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FIGURE 14.1 Identifying human specializations requires, first, determining that humans differ from the chimpanzee-bonobo group (the sister group of humans), and second, determining that the difference evolved in the human lineage. The latter judgement requires evidence about the character states of outgroup taxa, such as other apes and monkeys. For practical reasons, evaluations of human specializations often use a minimal set of comparisons, involving humans, chimpanzees, and macaque monkeys (Inset). Humans and chimpanzees diverged 6 to 8 Mya, and the lineage leading to macaque monkeys diverged from the human-ape lineage ~25 Mya. Modified from Preuss et al. (2004).

(Goodman and Sterner, 2010). The beginning of modern ideas about human molecular evolution, however, can be traced to a landmark paper published in Science by King and Wilson (1975). King and Wilson were struck by the great similarity between humans and chimpanzees in the amino acid sequences of proteins that had been sequenced to that point: the differences amounted to only 1%, which is less than that between anatomically indistinguishable sibling species in some animal groups. This great molecular similarity between humans and chimpanzees seemed paradoxical, given the profound differences in behavior, brain size, and gross anatomy between the two species, most of which, they assumed, evolved in the human lineage rather than the chimpanzee lineage. To resolve this paradox, they proposed that humans underwent a “relatively small number of genetic changes in systems controlling the expression of genes,” yielding a large phenotypic effect but only a small genetic signal (King and Wilson, 1975). Such changes could involve the sequences of

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