological adaptation, and pretty much whatever contributed to our ancestors’ ability to generate, employ, and transmit culture would have been selected for, with intelligence being a necessary
substrate. Culture is not necessarily opposed to biology; rather, it is humanity’s primary biological adaptation. The same may be true of intelligence. Moreover, it is a good bet that intelligence and culture are intimately related: Intelligence is a prerequisite for us to pick up whatever culture we are born into or migrate into. Hence, it may be highly significant that human beings are both the cultural creatures par excellence and the world’s most intelligent.

We may also be the only animals to have been substantially affected by group selection, a process that could well have been directly involved in the evolution of human intelligence. Thus, the payoffs associated particularly with enhanced communication and the teaching and learning of new skills could have enabled proto-humans to outcompete other groups lacking such skills and the benefits they provide. Closely allied to this is the reality of Lamarckian, nongenetic transfer of information, another payoff to intelligence, and one that could have rebounded not only to the benefit of individuals and genes but also to groups.

When it comes to asking “Why did human beings evolve to be so intelligent?” we are left, therefore, with an abundance of suspects. This isn’t necessarily a bad thing, since unlike investigators in a criminal case, those of us trying to solve the mystery of human intelligence are under no pressure to pin the rap on a single perpetrator. Maybe there were many.

Here are some other suspects.

When looking for the adaptive value of intelligence, researchers traditionally think in terms of its contribution to survival (hence hypotheses based on tools, language, environmental adaptation, warlike competition among groups, etc.). It is also possible, however, that much of our vaunted intellect evolved as a result of sexual selection. Earlier, we considered the suggestion—especially associated with evolutionary psychologist Geoffrey Miller—that the human capacity for and appreciation of the arts is attributable to mate choice, specifically the interaction between showing off and choosing the best possible mates.

Miller’s hypothesis was not developed with an eye toward explaining the arts, but rather as a way of solving the mystery of human intelligence more generally. The idea is that since brains are expensive, any traits—such as artistic or linguistic skills—that require big brains and hence intelligence would have been selected for as fitness indicators. The result would therefore have been more intelligent descendents sporting bigger brains, not so much because of any practical, survival-related benefit of intelligence as such, but because their ancestors had impressed members of the opposite sex with their overall genetic quality, perhaps including disease and parasite resistance. In the process, badges of physical fitness would have morphed into traits conferring evolutionary fitness as well, through the mediation of sexual choice. It is possible, as well, that studies measuring heritability of intelligence are at least in part measuring heritability of disease resistance instead.
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If theorists such as Miller are correct, then intelligence was favored not so much because of its intrinsic merits, but because it offers a relatively unfakeable and thus honest signal of biological sturdiness
⁹
—a bit of a takedown for those of us who value intelligence as an end in itself.

In this regard, it is worth noting that taxonomic groupings that contain large numbers of especially smart animals such as cephalopods, elephants, and apes typically aren’t notably abundant or ecologically successful in other respects. Maybe intelligence isn’t all that it is cracked up to be. As we have already seen—and as the sexual selection hypothesis requires—brains are very costly. Since larger brains are found only in creatures with larger bodies, perhaps those larger brained (hence, more intelligent) animals such as ourselves are successful because of our body size plus our opposable thumbs rather than our brainpower. After all, dinosaurs were successful for tens of millions of years—far longer than
Homo sapiens
have yet prospered—and their brains were notoriously unimpressive.

Maybe the evaluative deck is stacked in favor of those people who already value intelligence, perhaps more than it deserves. Thus,
Homo sapiens
who concern themselves with the adaptive value of big brains and heady intellection are especially likely to be those in universities, research institutes, or other avowedly intellectual communities, including the “intelligent reading public”
so beloved of book publishers such as Oxford University Press, and who are therefore likely to be strongly represented among those reading this book. But what if intelligence isn’t the immense asset that most of us in these communities like to think?

Here, accordingly, is some food for thought: At present, high intelligence correlates with
decreased
reproductive success.
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This may or may not have implications for human beings of the future, but nonetheless, it probably doesn’t detract from the generalization that among people in the past, intelligence and evolutionary fitness were evidently closely and positively linked for a million or more years, during which hominids were evolving such large brains, and becoming us.