by Carl Sagan
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In recent years it has been possible to go much further, to the very heart of life, to the Holy of Holies, and compare, nucleotide by nucleotide, the DNA molecules of two animals. We can now quantify the kinship of different species. We are able to establish molecular pedigrees, DNA genealogies, which provide the most powerful and compelling evidence that evolution has occurred, as well as tantalizing clues on its mode and tempo. The new tools of molecular biology have yielded insights wholly unavailable to previous generations.
Every animal with a backbone has a bloodstream in which hemoglobin is the oxygen carrier. Hemoglobin is composed of four different protein chains wrapped about one another. One of them is called beta-globin. A particular region of the ACGT sequence codes for beta-globin in all these animals, but only about 5 percent of the region is occupied by the actual instructions for this protein chain. Much of the remaining 95 percent are nonsense sequences—so here mutations can accumulate without being winnowed out by selection. When the beta-globin regions of the DNA are compared across the primate order,20 humans are found to be more closely related to chimps than to anyone else. (The human-gorilla connection comes in a close second.) A new basis for our chimp connection is uncovered: not just the bones, the organs, and the brains, but also the genes—the very instructions for making chimps and humans—are almost indistinguishable.
The DNA sequence that codes for beta-globin is roughly fifty thousand nucleotides long; that is, along a given strand of the DNA molecule, fifty thousand As, Cs, Gs, and Ts in a particular sequence describe precisely how to manufacture the beta-globin of the species in question. If the sequences of humans and chimpanzees are compared nucleotide by nucleotide, they differ by only 1.7%. Humans and gorillas differ by 1.8%, almost as little; humans and orangutans, 3.3%; humans and gibbons, 4.3%; humans and rhesus monkeys, 7%; humans and lemurs, 22.6%. The more the sequences of two animals differ, the more remote (both in relatedness and, usually, in time) is their last common ancestor.
When ACGT sequences that are mainly active genes are examined, a 99.6% identity is found between human and chimp. At the level of the working genes, only about 0.4% of the DNA of humans is different from the DNA of chimps.21
Another method is first to take the DNA from a human being, unzip the double helix, and separate the two strands. Then do the same for a comparable DNA molecule of some other animal. Put the two strands together and let them link up. You’ve now made a “hybrid” molecule of DNA. Where the complementary sequences are closely the same, the two molecules will tightly bind to each other, forming part of a new double helix. But where the DNA molecules from the two animals differ a great deal, the bonding between the strands will be intermittent and weak, and whole sections of the double helix will be flopping loosely. Now take these hybrid DNA molecules and put them in a centrifuge; spin them up so the centrifugal forces tear the two strands apart. The more similar the ACGT sequences are—that is, the more closely related the two DNA strands are—the more difficult it will be to tear them apart. This method does not rely on selected sequences of DNA information (that coding for beta-globin, for example) but on vast amounts of hereditary material, making up whole chromosomes. The two methods—determining the ACGT sequences of selected portions of DNA, and DNA hybridization studies—give remarkable overall agreement. The evidence that humans are most closely related to the African apes is overwhelming.
On the basis of all the evidence, the closest relative of the human proves to be the chimp. The closest relative of the chimp is the human. Not orangs, but people. Us. Chimps and humans are nearer kin than are chimps and gorillas or any other kinds of ape not of the same species. Gorillas are the next closest relatives, both to chimps and humans. The more remote the kinship—when we go to monkeys or lemurs or, say, tree shrews—the less the similarity in sequence. By these standards, humans and chimps are about as closely related as horses and donkeys, and are closer relatives than mice and rats, or turkeys and chickens, or camels and llamas.22
“All right,” you might say, “maybe chimp anatomy is almost the same as mine. Maybe the chimp’s cytochrome c and hemoglobin are almost the same as mine. But the chimp isn’t nearly as smart as I am, as well-organized, as hardworking, as loving, as moral, as devout. Maybe when the genes for these traits are discovered, bigger differences will be found.” Yes. Maybe you’re right. And even that 99.6% identity can be misleading. A 0.4% difference is substantial, because the DNA in any cell in either species is composed of some 4 billion ACGT nucleotides; of them conservatively 1% are in working, no-nonsense portions of the DNA and constitute the genes as such.
