Homo Mysterious: Evolutionary Puzzles of Human Nature (43 page)

This question has received considerable attention, from some very clever people, and yet, the answers remain elusive.

Even more elusive, it seems, is the mystery of proximate causation. Thus far we have pretty much ignored these “how” questions, and
although this chapter will continue in this vein, it is worth noting that when it comes to scientific mysteries, there may be none more daunting than
how
—in proximate terms—we achieve intelligence, consciousness, and the range of subjective perceptions and sensations that constitute every person’s innermost life. There seems an almost unbridgeable gap between physiological, anatomic, electrochemical events such as packets of neurochemicals and waves of ion-based depolarization passing along nerve cell membranes—which biologists are beginning to unravel and understand in astonishing detail—and those innermost sensations that all of us experience.

Connecting the “stuff” of the physical world with that of our subjective consciousness has long been the third rail of biology. Touch it and maybe you won’t die, but you are unlikely to get tenure! It helps, of course, if you are a Nobel laureate, such as Francis Crick or Gerald Edelman, but until recently it appeared that even their attempts to pin down the electrical-chemical-anatomical (or whatever) substrate of mental phenomena would go the way of Einstein’s doomed search for a unified theory of everything. This may yet be the case, but the situation has nonetheless changed dramatically of late, such that inquiry into the neurobiology of mental experience has become one of the hottest, best funded, and most media attracting of research enterprises, along with genomics, stem cells, and a few other newly favored subdisciplines.

For literally centuries, it was perfectly acceptable for philosophers to ponder consciousness, because after all, no one really expected them to come up with anything real. Descartes’ renowned
cogito ergo sum
(“I think, therefore I am”), for example, was modified thusly by Ambrose Bierce:
cogito cogito, ergo cogito sum
(“I think I think, therefore I think I am”), to which Bierce added that this was about as close to truth as philosophy is likely to get! Bierce, once again, noted that the chief activity of mind “consists in the endeavor to ascertain its own nature, the futility of the attempt being due to the fact that it has nothing but itself to know itself with.”

But now we have microelectrodes recording from individual neurons, computer modeling of neural nets, functional MRIs, and an array of even newer 21st-century techniques, all hot on the trail
of how mental processes emerge from “mere” matter.
ⁱ
Cartesian dualism is on the run, as well it should be.

Admittedly, there are some exceptions, proving that imbecility runs deep, especially in the curious world of the consciousness credulous. Take the remarkable popularity of the charlatan cinema “What the Bleep Do We Know?” with its faux scientific assertion that consciousness is an active force by which we can impact the world, not to mention showcasing such ludicrous—and persistently unreplicated—claims as this: Water supposedly forms different kinds of crystals as a result of being exposed to “fields of consciousness” embodied in written messages such as “You’re a fool” (no crystals or ugly ones) as compared to “I love you” (beautiful, heart-warming symmetrical delights). With such friends, the serious study of mental events hardly needs enemies.

This chapter, however, shall seek neither to bury nor to praise neurobiology, but to point instead to another side of bona fide inquiry that has received all too little attention, even as neurobiology has advanced. I refer to the question of why our higher mental processes exist at all. Accordingly, let’s grant a “how” to thought, intelligence, consciousness and those various perceptual events known as “qualia” (our experience of “red,” “cold” or “love,” for example) and agree that somehow or other, energy and matter come together and produce them, via electrochemical and anatomical events, some of which we understand and others yet unknown. And let’s get back to the “why.”

It is quite possible, after all, to imagine a world inhabited by highly competent zombies, who go about their days responding appropriately to stimuli—basking, perhaps, in the warm sun, obtaining suitable nutrients at opportune times, even repairing themselves
and producing offspring—but lacking intelligence or any inner mental life whatsoever.

