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