Genetic mutation suggests human ancestors often interbred A view of the Xujiayao site (below) and internal and external view of the Xujiayao 11 skull piece with its position indicated on the drawing of a complete skull (above). Credit: Erik Trinkaus LOUIS, March 18 (UPI) -- An ancient human skull found in China suggests inbreeding might well have been common among our ancestors, Chinese and U.S. researchers say. The 100,000-year-old skull unearthed at Xujiayao in northern China exhibits a congenital deformation -- an enlarged parietal foramen or "hole in the skull" -- that is consistent with modern humans diagnosed with a rare genetic mutation, the researchers report in the journal PLoS ONE. The mutation, which occurs in about one out of every 25,000 modern human births, prevents the closure of small holes in the brain case, a process normally completed within the first five months of fetal development. The evidence of the mutation is found unusually often in skulls of our ancient ancestors such a Homo erectus and other Pleistocene hominids, the researchers 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," study co-author Erik Trinkaus of Washington University in St. Louis said. That a mutation found in modern times has also been found in several different ancestors of modern humans suggests the possibility of significant inbreeding among them, the researchers said. "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 said. The study's coauthors include Xiu-Jie Wu and Song Xing of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.