Image caption The 1.8 million-year-old skull is the most complete hominid skull ever found

The idea that there were several different human species walking the Earth two million years ago has been dealt a blow.

Instead, scientists say early human fossils found in Africa and Eurasia may have been part of the same species.

Writing in the journal Science, the team says that Homo habilis, Homo rudolfensis and Homo erectus are all part of a single evolving lineage that led to modern humans.

But others in the field reject this.

A team looked at the most complete hominid skull ever found, which was uncovered in Dmanisi, Georgia.

It had a small braincase, large teeth and a long face, characteristics it shares with H. habilis. But many features from the braincase were also "unique" to H. erectus.

The 1.8-million-year old skull comes from a site that has given up the biggest collection of well-preserved early-human remains known anywhere in the world.

Image caption The skull had a very small braincase

The Dmanisi collection also represents the earliest evidence of primitive humans outside Africa, a group that emerged soon after early Homo diverged from Australopithecus, or "Lucy".

"We now have the best evidence for what early Homo really is," said lead author David Lordkipanidze from the Georgian National Museum in Tbilisi, Georgia.

"One of the most important things is that we have such a remarkable collection; it's very rare that you have that from one site."

The fossil remains showed a lot of variation that had previously puzzled researchers, but Prof Lordkipanidze said it was clear that these features were all from one population.

"When we looked at this variability and compared it with modern humans, you can see this is a normal range of variation," Prof Lordkipanidze told BBC News.

The skull was uncovered eight years ago and since then the team has compared it to other Homo fossils found in Africa from as early as 2.4 million years ago.

The comparative analysis of the hominid cranium revealed enough similarities for the team to consider the earliest Homo fossils as the same species as the Dmanisi hominids.

The Dmanisi hominids The Georgian hominids lived about 1.8 million years ago and represent an early expansion of human ancestors outside Africa

They are the most complete collection of a Homo species from any site older than 300,000 years old

species from any site older than 300,000 years old They had human-like spines and lower limbs that would have been well suited for long distance travel

The male of the species was much larger than the female

They also had relatively small brains and primitive upper limbs, traits which they shared with the earlier H. habilis, and even with the more primitive Australopithecus

A co-author of the study, Christoph Zollikofer from the Anthropological Institute and Museum in Zurich, Switzerland, said that if the braincase and the face of "Skull 5" had been found as separate fossils at different sites in Africa, they might have been attributed to different species.

"That's because Skull 5 unites some key features, like the tiny braincase and large face, which had not been observed together in an early Homo fossil until now.

"Furthermore, since we see a similar pattern and range of variation in the African fossil record, it is sensible to assume that there was a single Homo species at that time in Africa," Prof Zollikofer added.

"And since the Dmanisi hominids are so similar to the African ones, we further assume that they both represent the same species."

Other palaeoanthropologists, however, believe that at least three distinct species of humans co-existed in Africa.

They include Fred Spoor from University College London. He told BBC News that the methods of analysis that the team used were not sufficient to infer that these fossils were the same species.

Media playback is unsupported on your device Media caption David Lordkipanidze from the Georgian National Museum in Tblisi, led the team that made the discovery

"They do a very general shape analysis of the cranium which describes the shape of the face and braincase in broad sweeping terms," Prof Spoor said.

"The problem is that those Homo species are not defined using such a broad overview of what their general cranial shape is."

He added that the very specific characteristics that had been used to define H. erectus, H. habilis and H. rudolfensis "were not captured by the landmarks that they used".

"They did not consider that the thick and protruding brow ridges, the angular back of the braincase; and some details of the base of the cranium are derived features for H. erectus, and not present in H. habilis and H. rudolfensis."

Chris Stringer at the Natural History Museum in London said that the team had made an excellent case "that this remarkable new skull, with its huge jawbone", was part of the natural variation of the Dmanisi population.

But he said he was doubtful that all of the early Homo fossils could be "lumped into an evolving H. erectus lineage".

Image caption The Dmanisi site has uncovered the most complete collection of a Homo species

"Only H. erectus survives and becomes successful but at the origin, nature was experimenting with how to evolve humans in terms of increasing brain size," Prof Stringer told BBC News.

"Creatures were starting to use tools and eat meat, and this drove evolution, but I think it also drove diversity. The Dmanisi group is an example of the successful species that came out of that and then carried on to spread around the old world."