The discovery of a 1.8-million-year-old skull of a human ancestor buried under a medieval Georgian village offers new evidence that early man was a single species with a vast array of different looks, researchers have said.

With a tiny brain about a third the size of a modern human's, protruding brows and jutting jaws like an ape, the skull was found in the remains of a medieval hilltop city in Dmanisi, according to the study published in the journal Science.

This is important to understanding human evolution David Lordkipanidze, Director, the Georgian National Museum

It is one of five early human skulls - four of which have jaws - found so far at the site, about 100km from the capital, Tbilisi, along with stone tools that hint at butchery and the bones of big, saber-toothed cats.

Lead researcher David Lordkipanidze, director of the Georgian National Museum, described the group as "the richest and most complete collection of indisputable early Homo remains from any one site".

"This is important to understanding human evolution," he said.

The skulls vary so much in appearance that under other circumstances, they might have been considered different species, said co-author Christoph Zollikofer of the University of Zurich.

"Yet we know that these individuals came from the same location and the same geological time, so they could, in principle, represent a single population of a single species," he said.

For years, some scientists have said humans evolved from only one or two species, much like a tree branches out from a trunk, while others say the process was more like a bush with several offshoots that went nowhere.

The researchers have compared the variation in characteristics of the skulls and found that while their jaw, brow and skull shapes were distinct, their traits were all within the range of what could be expected among members of the same species.

"The five Dmanisi individuals are conspicuously different from each other, but not more different than any five modern human individuals, or five chimpanzee individuals, from a given population," said Zollikofer.

Evolutionary biology

Under that hypothesis, the different lineages some experts have described in Africa - such as Homo habilis and Homo rudolfensis - were all just ancient people of the species Homo erectus who looked different from each other.

It also suggests that early members of the modern man's genus Homo, first found in Africa, soon expanded into Asia despite their small brain size.

Before the site was found, the movement from Africa was put at about 1 million years ago.

"We are thrilled about the conclusion they came to. It backs up what we found as well," said Milford Wolpoff, a paleoanthropologist at the University of Michigan.

Wolpoff published a study in the journal Evolution last year that also measured statistical variation in characteristics of early skull fossils in Georgia and East Africa, suggesting a single species and an active process of inter-breeding.

But not all experts agree.

"I think that the conclusions that they draw are misguided," said Bernard Wood, director of the hominid paleobiology doctoral program at George Washington University.

"What they have is a creature that we have not seen evidence of before," he said, noting its small head but human-sized body.

"It could be something new and I don't understand why they are reluctant to think it might be something new."

In fact, the researchers did give it a new name, Homo erectus ergaster georgicus, in a nod to the skull as an early but novel form of Homo erectus found in Georgia.

Its discovery, in such close quarters with four other individuals, offered researchers a unique opportunity to measure variations in a single population of early Homo, and "to draw new inferences on the evolutionary biology" of our ancestors, co-author Marcia Ponce de Leon of the University of Zurich said.