Swiss scientists have announced that computer graphics of Neanderthals based on ancient fossils showed they were very different from early humans and did not mix with them.
Christoph Zollikofer, a neurobiologist at the University of Zurich, in collaboration with Marcia Ponce de Leon, a computer scientist and anthropologist, created virtual reality models of Neanderthal skulls from 16 fossils of the creatures who lived in Europe, North Africa and Asia from 125,000 to 40,000 years ago.
The images of the skulls appeared to have distinctive features established early in childhood which did not develop gradually through life.
Zollikofer says this indicates that the Neanderthals and humans co-existed but did not breed with each other.
"This is a strong argument for early separation on the species level, which means they had isolated populations," argues Zollikofer.
"There might have been some accidental inbreeding, but certainly not a big exchange of genes."
The aim of the research, say the pair of scientists, was to compare the development of Neanderthal man from childhood to that of early and modern humans.
Results of the research, published in the science journal "Nature", suggest that the distinctive features of the Neanderthal skull and face were present by the age of two years.
The research supports earlier evidence which showed that Neanderthals were too genetically distinct to have been an ancestor of modern humans.
"We think that together with the genetic data...it is quite reasonable to think that these are really two different species separated at least half a million years ago," suggests Zollikofer.
swissinfo with agencies