Buried for 100,000 years at Xujiayao in the Nihewan Basin of northern China, the recovered skull¹ pieces of an early human exhibit a now-rare congenital（先天的） deformation² that indicates inbreeding might well have been common among our ancestors, new research from the Chinese Academy of Sciences and Washington University in St. Louis suggests. The skull, known as Xujiayao 11, has an unusual perforation（穿孔） through the top of the brain case -- an enlarged parietal（头顶骨） foramen (EPF) or "hole in the skull" -- that is consistent with modern humans diagnosed with a rare genetic³ mutation⁴ in the homeobox genes⁵ ALX4 on chromosome⁶ 11 and MSX2 on chromosome 5.

 

These specific genetic mutations interfere⁷ 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.

 

Although this genetic abnormality is sometimes associated with cognitive⁸ deficits⁹, the older adult age of Xujiayao 11 suggests that any such deficits in this individual were minor¹⁰.

 

Traces of genetic abnormalities, such as EPF, are seen unusually often in the skulls¹¹ of Pleistocene humans, from early Homo erectus to the end of the Paleolithic.

 

"The probability of finding one of these abnormalities in the small available sample of human fossils is very low, and the cumulative¹² probability of finding so many is exceedingly small," suggests study co-author Erik Trinkaus, the Mary Tileston Hemenway Professor of Anthropology¹³ in Arts & Sciences at Washington University in St. Louis.

 

"The presence of the Xujiayao and other Pleistocene human abnormalities therefore suggests unusual population dynamics¹⁴, 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.

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