MorphoLogicaL interpretations
Since the time of Darwin and Huxley, anthropologists have recognized that humans' closest relatives are the African great apes, the chimpanzee and gorilla, with the Asian great ape, the orangutan, more distant. This conclusion is based principally on comparative anatomy of the hominoids. For a long time the question of the evolutionary relationship between humans, chimpanzees, and gorillas was debated. (It seems now to have been resolved, principally based on molecular evidence.)
For instance, the African apes share many anatomical similarities, particularly in their forelimbs, which show adaptations to their knuckle-walking mode of locomotion (see figures 15.1 and 15.2) and in their dentition (see figure 15.3) which has a thin layer of enamel on the cheek teeth. Modern humans and (most of) their extinct relatives have thick enamel (but see unit 19 for a qualification), as do many fossil apes. In several cladistic analyses of living hominoids (by, for example, Lawrence Martin of the State University of New
York at Stony Brook and Peter Andrews of the Natural History Museum, London), the shared limb anatomy and dental features of African apes were judged to be derived characters that linked chimpanzees and gorillas as a separate clade from humans. Under this scheme, humans were seen as having diverged first from the hominoid lineage, with gorillas and chimpanzees sharing a common ancestor in which knuckle-walking and thin tooth enamel evolved. A second scheme—a trichotomy in which African apes and humans diverged simultaneously from a common ancestor— was also said to be possible, though less likely.
The Martin/Andrews view of human/African ape affinity won wide support, although different views were expressed as well. For instance, one cladistic analysis grouped the orangutan with the African apes in a clade separate from humans, while another identified an African ape clade and a human/orangutan clade. In this plethora of morphological analyses, only one, published in 1986 by the Australian anatomist Colin Groves, concluded (weakly) that humans and chimpanzees are one another's closest relatives; this assessment was based on forelimb anatomy, particularly the wrist. That is, gorillas were suggested to have diverged first from the hominoid ancestor, with humans and chimpanzees sharing a common ancestor from which they later diverged. (A later, more detailed study, reached the same conclusion; see figure 15.4) As we shall see, this counterintuitive view was also emerging from molecular studies of the time, and it became ever more strongly supported throughout the following decade.
Morphologists resisted this latter interpretation, because the many anatomical similarities between gorillas and chimpanzees were assumed to be shared derived characters. If the human/chimpanzee association was indeed correct, then morphologists faced awkward puzzles. For instance, the many striking anatomical similarities of gorillas and chimpanzees must be explained either as homoplasies (independent, parallel evolution), which seems unlikely, or as shared primitive characters that were present in the common ancestor of apes and humans (see below). Furthermore, why have the
Adaptations to bipedal locomotion
FIGURE 15.1 Ape and human anatomy: The ape (left) is adapted to a form of quadrupedalism known as knuckle-walking, which is seen only in chimpanzees and gorillas. Rather than support the forelimb on the palm of the hand (like most primates) or the palmar surface of the fingers (like baboons), the African apes support it on the dorsal surface of the third and fourth digits of their curled hands.
Head held vertically
Reduced lower back
Short, wide pelvis
Femoral head angled and strengthened
Increased hindlimb length
Increased valgus angle of knee
Foot forms a platform structure with non-opposable great toe. Lateral toes not curved
Head held vertically
Reduced lower back
Short, wide pelvis
Femoral head angled and strengthened
Increased hindlimb length
Increased valgus angle of knee
Foot forms a platform structure with non-opposable great toe. Lateral toes not curved
Large, bulbous cranium Short face
Reduced anterior dentition, small canines, large cheek teeth coverd with thick enamel
Shortened forelimb
Hand with enlarged thumb, enhanced fingertip sensitivity, non-curved fingers (a manipulative, rather than a locomotor, structure)
FIGURE 15.1 Ape and human anatomy: The ape (left) is adapted to a form of quadrupedalism known as knuckle-walking, which is seen only in chimpanzees and gorillas. Rather than support the forelimb on the palm of the hand (like most primates) or the palmar surface of the fingers (like baboons), the African apes support it on the dorsal surface of the third and fourth digits of their curled hands.
FIGURE 15.2 Anatomy of the feet: The human foot is a platform, built for bipedalism, while the gorilla foot is more of a grasping organ. A key difference, therefore, is in the relationship of the great toe to the other toes of the foot. In humans, the great toe is parallel with the other toes; in apes, it is opposable.
The wrist and elbow anatomy is adapted so as to "lock," thus providing a firm support for the body weight. Human bipedalism (right) involves a number of anatomical differences from that seen in quadrupedalism, as indicated. Anthropologists are divided over whether the common ancestor of humans and African apes was a knuckle-walker.
homologous features that reveal the human/chimpanzee link been so hard to find? Groves was the lone voice in identifying any at all. Recently, however, analyses of fossil and living hominoids have added further evidence related to this point.
For instance, David Begun, of the University of Toronto, compared cranial and dental features in the Miocene ape Dryopithecus, an early member of the hominin clade, and living hominoids. He concluded that many characters in gorilla once considered to be derived are actually primitive, and that humans, chimpanzees, and australopithecines share several characters that are derived for the group as a whole. This finding links humans and chimpanzees as one another's closest relatives.
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