OH 7 and OH 8: One individual or two?

In the current AJPA, Randall Susman reviews the stratigraphic and morphological evidence concerning Olduvai Hominids 7, 8 and 35. Some history:

Fossil evidence of Homo habilis was recovered from Olduvai in 19601961 (Leakey, 1960, 1961a,b). When ?rst reported, a hand, foot, and clavicle, from site FLK NN Level 3, and a leg from FLK Zinj (Level 22) represented the ?rst early hominid postcrania recovered in East Africa. Together with a mandible and fragmentary skull the fossils from FLK NN Level 3 were reported to be those of a single juvenile whose age at death was [ca.] 12 years (Leakey, 1961b).1 After the initial reports, Day and Napier (1964) concluded that while the jaw, skull, and and were those of a child, the foot represented an adult and thus signaled the presence at FLK NN 3 of a second individual. Day and Napier s conclusion was based on the presence of what they considered to be age-related arthritis in the midfoot and metatarsus (M.H. Day, personal communication). The mandible, skull bones, and hand, were designated OH 7. These fossils became the holotype of the new species, Homo habilis (Leakey et al., 1964). The foot was catalogued as OH 8, and placed in the paratype along with other craniodental and postcranial fossils by Leakey et al. (1964). In their diagnosis and description, Leakey, Tobias and Napier maintained that the holotype, OH 7, was a subadult while other postcranial remains, including OH 8, OH 35 (a leg), and OH 48 (a clavicle), were the bones of adults (Susman 2008:356).

In the paper, Susman argues that OH 7 and OH 8 probably represent a single adolescent individual. His case depends on two points: (1) OH 8 is a subadult of equivalent age to OH 7, contrary to the earlier conclusion which suggested that OH 8 is an adult; and (2) it is “much more likely that OH 7 and OH 8 represent the same subadult individual than they do twin adolescents who died at the same time at FLK NN Level 3 and left complementary body parts (360).”

This review depends extensively on Susman and Stern’s (1982) article, “Functional morphology of Homo habilis”, but adds some new detail to the age assessment. The case that OH 8 is subadult is reasonable, and on that basis it is also reasonable to attribute the OH 7 remains to the same individual. Thus, the OH 8 foot would be part of the holotype (defining individual) of Homo habilis. The conclusion remains debatable, as all issues of attribution are, but it is reasonable.

Additionally, Susman suggests that the OH 8 foot may belong to the same individual as OH 35, an associated tibia and fibula. This is a harder case to make, because OH 8 and OH 35 come from distinct geological strata: OH 8 from FLK NN 3, and OH 35 from FLK “Zinj” (Level 22). Additionally, at least two different analyses of the OH 35 tibia and the OH 8 talus conclude that the two don’t match each other at the ankle joint. So, it’s an uphill climb.

Susman writes, based on a lithological resemblance of FLK NN and FLK “Zinj”, that we might reconsider whether the two necessarily sample different times. Still in the end he concedes that the association is geologically unlikely, and defers to current stratigraphic opinion. This takes the force out of his argument.

Even so, he makes a morphological case why OH 35 and OH 8 would make sense as a single individual. He points out deficiencies in earlier analyses that suggested they are inconsistent at the ankle joint, and claims that both exhibit similar signs of traumatic injury and similar taphonomic signs of carnivore and/or crocodile predation.

I think there’s some irony here. If we accept that the resemblances between OH 35 and OH 8 must be accidental – as the stratigraphy indicates — then we must conclude that morphological compatibility is at best a weak argument for identity. But Susman’s argument for OH 7 and OH 8 essentially boils down to a claim that morphological compatibility demonstrates identity. Whatever we might claim about what “parsimony dictates” (357), the comparison to the OH 35-OH 8 question undercuts the case for an OH 7-OH 8 identity. Even accepting that OH 8 is a subadult, the demonstration that they are of “consistent” relative ages means only that we cannot disprove the null hypothesis that they are the same.

Still, in this case, perhaps that’s enough. In particular, the OH 7=OH 8 hypothesis needs to be considered more carefully when we discuss H. habilis limb proportions. This discussion is otherwise dominated by OH 62, which has weaknesses of its own. In the absence of a convincing association of OH 7 with OH 8, others have observed similarities of OH 8 with robust australopithecine tali (notably Kromdraai TM 1517, c.f., Gebo and Schwartz 2006). If OH 8 is associated with OH 7, it argues for a wider view of diversity of talar morphology in early Homo. Since early Homo appears to be morphologically diverse in most other respects, it hardly makes sense to define a fossil talus as Homo or Australopithecus on the basis of similarities to later large-bodied humans.

References:

Susman RL. 2008. Evidence bearing on the status of Homo habilis at Olduvai Gorge. Am J Phys Anthropol 137:356-361. doi:10.1002/ajpa.20896

Susman RL, Stern JT. 1982. Functional morphology of Homo habilis. Science 217:931-934.

Gebo DL, Schwartz GT. 2006. Foot bones from Omo: Implications for hominid evolution. Am J Phys Anthropol 129:499-511.doi:10.1002/ajpa.20320