The KNM-ER 42700 calvaria

One of the highlights of the scientific program of the meetings was Fred Spoor's paper on the new cranial vault from Ileret, KNM-ER 42700. It is difficult to describe without a picture (which I don't have), but at a glance, the skull is very similar to another small, subadult skull that may be similar in geological age, Mojokerto. From the abstract:

Renewed fieldwork at Ileret, east of Lake Turkana, resulted in the discovery of a well-preserved calvaria KNM-ER 42700). The specimen derives from strata dated to an age between 1.5 and 1.6 Ma. The state of closure of the sutures, and of the spheno-occipital synchondrosis in particular, suggests that the individual was a subadult or young adult at death. Initial assessment of KNM-ER 42700 indicates affinities with Homo erectus (including H. ergaster). However, in its absolute vault dimensions it is closer to specimens assigned to H. habilis than to the traditional hypodigm of H. erectus. It also lacks some characteristic H. erectus features, including well-developed supraorbital tori and supratoral hollowing (Spoor et al. 2005:201).

Spoor did a very nice job presenting the anatomy of the skull and its metric comparisons with the known erectine and habiline sample. With a date of around 1.55 million years, it overlaps temporally with African early humans, and also potentially with habilines (although it would likely be the latest example of these, it would not be by too much).

This overlap is important to consider because of the very small size of the specimen. Spoor reported that its endocranial volume is estimated as 691 ml. Depending on its chronological age, the brain may not have quite reached its adult size; although Spoor argued for a older age estimate (and therefore smaller adult brain), he estimated that at a maximum, the brain may have reached 720 ml.

Anywhere in this range, around 700 ml, makes this specimen the smallest of the African erectines. It is in the size range of the Dmanisi erectines. But it is also within the size range of the habilines (KNM-ER 1470 has a volume of 752 ml), and unlike Dmanisi, this specimen has no face to set it apart from a large habiline. So its vault features and measurements must be examined to make clear what kind of hominid it is.

The specimen shares several nonmetric features with erectines, including a frontal sagittal keel, a high petrotympanic angle, and a flattened profile of the parietals in lateral profile. One of the most interesting aspects of Spoor's talk was that he found no compelling metric differences that would distinguish between the habilines and erectines. Although the habilines were consistently smaller in their measurements, they had basically the same relation of cranial measurements. Personally, I would say that disregarding the face, some of the habiline vaults are really similar in shape to erectines, expecially OH 16 and KNM-ER 1813.

Spoor argued on the basis of his bivariate metric comparisons that vault thickness, occipital curvature, and supraorbital torus size are not good H. erectus characters, and instead he accentuates nonmetric characters like keeling. To me, this is another dent in our understanding of what drove the evolution of speciation in early Homo.

References:

Spoor F, Leakey MG, Leakey LN. 2005. A comparative analysis of the KNM-ER 42700 hominin calvaria from Ileret (Kenya). Am J Phys Anthrol Supplement 40:200-201.