Years before the fundamental work of W. Hennig (1966), Phylogenetic Systematics, was published in English, W. E. Le Gros Clark (1959), had already distinguished between “characters of common inheritance” (i.e., primitive characters or plesiomorphies, in cladistic jargon) and “characters of independent acquisition” (derived characters or apomorphies). According to Le Gros Clark, in spite of sharing many primitive features with the great apes, the australopithecines are classified within the hominids on the basis of sharing a few derived characters. Although their grade was largely ape-like, the australopithecines already belonged to the human clade. “Since the pongid sequence of evolution has been much more conservative than the progressive hominid sequence, its terminal products (the modern anthropoid apes) have preserved more of the original characters of the ancestral stock. As divergent evolution proceeds, characters of common inheritance will become progressively supplemented or replaced by characters of independent acquisition in each line” (Le Gros Clark, 1959). The australopithecines, then, were recognized as primitive ancestors of our own species, and both Raymond Dart (the discoverer of the Taung child) and Darwin were vindicated. Finally, a missing link connecting humans with the great apes had been provided, but with which ones: All of the apes in general, only with the great apes, or only with some of the great apes in particular?
That modern humans shared some derived features only with African apes (but not the orangutans) was not realized at the time by Le Gros Clark. But things would soon change. At a summer conference in 1962 organized by the Wenner-Gren Foundation, three biologists presented the results of studies that grouped modern humans with African apes in particular and excluded orangutans and gibbons. The evidence relied upon was both cytogenetic (Klinger et al., 1963) and molecular: serum proteins (Goodman, 1963a) and hemoglobin (Zuckerkandl, 1963). In fact, the study of chromosomes went even further because it showed a closer relationship between modern humans and chimpanzees than between chimpanzees and gorillas, but Morris Goodman had published his results one year earlier (Goodman, 1962b).
George Gaylord Simpson, Theodosius Dobzhansky, and Ernst Mayr, all present at the conference, accepted the inclusion of humans in the African ape clade. However, the great primatologist Adolph H. Schultz (1963) continued to consider the great apes as forming a clade that excluded humans, who had branched off previously (more or less at the same time as the hylobatids and certainly before the orangutans separated from the African apes): “Such evidence [`of extremely close similarities between man and chimpanzee and (or) gorilla’], although of greatest interest, is more than counterbalanced by the mass of profound differences found in all sorts of other characters of recognized reliability.” On the other hand, Simpson