It’s hard to fault him. Like Copernicus, Galileo, and Descartes, he was about as brave as his age would allow. Many naturalists placed humans in a separate order; by Darwin’s time this would become the conventional wisdom. Many clerics (and some naturalists) placed us in a separate kingdom. The evidence may not have warranted it, but isolating humans in their own genus, their separate first-class compartment, was a popular step, reassuring to human vanity. In 1788, in a reflective and undefensive mood, Linnaeus wrote:I demand of you, and of the whole world, that you show me a generic character … by which to distinguish between Man and Ape. I myself most assuredly know of none. I wish somebody would indicate one to me. But, if I had called man an ape, or vice versa, I would have fallen under the ban of all the ecclesiastics. It may be that as a naturalist I ought to have done so.16
One of the scientific names for the common chimpanzee then was Pan satyrus. Pan was an ancient Greek deity, part man, part goat, associated with lust and fertility. A satyr was a closely associated chimera—at first represented as a man with a horse’s tail and ears and an erect penis. Clearly the rampant sexuality of chimps was the defining characteristic in this early naming of the species. The modern classification is Pan troglodytes, troglodytes being mythical creatures who live in caves and beneath the Earth—a much less appropriate designation, since chimps reside exclusively on (and slightly above) the Earth. (The Barbary apes of North Africa do sometimes live in caves; the only other primates known routinely to have lived in caves are humans.) Linnaeus had mentioned a Homo troglodytes, but it’s unclear whether he had ape or human in mind. Or something inbetween.
A systematic comparison of the anatomies of apes and humans was performed during the opening salvos of the Darwinian Revolution by T. H. Huxley. He described his research program in these words, notable among other respects for their extraterrestrial perspective:[L]et us endeavour for a moment to disconnect our thinking selves from the mask of humanity; let us imagine ourselves scientific Saturnians, if you will, fairly acquainted with such animals as now inhabit the Earth, and employed in discussing the relations they bear to a new and singular “erect and featherless biped,” which some enterprising traveller, overcoming the difficulties of space and gravitation, has brought from that distant planet for our inspection, well preserved, may be, in a cask of rum. We should all, at once, agree upon placing him among the mammalian vertebrates; and his lower jaw, his molars, and his brain, would leave no room for doubting the systematic position of the new genus among those mammals, whose young are nourished during gestation by means of a placenta, or what are called the “placental mammals” …There would remain then, but one order for comparison, that of the Apes (using that word in its broadest sense), and the question for discussion would narrow itself to this—is Man so different from any of these Apes that he must form an order by himself? Or does he differ less from them than they differ from one another, and hence must take his place in the same order with them?Being happily free from all real, or imaginary, personal interest in the results of the inquiry thus set afoot, we should proceed to weigh the arguments on one side and on the other, with as much judicial calmness as if the question related to a new Opossum. We should endeavour to ascertain, without seeking either to magnify or diminish them, all the characters by which our new Mammal differed from the Apes; and if we found that these were of less structural value, than those which distinguish certain members of the Ape order from others universally admitted to be of the same order, we should undoubtedly place the newly discovered tellurian [terrestrial] genus with them.I now proceed to detail the facts which seem to me to leave us no choice but to adopt the last mentioned course.17
Huxley then compares the skeletal and brain anatomies of apes and humans. The “manlike apes” (chimps, gorillas, orangutans, gibbons, and the gibbon-like siamangs—the first three called “greater” and the last two “lesser” apes) all have the same number of teeth as humans; all have hands with thumbs; none has a tail; all arose in the Old World. The skeletal anatomies of chimps and humans are strikingly similar. And “the difference between the brains of the Chimpanzee and of Man,” he concluded,18 “is almost insignificant.”
From these data, Huxley then drew the straightforward conclusion that contemporary apes and humans are close relatives, sharing a recent ape-like common ancestor. The conclusion scandalized Victorian England. The outraged reaction of the wife of the Anglican Bishop of Worcester was typicaclass="underline" “Descended from apes! My dear, let us hope that it is not true, but if it is, let us pray that it will not become generally known.”19 Here it is again: the fear that knowledge of the true nature of our ancestors might unravel the social fabric.
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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.