A computer analysis of the skulls of modern humans, Neanderthals, monkeys and apes shows that we are substantially different, physically, from those early humans.

New York University paleoanthropologist Katerina Harvati said Neanderthals should be considered a separate species from Homo sapiens, and not just a sub-species.

"We interpret the evidence presented here as supporting the view that Neanderthals represent an extinct human species and therefore refute the regional continuity model for Europe," she and colleagues wrote in the Proceedings of the National Academy of Sciences.

Some anthropologists believe that Neanderthals, who went extinct 30,000 years ago, may have at least contributed to the ancestry of modern Europeans.

There is strong evidence that Homo sapiens neanderthalis, as they are known scientifically, interacted with the more modern Cro-Magnons, who eventually displaced them. Cro-Magnons are the ancestors of modern humans, Homo sapiens sapiens.

A Neanderthal skull above, compared with a human skull below

Some research has suggested they may have interbred to a limited degree, although this is hotly disputed in anthropological circles.

At least one study that looked at fragments of Neanderthal DNA suggested any Neanderthal-Cro-Magnon offspring did not add to the modern gene pool.

Harvati and colleagues combined modern computer technology and the tried-and-true method of determining species that uses physical comparisons.

They examined the skulls of modern humans and Neanderthals and 11 existing species of non-human primates including chimpanzees, gorillas and baboons.

They measured 15 standard skull and face landmarks and used 3D analysis to superimpose each one on the other.

"From these data, we were able to determine how much variation living primate species generally accommodate, as well as measure how different two primate species that are closely related can be," Harvati said in a statement.

Their computer analysis showed that the differences measured between modern humans and Neanderthals were significantly greater than those found between subspecies of living monkeys and apes.