Shadows of Forgotten Ancestors

by Carl Sagan

  Males in these species are often strongly attracted to infants. Immediately after birth, they have been observed trying to sniff, touch, or hold the still-bloody newborn, and they sometimes even lick off the covering birth fluids … Within hours of birth, males carry infants on their backs, groom them, and protect them … Large portions of a male’s day are devoted to infant care, and the most devoted fathers return their infants to the mother only to suckle …

  Males also permit infants to take food from their hands and mouths … The food items shared are those that infants have difficulty obtaining or processing themselves, such as large mobile insects or hard-shelled fruit …

  Fiercely protective, males will defend infants against any real or imagined threat. In captivity, tiny lion tamarin males have flung themselves against intruders as intimidating as woolly monkeys, macaques, and humans.38

  * Apes are bigger and smarter than monkeys, and lack tails. The apes are the chimpanzees, gorillas, gibbons, siamangs, and orangutans. The siamangs are about as closely related to gibbons as chimps are to humans.

  * The fact that in every human ethnic group and culture males have been on average larger than females has not escaped the notice of primatologists. It may have something to do with the penchant of men for sexism, coercion of women, rape, and harems when they can get away with it. The key question is to what extent anatomy is destiny, a point to which we will return.

  * Something similar happened when a number of fugitive Englishmen, without a well-established dominance hierarchy (the alpha male and his close followers had been put overboard in a small boat), along with a few Polynesian women settled tiny Pitcairn Island in 1790, after the mutiny on H M S Bounty.

  * Those who study chimps and bonobos, so the joke goes, are called panthropologists.

  Chapter 18

  THE ARCHIMEDES OF THE MACAQUES

  Some ascribe this to his natural genius; while

  others think that incredible effort and toil

  produced these, to all appearances, easy and

  unlabored results. No amount of investigation

  of yours would succeed in attaining the proof,

  and yet, once seen, you immediately believe

  you would have discovered it—by so smooth

  and so rapid a path he leads you to the

  conclusion … Such was Archimedes.

  PLUTARCH

  “Marcellus,” in The Lives of the Noble

  Grecians and Romans1

  We 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.2 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.3 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.4 By 50 million years ago (six months ago in the life of our fifty-year-old) there were arboreal primates living in subtropical Wyoming.5 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.* 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 smell6 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 depend
ence; 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.7 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.8 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.9 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.10 Evading predators must be very high on the primate agenda—and communal life provides early warning and collective defense.

  Vervet monkeys have ventured a little out of the comparative safety of the forest and into the open savanna, where there is less cover for them, and more danger. By playing recordings of their calls back to them, they reveal that they have specific, readily understood alarm cries that elicit specific actions—for a python or black mamba (whereupon all stand on tiptoes and peer anxiously about them in the grass), for a Martial eagle, (whereupon all look up into the sky and dive into deep foliage), and for a leopard (whereupon all quickly scramble up into the trees). Different predators elicit different cries and different evasive behavior. The responses are in part learned. Infants frantically sound the eagle alarm even when a non-raptor is spied flying overhead, and sometimes in response to a falling leaf. Gradually, they get better at making these distinctions. They learn from experience and from others. They have a range of other grunts, some of which scientists think they understand; vervets leave at least a superficial impression of conversing with each other. Gregariousness, by several different routes, spurs social intelligence, which seems to be, of all the species of life on Earth, most highly evolved in the primates.

  The vervet fear of snakes is shared by baboons, chimps, and many other primates. You expose wild rhesus monkeys to snakes and objects that look like snakes and they jump out of their skins. Do the same experiment with laboratory-raised rhesus monkeys who have never seen a snake and, although some of them are afraid, you find that they’re much less distraught. In one experiment the wild chimps’ snake phobia became almost manageable when every time the chimp saw a snake it also was offered a banana.11 So is the fear of snakes not hereditary, but somehow taught by mothers to their babies? Or is there an inborn fear that’s softened in laboratory monkeys because they become habituated to harmless, snake-like objects—hoses, for example? Which is it: heredity or environment? Is knowledge of what a snake looks like, and that snakes mean primates no good, encoded in the DNA? Or are baby primates just watching adults closely and copying what they do?

  Almost certainly the answer is a mix between the two. There seems to be an inborn snake-aversion program in the brains of primates. But this is not a closed program, inaccessible to new information from the outside world. Instead it’s an open program that can be modified by experience—for example, “I’ve seen a lot of snakes in my time that don’t do me much harm, so I’ll be a little more relaxed around them,” or, “Every time I see a snake, a banana miraculously appears; snakes have their good points too.” Most primate programs are open, adaptive, malleable, adjustable to new circumstances—and therefore necessarily partaking of ambivalence, complexity, inconsistency.

  In a typical modern chronology,12 the line that would lead to us split off
from Old World Monkeys about 25 million years (m.y.) ago; from the gibbons, 18 m.y. ago; from orangutans around 14 m.y. ago; from gorillas some 8 m.y. ago; and from the chimps approximately 6 m.y. ago. Bonobos and common chimps went their separate ways only about 3 m.y. ago. Our genus, Homo, is 2 million years old. Our species, Homo sapiens, is maybe 100,000 to 200,000 years old—the equivalent of the last day in the life of that fifty-year-old.

  Committed to a communal social life, under intense selection pressure from predators, with brains evolving rapidly and education of the young effectively institutionalized, the primates have been developing new forms of intelligence. Their curiosity, experimental bent, and intellectual quickness are partly responsible for their success.

  ——

  Here is an account, by a Japanese primatologist, of a remarkable set of events that transpired in a colony of macaques isolated on a small island called Koshima. Initially, in 1952, there were only twenty of the monkeys; the number almost trebled over the following decade. The natural food supply on Koshima was inadequate, so the monkeys had to be provisioned—with sweet potatoes and wheat dumped on the shore by the primatologists who were observing them.

  As anyone knows who’s ever been to a picnic at the beach, sand sticks to food and makes it unpleasantly gritty. In September 1953 a one-and-a-half-year-old female named Imo figured out that she could rinse the sand off her sweet potatoes by dunking them in a nearby brook.

  After Imo, the next individual to learn potato washing was Imo’s playmate, who did so in October. Imo’s mother and another male peer began to wash in January 1954. In subsequent years (1955 and 1956), three of Imo’s lineage (younger brother, elder sister, and niece) and four animals from other lineages (two were a year younger and two were a year older than Imo) started to do so. Thus, with the exception of her mother, all the individuals that learned potato washing quickly were either peers or young close relatives of Imo …