Buried for 100 000 years at Xujiayao in the Nihewan Basin of
northern China, the recovered skull pieces of an early human exhibit a now-rare
congenital deformation 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
through the top of the brain case - an enlarged parietal foramen (EPF) or
"hole in the skull" -- that 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
foetal development. It occurs in about one out of every 25 000 modern human
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 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," suggests study
co-author Erik Trinkaus, the Mary Tileston Hemenway Professor of Anthropology
in Arts & Sciences at Washington University in St. Louis.
"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.