There is an old and not terribly funny joke—of the type known generically as a “shaggy-dog story”—that involved a “potfer.” After several minutes of lengthy and irrelevant narration, the joke’s victim is led to ask, “What’s a potfer?” whereupon the joke teller triumphantly announces the punch line: “Cooking.” So, what’s a brainfer? Most people would answer “Thinking.” Or maybe “feeling.” Or “controlling one’s body.” Most evolutionary biologists, however, are likely to disagree with all of these, pointing out that the adaptive significance of brains is both more basic and more multidimensional and complex: promoting the fitness of bodies within which they reside or, more precisely, the fitness of those genes that are responsible for producing the brains in question.

Brains may or may not be good at making sense of the world, or thinking great thoughts, or providing vivid subjective experiences to their possessors, or adroitly controlling their bodies. It is even possible, one can imagine, to be too brainy for one’s own good, which brings up another story, this one told by the landscape architect Ian McHarg: It was the aftermath of World War III and our planet had been reduced to radioactive cinders. In the deepest recesses of the ocean, the few exiguous survivors—a motley group of primitive, amoeboid creatures—have just decided they are going to try once again, but before they separated, ready to initiate, once more, that old evolutionary process, they take a solemn vow: “This time, no brains!”

Brains, in short, can be a problem. For evolutionary biologists, they definitely are. The question is, “Why did our brains become so large, so quickly?” which often boils down to “How do/did they contribute to fitness?” The answers have not been easy to obtain. Or rather, they have been too forthcoming. Just as Mark Twain once pointed out that it was easy to stop smoking—he had done it hundreds of times!—it is easy to identify the adaptive significance of the extraordinarily large human brain: It has been done dozens of times.

As we’ll see, there are hypotheses suggesting that human braininess is a result of selection for tool use, tool making, cooperative hunting, defense against predators, defense against other protohumans, and so forth, not to mention the suggestion by Alfred
Russell Wallace that the elaborate functional complexity of our cerebrum, and especially its remarkable cognitive and artistic capacities, must be due to something supernatural (intelligent design for intelligence itself). There is also Stephen Jay Gould’s patently absurd suggestion that our mental capacities must be a random, nonadaptive happenstance, specifically a result of “surplus” brain tissue.

Why is this absurd? For the same reason that evolutionary biologists agree that high intelligence and large brains need to be explained at all, in contrast to the assumption of most people that intelligence is always a good thing and therefore needn’t be “explained.” It is possible for natural processes to accumulate large quantities of stuff, like gravel at the base of a glacier, or fresh water flowing down a river, but not if the process is governed by natural selection, in which benefits must exceed costs. And a brain is very, very costly. Its 100 billion nerve cells are highly nonrandom, hooked together via perhaps 100 trillion carefully orchestrated connections. Such a device is devilishly difficult to encode, requiring more than its fair share of precious DNA. Moreover, even after it is constructed, the human brain is extraordinarily expensive to maintain.
¹
It uses up an inordinate amount of metabolic energy: Although it occupies only about 2% of the body’s weight, it accounts for roughly 20% of our total metabolism, compared to 10% or so for most mammals, including chimpanzees. If brain tissue has ever been surplus, it would long ago have been selected against, or turned into fat, not mental athleticism. (And although some people can be accused of being “fat-headed,” this isn’t the usual implication.)

For an animal like ourselves, a product of natural selection like all other living things, to have evolved a brain like this, we must have needed it very, very badly. But for what?

There is little doubt that brain size is importantly linked to intelligence as well as to other higher mental faculties. In 1935, J. A. Hamilton conducted laboratory-based breeding experiments, selecting for maze-bright and maze-dull rats. By the 12th generation of such artificial selection, the maze-bright and maze-dull strains had brain weights that differed by 2.5 standard deviations. This experiment is a sort of accelerated test of evolutionary change, showing a dramatic association between brain size and cognitive ability.
²

On the other hand, although brain size generally correlates with intelligence, the pattern is not invariant. Albert Einstein, for example, had an unusually small brain, measured by volume … but not by output. And Neanderthals had larger brains than Cro-Magnon
Homo sapiens
, who eventually replaced them, perhaps via direct head-to-head (brain-to-brain?) competition. But Neanderthal brains were also constructed somewhat differently, with relatively fewer neurons in the frontal and prefrontal lobes, which is where higher intellectual pursuits evidently reside.

