Malapa and the "problem" skull KNM-ER 1813

The announcement of the Malapa skeletons has many of us going back to descriptions of early Homo. After the paper by Berger and colleagues came out last month, I wrote up some notes on KNM-ER 1813. This is another skull, often argued to be Homo, that has struck many people as similar to the samples from Sterkfontein and Makapansgat.

KNM-ER 1813 has some of the smallest postcanine teeth of any maxillary specimen attributed to Homo habilis. The Malapa MH1 specimen is also small compared to the Homo habilis sample, but not KNM-ER 1813 – MH1 is larger than KNM-ER 1813 in at least one dimension of all its maxillary teeth.

Over the past 20 years, led by Wood (1991), most commentators have placed KNM-ER 1813 in Homo. But that assignment reversed many of the opinions on the skull’s morphology that had been expressed since its discovery. Richard Leakey (1974) emphasized the differences between KNM-ER 1813 and KNM-ER 1470, which he had earlier attributed to Homo habilis. KNM-ER 1813 is quite a lot smaller in its endocranial volume – only 509 ml, where KNM-ER 1470 is 752 ml (Holloway 1983).

Wood framed his discussion of the relationships of the specimen by explicitly listing the features in which KNM-ER 1813 differs from Australopithecus africanus. He began by acknowledging that the overall size and shape of the skull aligns it with A. africanus, whether we consider metric or nonmetric traits. Then he discusses several derived similarities that detract from that simple picture:

However, detailed differences between KNM-ER 1813 and A. africanus suggest that the general phenetic resemblance may be misleading. One of these detailed differences involves the frontal, and two concern the morphology of the occipital. The frontal of KNM-ER 1813, unlike that of A. africanus, shows a modest, but unmistakable post-toral sulcus and it is also braoder thn that of A. africanus. The occipital of KNM-ER 1813 makes a relatively greater contribution to the sagittal profile than in A. africanus, no matter which of the two locations of lambda is used. In addition, it bears an incipient torus, the shape of which has been interpreted by two independent authors as being reminiscent of H. erectus. All three of these detailed differences are such that the condition in KNM-ER 1813 is derived in the direction of H. erectus (Wood 1991: 92-93, citations omitted).

The extent of the occiput may be a simple correlate of a larger vault, but the other two characters – supratoral sulcus and occipital torus, are not.

When the morphological features of the cranial base of KNM-ER 1813 are assessed against this comparative background, the evidence suggests that the cranial base of KNM-ER 1813 differs from that of A. africanus. Without exception, the expressions of these characters in KNM-ER 1813 are more derived in the direction of Homo than are the homologous characters of A. africanus. The angulation of the petrous temporal and the inclination of the foramen magnum are two particularly crucial indicators of the very different arrangement of the cranial base of KNM-ER 1813 and A. africanus. Further evidence of the relative shortness of the base of KNM-ER 1813 comes from the position of the porion with respect to the anteroposterior axis of the cranium, and the evidence of a relatively wide sphenoid (e.g. the location of the foramen ovale and spinosum and the make-up of the entoglenoid) must also be taken into account. This is not to say that all features of the cranial base of KNM-ER 1813 are Homo-like, for Dean (1984) has shown that the relative sizes of the insertions of the nuchal muscles are still remarkably pongid-like (as in deed they are for many early hominids). However, for several features for which we have good comparative evidence, their expression in KNM-ER 1813 must lead us to reject a close association between it and A. africanus. The anatomy of the mandibular fossa region is also derived with respect to the australopithecines, and Picq has claimed to see in KNM-ER 1813 the basis of a temporomandibular joint morphology that can be traced through KNM-ER 3733 to the condition seen in H. erectus from Asia (Wood 1991: 93, citations omitted).

Wood here went into a lot of detail about the cranial base because of his earlier work on this part of the anatomy (for example, Dean and Wood 1982). But even this description may seem cursory in light of the variability within A. africanus of these cranial base characters. KNM-ER 1813 is not ideally preserved, missing most of the basisphenoid and basiocciput, and without a good join along the midline back cracking across the occiput to the right asterion. The petrous orientation is very different from Sts 5, which also has a more posteriorly placed foramen magnum. But Sts 19 is not nearly so different in these respects from KNM-ER 1813.

Now, the cranial base of the MH1 skull from Malapa is still embedded in matrix, so we can’t do this comparison yet with that skull. Will it look Homo-like in the ways that KNM-ER 1813 apparently does, or will it fit squarely within the range of Sterkfontein sample? If I were going to put money on the question, I would bet that the cranial base is influenced by endocranial volume. If so, then the small brain of MH1 will determine an essentially australopithecine-like cranial base. We’ll see when the scans are examined.

The point of discussing this anatomy is not because the cranial base is itself intrinsically important. It fades next to more familiar traits such as brain size and dental morphology. But brain and teeth can’t answer the question alone – they need corroborating evidence from other characters. Particularly in cases like KNM-ER 1813, and MH1, with a more Homo-like dentition than brain, we want to find a phylogenetic hypothesis that maximizes consistency across the entire skeleton.

To understand why, consider another fossil: D2700 from Dmanisi. Rightmire, Lordkipanidze and Vekua (2006) explicitly noted the similarity of the subadult D2700 skull with KNM-ER 1813, including the size, the contours of the facial and vault profile, the size, shape and depth of the palate. The picture reflects the broad similarities noted in the text of that paper:

KNM-ER 1813 and D2700

KNM-ER 1813 (left) and D2700 (right), from Rightmire et al. 2006.

Rightmire and colleagues (2006) note that KNM-ER 1813 is more like H. erectus in some respects than is the Dmanisi specimen – chiefly, D2700 has little if any development of an occipital torus and has a longer clivus and wider interorbital distance than D2700. Rightmire and colleagues (2006) assume that KNM-ER 1813 represents H. habilis for the purposes of this comparison, and they then show that most of the resemblances between this specimen and D2700 are primitive. That amounts to an argument that D2700 is not H. habilis.

It’s funny that this key point in human evolution is best documented by three skeletons that could all represent 11-year-old boys – D2700, MH1, and KNM-WT 15000. In the case of D2700, the contrast with KNM-WT 15000 is possibly great, but hard to interpret because of the imperfect state of our developmental knowledge. Along those lines, it becomes clear just how much there is yet to learn about MH1.

References:

Dean MC, Wood BA. 1982. Basicranial anatomy of Plio-Pleistocene hominids from East and South Africa. Am J Phys Anthropol 59:157-174.

Rightmire GP, Lordkipanidze D, Vekua A. 2006. Anatomical descriptions, comparative studies and evolutionary signi?cance of the hominin skulls from Dmanisi, Republic of Georgia. J Hum Evol 50:115-141. doi:10.1016/j.jhevol.2005.07.009

Wood B. 1991. Koobi Fora Research Project. Vol. 4. Hominid Cranial Remains. Clarendon Press, Oxford.