Shadows of Forgotten Ancestors (50 page)

THE ARCHIMEDES OF THE MACAQUES

W
e humans have not evolved from any of the two hundred other primate species alive today; rather, we and they have evolved together from a succession of common ancestors. As we reconstruct the primate family tree, we discover who our closest relatives are. The behavior of the primates varies so widely, even between species in the same genus, that it really does make a difference for our view of ourselves which ones are our nearest relatives.

The answer, as we’ve already described, seems to be that the chimps are our closest kin, sharing some 99.6% of our active genes. We know from DNA sequencing, as you would of course suspect, that bonobos and ordinary chimps are a lot more like each other than either of them are like us.
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But 99.6% is very close. We must share many characteristics of both. (Indeed, there must be behavioral traits that we share with our most
distant
primate cousins.)

By using molecular and anatomical evidence, together with the record in the rocks, the entire family tree of primates can be drawn, at least approximately, and a timeline placed upon it. The evidence from the bones and from the molecules are not in perfect accord, although they are beginning to converge; in this book, we have given weight to gene sequencing and DNA hybridization data. According to the molecular evidence, gorillas branched off from the evolutionary line leading to us about 8 million years ago; the still unidentified, now-extinct common ancestor of humans and chimpanzees separated from the gorillas maybe a million years later. Very quickly thereafter, the lines to chimps and humans began evolving toward their separate destinies.
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On a planet that’s been inhabited a thousand times longer, that’s pretty recently, as recent as the last two weeks in the life of a fifty-year-old human. This doesn’t mean that humans and chimps
themselves
began 6 million years ago; only that our common twig in the evolutionary tree branched out then.

To understand a little more about our primate nature and its development, let’s cast our minds back toward the end of the Mesozoic Age, around 100 million years ago. That would be about a year ago in the life of a middle-aged person. There were mammals even then; but they were not easy to find. The daytime was ruled by the dinosaurs; among them, some of the most fearsome killing machines ever to evolve on land. Our mammalian ancestors, it is thought, were timid, weak, and small; they were in fact typically the size of mice. Like all reptiles and amphibians today, some of the dinosaurs may (this is still a controversial point) have been cold-blooded; if so, in the chill of the evening, especially in winter, they closed up shop—especially the smaller ones that both preyed on mouse-sized mammals and were more vulnerable to the cold. But the mammals themselves were warm-blooded, and so could stay out all night.

Imagine a moonlit darkness in which their adversaries lay senseless, strewn across the landscape in stupors of sleep. This was the chance for our ancestors to scamper about their humble business—catching grubs, nibbling leaves, mating, caring for the young. But to function well in the dark, they had to be very good at using senses other than sight; and in that epoch the mammalian brain evolved along with elaborate machinery for enhanced hearing and smell, their hedge against whatever dinosaurs hunted at night.  .

Asleep in burrows during the day, our ancestors perhaps tossed fitfully, dreaming daymares filled with row after row of needle-like teeth and nimble, hair-raising scampers to safety. They may have been frightened all their lives, their hearts in their throats at every daylit step, longing for nightfall.

Sixty-five million years ago, a bolt from the blue—the impact of a small world—seems to have cataclysmically altered the planetary environment, wiping out the dinosaurs and permitting the mammals, wholly insignificant until then, to flourish and diversify. We do not know if there were primates so early, or if some other mammal quickly evolved into the first primate. We do know from fossil evidence that tiny monkey-like beings, weighing perhaps a few ounces, with teeth about a millimeter long, lived in what is today Algeria just after the extinction of the dinosaurs.
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By 50 million years ago (six months ago in the life of our fifty-year-old) there were arboreal primates living in subtropical Wyoming.
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The canine teeth of the males were twice as
long as the females’. If we can judge by what this difference means in contemporary monkeys, the males bullied the females, established dominance hierarchies, competed with each other, and probably maintained harems. All that’s been with us since the beginning of the primate order.

The first primates are judged to have been much more like early mammals (with longer snout, eyes to the sides of the head, and claws) than are modern monkeys, apes, and humans. The so-called “lower” primates, or prosimians—lemurs or lorises, say—may be something like the earliest primates. You can see that they’re nocturnal at a glance: Their eyes are appealingly large for their faces, the larger aperture being an adaptation for night vision in a world illuminated only by the moon and the stars.

They probably communicated in part by spraying scents from specialized glands.
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They had brains—large for their body size—to think with, stereoscopic vision to see with, and hands to manipulate the environment. Typical primate dominance hierarchy rituals had probably already appeared, including both sexes presenting their rears as a gesture of submission to the dominant male.

