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concluded that “the structural differences which separate Man from the Gorilla and the Chimpanzee are not so great as those which separate the Gorilla from the lower apes” (p. 103). The next significant advance in our understanding of the relationships among the great apes came when developments in biochemistry and immunology made in the first half of the 20th century allowed the focus of the search for evidence to be expanded beyond traditional gross morphological evidence to the properties of molecules (Goodman, 1963a; Zuckerkandl, 1963; Sarich and Wilson, 1967a), to the structure of proteins (King and Wilson, 1975), and most recently to the composition of the genome (Ruvolo, 1997; Bradley, 2008). A recent molecular supermatrix analysis based on 15 mitochondrial and 43 nuclear genes (Fabre et al., 2009) provides strong support for modern humans being more closely related to chimpanzees and bonobos than to any other living great ape. Gorillas are more distantly related to modern humans than to chimpanzees and bonobos, and a recent report notwithstanding (Grehan and Schwartz, 2009), the orangutan is the great ape most distantly related to modern humans; these relationships can also be expressed in the form [Pongo (Gorilla (Pan, Homo))]. This recent molecular supermatrix analysis effectively removes any reasonable doubt that extant Pan species are more closely related to modern humans than they are to extant Gorilla taxa. This is an important advance in our understanding of human evolution because, in combination with the principle of parsimony, it enables researchers to generate hypotheses about character evolution within the great ape clade. These hypotheses can then be used as the equivalent of a null hypothesis when considering where to place newly discovered fossil great ape taxa.

The Human Fossil Record

The fossil record of the human clade consists of fossil evidence for modern humans plus that of all extinct taxa that are hypothesized to be more closely related to modern humans than to any other living taxon. Not so long ago nearly all researchers were comfortable with according the human clade the status of a family, the Hominidae, with the nonhuman extant great apes (i.e., chimpanzees, bonobos, gorillas, and orangutans) placed in a separate family, the Pongidae. But given the abundant evidence for a closer relationship between Pan and Homo than between Pan and Gorilla (see above), many researchers have concluded that the human clade should be distinguished beneath the level of the family in the Linnaean hierarchy. These researchers now use the family Hominidae for all of the extant great apes (including modern humans), and they use the subfamily Homininae either for Gorilla, Pan, and Homo [e.g., Harrison (2010)] or for just Pan and Homo. Some of the researchers



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