The number of operational ACGT nucleotide pairs that are different between humans and chimpanzees must then be roughly 0.4% times 1% times 4 billion, or 160,000. If these are the working parts of genes each 1,000 nucleotides long, each of which codes for a separate enzyme, then the number of completely different kinds of enzymes that humans have and chimps don’t, or vice versa, would be somewhere around 160,000/1,000 or 160. We recall that enzymes have a powerful leverage; they preside over changes in the chemistry of the cell, which can happen very fast; one enzyme can process a multitude of molecules. A hundred enzymes, if they’re the right enzymes, might make a very big difference. A hundred enzymes seems more than enough to account for Huxley’s metaphorical description of the difference between apes and humans: “a hair in the balance-wheel, a little rust on a pinion, a bend in a tooth of the escapement, a something so slight that only the practised eye of the watchmaker can discover it.” Some enzymes would affect estrus, some stature, some fur, some climbing and leaping abilities, some development of the mouth and larynx, some changes in posture, toes, and gait. Many of them would be for a bigger brain with a bigger cerebral cortex, and new ways of thinking beyond the reach of apes.
What’s more, a hundred enzymes changed is certainly an underestimate. Probably none of the differences between chimps and humans requires entirely new enzymes to be evolved. A small number of changes, maybe only a change in a single nucleotide, is adequate to render an enzyme inoperable or to change its function. And many of the differences may not be in the genes themselves, but in the promoters and enhancers, the regulatory elements of the DNA that control when and for how long certain genes should be operational. So even a 0.4% difference could, for all we know, imply profound differences in certain characteristics.
Still, chimps are nearer relatives to us than any other animal on Earth. A typical difference between your DNA—all of it, including the untranscribed nonsense—and that of any other human being23 is roughly 0.1% or less. By this standard, chimps differ from humans only about 20 times more than we differ from one another. That seems awfully close. We must be very careful that those “mortifying reflections” of which Congreve spoke do not make us exaggerate the differences and blind us to our kinship. If we want to understand ourselves by closely examining other beings, chimps are a good place to start.
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Fledgling students of animal behavior are warned against anthropomorphizing. The word literally means changing into human form—attributing human attitudes and states of mind to other animals whose thoughts are not vouchsafed to us. Fairy tales, Aesop, La Fontaine, Joel Chandler Harris, and Walt Disney are among the foremost exponents of the genre. Darwin was guilty of a kind of anthropomorphizing and, even more flagrantly, so was his student George Romanes. The temptation of sentimental self-deception was considered so insidious, and the sin of anthropomorphizing so grave an error, that an influential school of American psychology arose in the first half of the twentieth century which taught that animals enjoyed no internal mental states, no thoughts and no feelings. Its practitioners talked about “the myth of consciousness.” We must, its founder said, “make a clean break with the whole concept of consciousness.” Real scientists, it was claimed, are concerned with no more than what can be observed of the actual behavior of animals. Sensory inputs go in, behavioral outputs come out, and that’s that.
Animals feel no pain. Animals are mechanical black boxes. Behaviorism, as it was called, was an example of the ultrapragmatic streak in American science. It had something in common with Descartes’s automata, although it allowed far less room for free inquiry. It came close to deciding that humans don’t have any thoughts or feelings either.
A concerted but fair-minded attack on at least the more extreme forms of behaviorism has been mounted by the biologist Donald Griffin. In the following passage, Griffin refers to “parsimony”—in seience, the doctrine that in deciding between two adequate explanations, we should choose the simpler. It’s also called “Occam’s Razor.”
