Shadows of Forgotten Ancestors (41 page)

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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,
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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 being
²³
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.”

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.
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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 orangs
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), 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.