March 25, 2013

Rare early North Chinese skull suggests inbreeding depression

Again very quickly on a matter that seems to be of some interest:

Xiu-Jie Wu, Song Xing & Erik Trinkaus, An Enlarged Parietal Foramen in the Late Archaic Xujiayao 11 Neurocranium from Northern China, and Rare Anomalies among Pleistocene Homo. PLoS ONE, 2013. Open accessLINK [doi:10.1371/journal.pone.0059587]

Abstract

We report here a neurocranial abnormality previously undescribed in Pleistocene human fossils, an enlarged parietal foramen (EPF) in the early Late Pleistocene Xujiayao 11 parietal bones from the Xujiayao (Houjiayao) site, northern China. Xujiayao 11 is a pair of partial posteromedial parietal bones from an adult. It exhibits thick cranial vault bones, arachnoid granulations, a deviated posterior sagittal suture, and a unilateral (right) parietal lacuna with a posteriorly-directed and enlarged endocranial vascular sulcus. Differential diagnosis indicates that the perforation is a congenital defect, an enlarged parietal foramen, commonly associated with cerebral venous and cranial vault anomalies. It was not lethal given the individual’s age-at-death, but it may have been associated with secondary neurological deficiencies. The fossil constitutes the oldest evidence in human evolution of this very rare condition (a single enlarged parietal foramen). In combination with developmental and degenerative abnormalities in other Pleistocene human remains, it suggests demographic and survival patterns among Pleistocene Homo that led to an elevated frequency of conditions unknown or rare among recent humans.

The authors argue, according to previous research that Xujiayao remains are neither Homo erectus nor early Homo sapiens but it's difficult to say, as the Chinese Academy tends to be ambiguous on these matters and so is Trinkaus.

The most recent datings (OSL) for the site suggest MIS 4 ages (60±8 and 69±8 ka BP). However earlier datings (U-Th series) claimed a much older age: 104-125 Ka BP (MIS 5). The later dates would certainly fit with my expectations for the arrival of H. sapiens to the area.

Whatever the case, the authors conclude that this EPF anomaly is in line with a long list of Paleolithic cases of developmental anomalies that they suspect caused by inbreeding within the parameters of small hunter-gatherer populations. 

To the extent that these abnormalities can be considered congenital or cannot be securely diagnosed, these considerations raise questions regarding the population dynamics of Pleistocene humans. To what extent could this pattern reflect small, highly inbred populations, which were also demographically unstable, resulting in both the increased appearance of congenital deleterious conditions and in their subsequent disappearance through local population extinction? Demographic instability appears to have been characteristic of most Pleistocene human populations [78][80]. It remains unclear, and probably untestable, to what extent these populations were inbred, but close genetic relationships have been suggested for one Neandertal sample [81] and some Upper Paleolithic burial groups [76], [82], [83].

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