According to the strict behaviorists, it is more parsimonious to explain animal behavior without postulating that animals have any mental experiences. But mental experiences are also held by behaviorists to be identical with neurophysiological processes. Neurophysiologists have so far discovered no fundamental differences between the structure or function of neurons and synapses in men and animals. Hence, unless one denies the reality of human mental experiences, it is actually parsimonious to assume that mental experiences are as similar from species to species as are the neurophysiological processes with which they are held to be identical. This, in turn, implies qualitative evolutionary continuity (though not identity) of mental experiences among multicellular animals.
The possibility that animals have mental experiences is often dismissed as anthropomorphic because it is held to imply that other species have the same mental experiences a man might have under comparable circumstances. But this widespread view itself contains the questionable assumption that human mental experiences are the only kind that can conceivably exist. This belief that mental experiences are a unique attribute of a single species is not only unparsimonious; it is conceited. It seems more likely than not that mental experiences, like many other characters, are widespread, at least among multicellular animals, but differ greatly in nature and complexity.
… Extreme forms of behaviorism tend to become little more than irrelevant pleas of willful ignorance …
Some behavioral scientists vigorously proclaim that they are not interested in animal awareness even if it does occur. Their antipathy sometimes seems to be so strong as to suggest that they really do not want to know about any thinking in which animals might engage.24
It’s possible, we submit, to carry the fear of anthropomorphism too far. There are excesses worse than a surfeit of sentiment. There must be some interior state, some thoughts and feelings among the monkeys and apes, and if they are genetically our close relatives, if their behavior is so similar to ours as to be familiar, it’s not unreasonable to attribute to them feelings similar to ours as well. Of course, until better communication with them is established, or until we understand much more about how their brains and hormones work, we can’t be sure. But it’s plausible, it’s an effective teaching tool, and in this book on a few occasions we attempt to portray what it might be like inside the head of another animal.
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By now, the reader will have at least suspected that the interior monologues of the preceding chapter—the first and third by a middle-ranking female, the second by a high-ranking male—are not intended to refer, exactly, to humans. Instead, we’ve tried to depict what it’s like to be a chimp in chimp society. Systematic, long-term observation of chimp groups in the wild is a new field of science. We’ve relied chiefly on the courageous, insightful, and pioneering work of Jane Goodall at the Gombe Reservation in Tanzania, as well as studies by Toshisada Nishida and his colleagues in the Mahale Mountains, also in Tanzania, and by Frans de Waal, who investigated a troop of chimpanzees in a two-acre enclosure in the Arnhem Zoo in the Netherlands.25 Every event dramatized in the last chapter is based on the accounts of these scientists. Their observations speak to us of a way of life that is unmistakably familiar, rich with the Sturm und Drang of human relations. Of course, no human has ever been inside a chimp’s mind, and we cannot be sure how they think. We have taken liberties. We make no apologies for doing so, but stress that it is intended only as a way to think about the chimps.
We must be careful of circular reasoning here—foisting human mental and emotional processes on the chimps, and then triumphantly concluding at the end of our narrative how much like us they are. If we’re to understand ourselves better by looking closely at chimpanzees, we’ll have to give great weight to what they do and comparatively little to what we imagine is going on inside their heads. We must be careful not to deceive ourselves. The behaviorists were not wholly misguided.
We didn’t mention that chimps sleep in trees and that they spend a great deal of time grooming one another. Although chimps do not seem as much transfixed by oral sex as some other primates (cunnilingus is an almost invariable part of foreplay among the orangs26), we used the now-popular phrase to “suck up” to someone because it seems to us, in its present English-language associations at least, to approximate some of the nuance of chimpanzee submission. (The gestural vocabulary of chimpanzee submission does include kissing the alpha’s thigh.)
