By Bernard Wood
Among the fossils recovered from Swartkrans in 1949 were two fragments that preserve the proximal end of the right femur. The two specimens, SK 82 and SK 97, were not initially recognized as hominin. For example, neither fossil was included in Broom and Robinson’s (1970) discussion of the postcranial evidence for the taxon that was then referred to as Paranthropus crassidens.
In the first blog in this series I related how Wilfrid Le Gros Clark had journeyed to South Africa soon after end of WWII to see at first hand the hominin fossils that Raymond Dart, Robert Broom and others had been describing in journal articles and monographs. Although I do not know whether John Napier undertook the same pilgrimage, he was given access to casts of some of what were considered to be the obviously hominin postcranial remains plus the remains of the non-hominin primates recovered from the South African cave sites. It was probably Napier’s familiarity with Miocene hominoids that led him to recognize that although SK 82 and SK 97 were less modern human-like than the proximal femora assigned to Australopithecus, he was sure they were hominin and thus were evidence of the proximal femur of P. crassidens.
Napier published his analysis of the two proximal femora (his “SK 83” is actually SK 97) in a comparatively obscure journal (Napier 1964), but his paper deserves more attention and recognition than it receives. He not only remarked on the “absolute and relative smallness of the head” (ibid, p. 691) of the two femora, but he provided compelling quantitative evidence of the differences between the relative size of the head and the relative length of the neck of the two femora and the proximal femora of two samples of modern humans (ibid, Table IV, p. 692 and Table V, p. 693). The smaller of the two samples consisted of what he describes as “Bushman”. So Napier was also aware of the importance that absolute size might play in the differences between the small fossil femora and his sample of European modern humans. Napier combined what was then known of the morphology of the pelvis and the proximal femur of P. crassidens to suggest that although he accepted that the latter taxon was bipedal he interpreted its gait as “waddling” (ibid, p. 696) rather than like the distinctive “striding” gait of modern humans. Napier was also ahead of his time by suggesting that the “most likely explanation” for the differences between Australopithecus and P. crassidens would seem to have been “an ecological one” (ibid, p. 696) and he sets out his ecological hypothesis with care.
Although Napier was the first to suggest that early hominins might have practiced a form of bipedal locomotion that was different from that habitually practiced by modern humans, the explanation of how such an absolutely and relatively small femoral head and an absolutely and relatively long femoral neck could be consistent with habitual bipedalism was not provided until 1973. In a landmark paper published in that year, Owen Lovejoy, Kingsbury Heiple and Albert Burstein suggested that it was the reduced width of the pelvic outlet (Lovejoy et al. 1973, Fig. 13, p. 778) that enabled P. crassidens and Australopithecus (N.B., Lovejoy et al. refer to both taxa as Australopithecus) to be bipedal even though individuals belonging to those taxa lacked an iliac pillar and had a substantially smaller femoral head than modern humans. Lovejoy et al. argued that the smaller birth canal of P. crassidens and Australopithecus allowed the head of the femur to be located closer to the midline and this thus improved the efficiency of the abductor muscles that prevented the pelvis from dropping on the side of the non-supporting leg during the swing phase of bipedal walking (or running).
Until these influential studies the only form of hominin bipedalism that was contemplated was our own, modern human, form and the only morphology consistent with hominin bipedal locomotion was the distinctive modern human pelvo-femoral complex. Napier and Lovejoy et al. showed this to be a flawed logical conjunction. If you were a hominin (or a hominid) you could be bipedal without having to look, from the pelvis downwards, like a modern human. Both papers, but especially Lovejoy et al., should be required reading for all paleoanthropology graduate students.
Le Gros Clark, W.E. (1947) ‘Observations on the anatomy of the Australopithecinae’. Journal of Anatomy, 81: 300-333.
Lovejoy, C.O, Heiple, K.G. and Burstein, A.H. (1973) ‘The gait of Australopithecus’. American Journal of Physical Anthropology, 38(3): 757-780.
Napier, J.R. (1964) ‘The evolution of bipedal walking in the hominids’. Archives de Biology (Liège), 75 (Suppl.): 673-708.