The early evolution of primates was marked by a profound transformation of creatures of the night into habitués of daylight; a corresponding suppression of the sense of smell
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and elaboration of vision; developing facial muscles so moods could be communicated by expressions; a still more powerful bond between mother and child; a longer period of infantile dependence; and an improving ability of the newer, higher brain centers of the cerebral cortex to moderate aggression and other behavior patterns emanating from the older, lower layers. All this in turn led to major changes in primate society: The less aggression, the more a true communal life is possible; the longer the childhood, the more parents can teach their young. Alliances and support groups, reconciliation, reassurance, forgiveness, remembering
the past behavior of specific individuals, and planning future actions swiftly evolved. Our ancestors were by now well along a path toward greater alertness, intelligence, communications skills, love.

After the extinction of the dinosaurs, mammals moved out into the daylight. For a while, they must have felt safe and free. But the growing, multiplying, and diversifying mammals eventually became too good a meal to pass up. They began to eat each other. And new predators evolved, including birds of prey. The day shift became increasingly dangerous. For example, in a study of modern South American harpy eagles, 39% of the “prey items” returned to the nest turn out to be body parts of monkeys.
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In daylight you have to be on your toes. Mutual defense—scanning the skies, say, and air raid sirens when an eagle is spied—becomes vital.

Foraging baboons, faced with predators, typically respond by closing ranks and moving faster.
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Certain collective behavior that we readily describe as military constitutes an adaptive response of very ancient standing to the threat of predation. Competent predators can force the potential prey to evolve rapidly—toward binocular vision, arboreal acrobatics, mutual support, quickly disinhibited combat skills, intelligence, and general military virtues.

Monkeys are born with an ability to recognize the significance of various facial expressions—although just how to respond to such expressions depends on experience and training. There are single brain neurons that are preferentially triggered when the monkey sees the eyes or mouth or fur of another monkey. There is even a kind of brain cell specifically responsive to a crouching or bowing posture. Facial expressions and body posture have a meaning in the primates that’s hardwired, and not merely a matter of social convention. The male rhesus monkey’s come-hither look is to thrust out his chin and pucker his lips; if you’re a rhesus monkey (of either sex), it’s important, even early in your career, to know what this means.

One of the uses to which the evolving primate brain has been put is the storing up of grudges. Monkeys generally make up—often by ceremonially mounting each other—within minutes after a fight. Chimp males, with females frequently in a peacemaking role, may take hours or days. But among themselves the females are less forgiving; they may hold grudges for the rest of their lives. Humans of both sexes can take anywhere from moments to millennia. Even among monkeys, a
smoldering resentment against an individual is often broadened to encompass his or her relatives. Among the many new social forms invented by the primates are feuds and vendettas, sometimes extending over many generations—intimations of the beginnings of history.

As in most mammals, primate aggression, dominance, territoriality, and the sex drive are mediated by testosterone circulating in the blood, and generated mainly by the testicles. Almost certainly this was true of the earliest primates, and long before. The more testosterone and other androgens the developing fetal brain receives, the more of these masculine characteristics the animal will exhibit when he grows up. The lower the testosterone levels in a male, the more subdued will be these proclivities and the more likely that he will present himself for mounting by other males. But the testosterone levels also respond to the mantle of leadership. When presented with females in estrus and no high-ranking males around, the testosterone level of lower-ranking males soars. Within certain limits, primates rise to the occasion. The office makes the monkey.

Males of many primate species (although, on average, not humans) show a marked preference for female sexual partners who have already produced offspring; younger females may have to make special efforts to be alluring.
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We have described the vigilance with which chimp alpha males guard their females, but only during ovulation. Nevertheless, sex has evolved in primates into something much more than simply the means for the replication and recombination of DNA sequences. Year-round, virtually compulsive sex with many partners—described by human observers as “promiscuous,” “depraved,” “perverse,” and “indiscriminate”—is there for a reason. It serves as a mechanism of socialization. This is clearest among the bonobos. Despite sexual jealousy, it holds the group together. It provides bonds of affection, common goals, means of identification with others, and a gentling of dangerous aggression. The essence of primate living arrangements is a gregarious communal life, which partakes of many recognizable aspects of human culture and society. One of the chief motivations for this communal life is sex.

Adult role models are essential among animals in which childhood learning plays so central a role. Dominance hierarchies soften violence (but not aggression) within the group. Cooperation is important in any hunt, critical in hunting large animals, and sometimes essential
in evading predators. In a survey of thirty primate species in the wild, the probability that any given individual will be eaten by year’s end is found to be one chance in sixteen.
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Evading predators must be very high on the primate agenda—and communal life provides early warning and collective defense.