Many behavioral differences exist between chimps and humans, just as between chimps and gorillas or between gibbons and orangutans. But we are struck by how much the core of chimpanzee social life in the wild resembles some forms of human social organization, especially under great stress—in prisons, say, or urban and motorcycle gangs, or crime syndicates, or tyrannies and absolute monarchies. Niccolò Machiavelli, chronicling the maneuvering necessary to get ahead in the seamy politics of Renaissance Italy—and shocking his contemporaries, especially when he was honest—might have felt more or less at home in chimpanzee society. So might many dictators, whether they style themselves of the right or left persuasion. So might many followers. Beneath a thin varnish of civilization, it sometimes seems, there’s a chimp struggling to bust out—to take off the absurd clothes and the restraining social conventions and let loose. But this is not the whole story.
They’re a little shorter, somewhat hairier, much stronger, and a lot more sexually active than most humans are. They have brown hair and brown eyes. In their natural habitats, they may live to be forty or fifty years old—which is longer than the average in any human society before the Industrial and Medical revolutions. But their average life expectancy is much less. Unlike modern humans, females past infancy are not likely to live as long as males. They alternate between walking on two feet and on all fours, using their knuckles. Chimpanzee males tend to have short fuses. They give off a faint but characteristic odor when they’re nervous or excited, revealing emotions they sometimes try to hide. Chimps are not ashamed of displaying their sexual parts. By our lights they’re a lot dumber than we are, but they do use and even make tools. They apparently hold grudges, nurse resentments, and harbor thoughts of revenge. They plan future courses of action.
Family ties may be strong and lasting. Aged mothers will rush to the defense of their children, even full-grown sons. Orphaned infants are tenderly raised by older siblings. They experience prolonged grief at the loss of a loved one. They suffer from bronchitis and pneumonia, and can be infected with almost any human disease, including the AIDS virus. The elderly turn gray, get wrinkles, lose teeth and hair. Chimps get drunk. They’re able to learn more words of a human language than we have of any chimp language. When they look in the mirror, they recognize themselves. They are, at least to some degree, self-aware. Infants get cranky and irritable when they’re weaned. Chimps form friendships, often with comrades-in-arms who hunt together and guard their turf against intruders. They share food with relatives and friends.
When raised among humans, they have been known to masturbate to pictures of naked people. (This is probably true only of those who, through prolonged contact, have come to consider themselves human. Wild chimps would no more masturbate to erotic images of humans than vice versa.) They keep secrets. They lie. They both oppress and protect the weak. Some, despite many setbacks, persistently strive for social advancement and
career opportunities. Others, less ambitious, are more or less content with their lot.
Among much other innate knowledge, they are born with an understanding about how to make a bed of leaves each night up there in the trees. They are much better climbers than we, partly because they haven’t lost, as we have, the ability to grasp branches with their feet. The youngsters love to climb trees and rival one another in spectacular feats of gymnastic derring-do. But when an infant has climbed too high, its mother—socializing with her friends at the base of the tree—decisively taps the trunk and the baby obediently scampers down.
The forest is crisscrossed with a network of trails made by generations of chimps going about their daily business. Each knows the local geography at least as well as the average human city-dweller knows the neighborhood streets and shops. They almost never get lost. Here and there along the trails are trees with acoustically resonant trunks. When a party of foragers spies such a tree, many run forward and drum away—both sexes, children as well as adults. There are no strings, woodwinds, or brass yet, but the percussion section is in place.
Chimps recognize one another’s individual voices, and a distinctive pant-hoot may summon an ally or relative from a considerable distance. In answering a pant-hoot from, say, an adjacent valley, they lift their heads and purse their lips as if they were on stage at La Scala. Up close, they have an uncanny ability—“uncanny” means only that we haven’t been smart enough to figure it out yet—to communicate with one another, not just about such straightforward matters as sex or dominance, but about much more subtle matters, such as hidden dangers, or buried food supplies. A classic set of experiments was done by the psychologist E. W. Menzel: