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Ancient human skulls unearthed in northern China reveal that inbreeding may have been a common practice among our ancestors, according to new research.

Skull pieces discovered at Xujiayao in the Nihewan Basin of northern China show a now-rare congenital deformation that suggests inbreeding might well have been common among early humans, according to researchers at the Chinese Academy of Sciences and Washington University in St. Louis.

Scientists said the skull, known as Xujiayao 11, has an unusual perforation through the top of the brain case.  This enlarged parietal foramen (EPF) or "hole in the skull" is similar to skulls from modern humans diagnosed with a rare genetic mutation in the homeobox genes ALX4 on chromosome 11 and MSX2 on chromosome 5.

Researchers explain that these specific genetic mutations interfere with bone formation and prevent the closure of small holes in the back of the prenatal brain case.  Normally, this development process is completed within the first five months of fetal birth. 

According to researchers, this specific deformity occurs in about one out of every 25,000 modern human births. While this genetic abnormality is sometimes associated with cognitive deficits, the older adult age of Xujiayao 11 indicates that cognitive deficits in this individual were minor.

Researchers say traces of genetic abnormalities, like EPF described in the recent study published March 18 in the journal PLOS ONE, are seen unusually often in the skulls 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 cumulative probability of finding so many is exceedingly small," study co-author Erik Trinkaus, the Mary Tileston Hemenway Professor of Anthropology in Arts & Sciences at Washington University in St. Louis, said in a statement.

"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," Trinkaus explained, adding that the latest analysis provides a background for understanding populational and culture throughout human evolution.