Newly discovered fossils indicate that early human ancestors may have commonly practiced inbreeding.

The conclusion was made after studying a 100,000-year-old skull found in Xujiayao in northern China. Named Xujiayao 11, the fossil has a deformity that likely arose through inbreeding. It is but one of many examples of human remains that show rare abnormalities.

"These populations were probably relatively isolated, very small and, as a consequence, fairly inbred," study leader and anthropologist at Washington University in St. Louis, Erik Trinkhaus, told LiveScience.

Scientists say that the skull they discovered had an uncommon perforation through the top portion. Called an enlarged parietal foramen (EPF) or "hole in the skull," the perforation is also found in the skulls of modern humans who have a rare genetic mutation that prevents fragments in the skull from closing. Currently only about one out of every 25,000 babies is born with the mutation.

It appears as though the skull belonged to a person who was middle-aged, which means that the deformity probably had minimal effect on brain function. Such abnormalities were unusually common during the Pleistocene epoch, which covers the period about 2.6 million to 12,000 years ago. This high rate of mutations "reinforces the idea that during much of this period of human evolution, human populations were very small," Trinkhaus says.

It remains unknown, however, how far the implications of this discovery will reach.

"It remains unclear, and probably un-testable, to what extent these populations were inbred," the researchers state in their study, published on Monday March 18 in the journal "PLOS ONE."

If these types of small, inbred groups of people did exist, however, then that would pose major implications for the study of evolution. Previous inferences about human genetic development assumed the predominance of groups of people that existed in large, stable environments.