Dmanisi reconsidered

Implications of LD 50-1 jawbone and Spoor H. habilis study for ‘variable single species’ theory

In October 2013, Lordkipanidze and his colleagues reported the discovery of an adult skull from Dmanisi, Georgia. The fifth skull to be discovered at the site, it was complete and undeformed; it is the only known fully-preserved adult hominin skull from the early Pleistocene. They also put forward the radical suggestion that the various species often proposed for early African Homo (Homo habilis, Homo rudolfensis, Homo ergaster and Homo erectus) were all actually variants of the same species, and that early Homo was a single lineage which evolved over time without differentiating into multiple species. This conclusion is based on a claim that shape variation between the five Dmanisi skulls is roughly the same as that seen among the various early Homo skulls from East Africa, even though the former represents a single species and the latter are generally thought to represent several (Lordkipanidze, et al., 2013).

This suggestion must now be reconsidered in the light of last week’s announcement that the LD 50-1 partial lower jawbone places the origins of Homo at least 2.8 million years ago (Villmoare, et al., 2015); and Spoor and colleagues’ (2015) reappraisal of the OH 7 Homo habilis type specimen. The latter report that:

1.       The cranial capacity of OH 7 is estimated at between 729 and 824 cc, which is substantially larger than 500 to 700 cc typically cited and within the range of early Homo erectus;

2.       OH 7 is more primitive than the 2.33-million-year-old AL 666-1 upper jawbone provisionally assigned to Homo habilis, despite being 500,000 years younger.

Spoor and colleagues suggest that AL 666-1 cannot be placed within either Homo habilis or Homo rudolfensis. They do not investigate its true affinities further but suggest that their data is consistent with it belonging to Homo erectus. That it is more derived than Homo habilis implies that the origins of the latter must be sought even further back in time than 2.33 million years ago. The reporting that LD 50-1 is transitional between Australopithecus and Homo places the origin of the latter at around 2.8 million years ago. Putting the two reports together, the implication is that Homo was already diverse lineage by 2.33 million years ago, and that early human types were distinguished by facial morphology rather than by brain size.

Lordkipanidze and his colleagues noted that the morphological diversity within the five skulls recovered at Dmanisi is greater than that recorded for specimens recovered in Africa and assigned to different species. On the basis that the Dmanisi hominins all belong to the same species, they suggest that the morphological diversity in African fossil record of Homo 1.8 million years ago is better interpreted as demes of a single evolving lineage of Homo erectus rather than multiple species.

Given that Homo habilis, Homo rudolfensis and Homo erectus might all have emerged by 2.33 million years ago, an ancestor/descendant relationship between these species seems unlikely. It could be argued that the deme interpretation is more parsimonious than the traditional multiple species view.

Against this view, it has been suggested that the limb proportions of Homo habilis are more apelike (longer arms, shorter legs) than those of Homo erectus (Richmond, et al., 2002). The main problem is that in comparison to classic fossils such as ‘Lucy’ (Australopithecus afarensis) and the Turkana Boy (Homo erectus), we do not have a well-preserved example of Homo habilis. The evidence for the apelike limb proportions of Homo habilis largely rests on the two fragmentary skeletons KNM-ER 3735 and OH 62.

In the case of OH 62, the leg is represented by an upper section of the femur, the true length of which is uncertain. KNM-ER 3735 preserves even less limb detail. A re-evaluation of these specimens in comparison to fossil limb parts OH 34 and OH 35 suggests that the upper-to-lower limb ratio of OH 62 lies within the upper range of modern humans and lower range of chimpanzees; and that KNM-ER 3735 lies in the middle of the modern range, entirely outside the chimpanzee range. Based on these results, the limb proportions of Homo habilis were modern rather than apelike (Haeusler & McHenry, 2004).

Overall, these results appear to refute suggestions that Homo habilis should be reclassified as an australopithecine (Wood & Collard, 1999) and are consistent with the conclusions of Lordkipanidze and his colleagues.

References:

1.      Haeusler, M. & McHenry, H., 2004. Body proportions of Homo habilis reviewed. Journal of Human Evolution, Volume 46, pp. 433-465.

2.       Lordkipanidze, D. et al., 2013. A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo. Science, 18 October, Volume 342, pp. 326-331.

3.       Richmond, B., Aiello, L. & Wood, B., 2002. Early hominin limb proportions. Journal of Human Evolution, Volume 43, pp. 529-548.

4.       Spoor, F. et al., 2015. Reconstructed Homo habilis type OH 7 suggests deep-rooted species diversity in early Homo. Nature, 5 March, 7541(519), pp. 83-86.

5.       Villmoare, B. et al., 2015. Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia. Science, 5 March.

6.       Wood, B. & Collard, M., 1999. The Human Genus. Science, 2 April, Volume 284, pp. 65-71.