Was inbreeding common among early humans?

de bene esse: literally, of well-being, morally acceptable but subject to future validation or exception

Skull pieces from an early human who lived 100,000 years ago in northern China suggest that inbreeding might have been common among our ancestors, a new study claims.

The remains found at Xujiayao, in the Nihewan Basin, have a now-rare congenital deformation usually only found in incestuously conceived offspring. It adds to a growing body of archaeological evidence that human ancestors regularly inbred at least until the end of the Palaeolithic era.

Incest: A view of the Xujiayao site and internal and external view of the Xujiayao 11 skull piece with its position indicated on the drawing of a complete skull

Examination of the skull, known as Xujiayao 11, revealed an unusual perforation through the top of the brain case - an enlarged parietal foramen (EPF) or 'hole in the skull'. The symptom is consistent with modern humans diagnosed with a rare genetic mutation in the homeobox genes ALX4 on chromosome 11 and MSX2 on chromosome 5.

 

These specific genetic mutations interfere with bone formation and prevent the closure of small holes in the back of the prenatal braincase, a process that is normally completed within the first five months of fetal development. It occurs in about one out of every 25,000 modern human births and is associated with inbreeding.

Abnormality: Examination of the skull, known as Xujiayao 11, revealed an unusual perforation through the top of the brain case - an enlarged parietal foramen (EPF) or 'hole in the skull'

Although this genetic abnormality is sometimes associated with cognitive deficits, the older adult age of Xujiayao 11 suggests that any such deficits in this individual were minor.

Traces of genetic abnormalities, such as EPF, are seen unusually often in the skulls of Pleistocene humans, from early Homo erectus to the end of the Palaeolithic.
Study author Erik Trinkaus, professor of anthropology at Washington University, said: 'The probability of finding one of these abnormalities in the small available sample of human fossils is very low, and the cumulative probability of finding so many is exceedingly small.

'The presence of the Xujiayao and other Pleistocene human abnormalities therefore suggests unusual population dynamics, most likely from high levels of inbreeding and local population instability.' It therefore provides a background for understanding populational and cultural dynamics through much of human evolution, he added.

Genetic: The symptom is consistent with modern humans diagnosed with a rare genetic mutation in the homeobox genes ALX4 on chromosome 11 and MSX2 on chromosome 5

Speaking to LiveScience, Dr Trinkaus said: 'These populations were probably relatively isolated, very small and, as a consequence, fairly inbred.'  The apparently common occurrence of such genetic abnormalities 'reinforces the idea that during much of this period of human evolution, human populations were very small', he added.

The findings of the study, co-authored by Xiu-Jie Wu and Song Xing of the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, were published yesterday in the open-access journal PLOS ONE.