Gather ’round the fire, children, and I’ll tell you the whole sad story.
Once upon a time, Mary-Claire King and the late Allan Wilson published a paper — that became a widely-cited classic — about the genetic similarity of chimps and humans. “Evolution at Two Levels in Humans and Chimpanzees,” Science 188 (1975):107-116 was, alas, cited far more for proving the genetic near-identity of chimps and humans than for its much more interesting, deeper and more disturbing message: no one really understands how macroevolution occurs.
In brief: King and Wilson compared the chimp vs. human amino acid sequences of several proteins (such as cytochrome c, hemoglobin, and myoglobin), and found the sequences either identical, or very nearly so. Their conclusion? “…the sequences of human and chimpanzee polypeptides examined to date are, on the average, more than 99 percent identical” (p. 108). And thus was born what Jon Cohen, in the latest issue of Science, calls “The Myth of 1%,” namely, that Homo sapiens and Pan troglodytes are “genetically 99% the same.” (See Jon Cohen, “Relative Differences: The Myth of 1%,” Science 316 [29 June 2007]:1836.)
But one cannot hold King and Wilson responsible for what lazy readers did with their powerful paper. The “one percent” message comes on the second page of the paper (p. 108). If being mostly chimp, genetically speaking anyway, is what matters to a reader, chances are he’ll do what Simon and Garfunkel sang about in “The Boxer” — “a man hears what he wants to hear, and disregards the rest” — and stop reading.
The real message of King and Wilson 1975 arrives later in the paper, where casual readers don’t bother to follow:
The molecular similarity between chimpanzees and humans is extraordinary because they differ far more than sibling species in anatomy and way of life. Although humans and chimpanzees are rather similar in the structure of the thorax and arms, they differ substantially not only in brain size but also in the anatomy of the pelvis, foot, and jaws, as well as in relative lengths of limbs and digits. Humans and chimpanzees also differ significantly in many other anatomical respects, to the extent that nearly every bone in the body of a chimpanzee is readily distinguishable in shape or size from its human counterpart. Associated with these anatomical differences there are, of course, major differences in posture…, mode of locomotion, methods of procuring food, and means of communication. Because of these major differences in anatomy and way of life, biologists place the two species not just in separate genera but in separate families. So it appears that methods of evaluating the chimpanzee-human difference yield quite different conclusions. (p. 113, footnote numbers omitted; emphasis added)
There must be more to macroevolution — e.g., the origin of chimpanzees and humans from a common ancestor — than site-by-site amino acid changes in proteins, which was largely the picture drawn in textbook neo-Darwinism at the time (1975). Chimp hemoglobin is pretty much human hemoglobin, and so on, yet it’s always the chimps behind the bars, gazing out, when one visits the zoo:
The contrasts between organismal and molecular evolution indicate that the two processes are to a large extent independent of each other. Is is possible, therefore, that species diversity results from molecular changes other than sequence differences in proteins? (p. 114)
What genetic changes have caused the manifold organismal differences between chimps and humans? After all, that’s really what we want evolution to explain.
King and Wilson speculated about “regulatory mutations,” which is where evolutionary biology finds itself today, 32 years later. Over to Cohen:
Yet it remains a daunting task to link genotype to phenotype. Many, if not most, of the 35 million base-pair changes, 5 million indels in each species, and 689 extra genes in humans may have no functional meaning. “To sort out the differences that matter from the ones that don’t is really difficult,” says David Haussler, a biomolecular engineer at UC Santa Cruz…(p. 1836)
Always read a paper through to the end.