A study on a human skull found in a cave in Nigeria shows that Stone Age humans in the area had features similar to much older hominids, suggesting interspecies breeding.

A skull from the Iwo Eleru cave, dated to 13,000 years ago, shares characteristics of both modern humans and more primitive species.

Professor Chris Stringer of the Natural History Museum in London, who led the research, said: "If we didn't know how old it was we would guess it was over 100,000 years old due to its shape, but it is a lot younger than that."

Palaeontologists see the similarities as more evidence that modern humans and older subspecies of human might have lived at the same era and even reproduced with other in Africa.

Modern humans and older forms are distinguished by their anatomic structure and features, with modern humans' remains being more refined in shape and form.

Modern humans are estimated to have first appeared in Africa 200,000 years ago. Some scientists now believe older humans must have lingered and interacted with the new generation of human life.

"It is a combination of recent Africans and more primitive humans. There is a suggestion that older forms of evolution were hanging on quite a lot later than had been previously thought... The Iwo Eleru skull is either a descendant of those forms or I think it is much more likely that it is a modern human that has had a bit of interbreeding with these lingering more ancient forms of human that has given it this mixture of features," Stringer said.

The study, which is published in the scientific journal PLoS One, used digital scanning techniques to reanalyse a skull found in the Iwo Eleru cave in 1965 by Stringer.

"Somewhere lurking in bits of Africa were these more archaic people, and we are starting to get a picture of that," he said.

Even as not all palaeontologists are convinced of the interbreeding theory, other genetic research has shown that modern humans in Europe interbred with Neanderthals. In fact, traces of their DNA can still be found in Europeans living today.