Primates are smaller bodied than many other mammalian groups, but even little ones such as squirrel monkeys are typically smarter than their big grazing cousins such as antelopes or giraffes. As a result, biologists interested in comparing the intellectual anatomy of different species have been inclined to employ, among various measures, one that reflects relative brain size and is known as the “encephalization quotient,” defined as the ratio between actual brain mass and predicted brain mass for an animal of a given size. The encephalization quotient for reptiles is roughly 0.05; for birds, 0.75; for chimps, around 2.3; for gorillas, 1.6; and for people, 7.5. Hominid encephalization began increasing roughly about 1 million years ago, peaking roughly 35,000 to 20,000 years before the present. The modern human brain evolved not only in size, however, but also in complexity, such that it is fully three times the size of the chimpanzee brain, but has only a 25% advantage in the number of neurons. On the other hand, human brains have much higher numbers of synapses and interconnecting branches, such that it is often said that our brains are the most complex things in the universe (although in all fairness, we probably should consider who—or what—is telling us this “fact”).

The picture is genuinely complex, almost as much as thought itself, but it is nonetheless clear that (1) within certain limits, there is a correlation between brain mass and overall intelligence and (2) both these traits increased quite dramatically in the course of human evolution. It is also clear that for this to have happened, smarter individuals must have somehow experienced a fitness advantage over those who were less intellectually endowed. Altogether unclear, however, is the basis for this advantage, although there are many contenders.

We can start with some of the obvious ones. For a long time, it was believed that the use of tools was a unique human specialty, to the extent that the Latin term
Homo faber
(“man the maker” or “builder”) had seriously been proposed. Moreover, it is easy to imagine that our ancestors’ unusual skills in this regard could well have set up a kind of self-catalyzing positive feedback, in which those early proto-humans who were better able to employ tools—and who gained a distinct survival and reproductive advantage as a result—were likely smarter than their less handy fellows. The result was a neat conceptual scheme that explained the rapid evolution of human brains and intelligence.

Then Jane Goodall discovered that chimpanzees use sticks to “fish” for termites, which they avidly consume. Her mentor, Louis Leakey, was ecstatic at the news, responding that “Now we must redefine ‘tool,’ redefine ‘man’ or accept chimpanzees as humans.” Those struggling to retain the status of human uniqueness without engaging any of Leakey’s redefinitions quickly adopted a fall-back position: Human beings were unique in
making
tools, not just using them. But then Goodall pointed out that prior to their tool-assisted fishing expeditions, chimps run their fingers along suitable sticks, removing leaves and twigs, thereby fashioning a bare implement all the better to insert into termite mounds. So much for the uniqueness of tool making. (Perhaps one benefit of recurring efforts to define human uniqueness is that they spur field primatologists to keep disproving the latest definitions!)

Even if neither tool use nor tool making is unique to
Homo sapiens
, there is no question that human beings are extraordinary among animals in the extent to which they create and employ tools, leading to the prospect that adroitness with tools constituted an important selective agent for increased intelligence and brain size. After all, we are physically modest creatures, lacking impressive canine teeth or claws, unable to fly or burrow rapidly underground, not especially fleet of foot, vulnerable to being killed and eaten by a range of predators, and not terribly well equipped to capture or kill other animals. It seems likely that intelligence helped make up for our numerous physical deficits: Those among our ancient primate ancestors who were smart enough to fashion
and use tools—defined broadly to include weapons, digging implements, carrying devices for small vegetables or invertebrates, animal hides as primitive clothing, and so forth—would likely have experienced a distinct reproductive advantage. The result would be selection for intelligence and thus big brains.