After eight years spent studying a 1.8-million-year-old skull uncovered in the republic of Georgia, scientists have made a discovery that may rewrite the evolutionary history of our human genus Homo.

It would be a simpler story with fewer ancestral species. Early, diverse fossils — those currently recognised as coming from distinct species like Homo habilis, Homo erectus and others — may represent variation among members of a single, evolving lineage. In other words: Just as people look different from one another today, so did early hominids look different from one another, and the dissimilarity of the bones they left behind may have fooled scientists into thinking that they came from different species.

This was the conclusion reached by an international team of scientists led by David Lordkipanidze, a paleoanthropologist at the Georgian National Museum in Tbilisi, as reported on Thursday in the journal Science.

The key to this revelation was a cranium excavated in 2005 and known as Skull 5, which scientists described as “the world’s first completely preserved adult hominid skull” of such antiquity. Unlike other Homo fossils, it had a number of primitive features: a long apelike face, large teeth and a tiny braincase, about one-third the size of that of a modern human being. This confirmed that, contrary to some conjecture, early hominids did not need big brains to make their way out of Africa.

The discovery of Skull 5 alongside the remains of four other hominids at Dmanisi, a site in Georgia rich in material of the earliest hominid travels into Eurasia, gave the scientists an opportunity to compare and contrast the physical traits of ancestors that apparently lived at the same location and around the same time.

Mr. Lordkipanidze and his colleagues said the differences between these fossils were no more pronounced than those between any given five modern humans or five chimpanzees. The hominids who left the fossils, they noted, were quite different from one another but still members of one species.

“Had the braincase and the face of Skull 5 been found as separate fossils at different sites in Africa, they might have been attributed to different species,” a co-author of the journal report, Christoph Zollikofer of the University of Zurich, said in a statement. Such was often the practice of researchers, using variations in traits to define new species.

“Since we see a similar pattern and range of variation in the African fossil record,” Mr. Zollikofer said, “it is sensible to assume that there was a single Homo species at that time in Africa.” Moreover, he added, “Since the Dmanisi hominids are so similar to the African ones, we further assume that they both represent the same species.”

In their report, the Dmanisi researchers said the Skull 5 individual provided “the first evidence that early Homo comprised adult individuals with small brains but body mass, stature and limb proportions reaching the lower range limit of modern variation.”

Skeletal bones associated with the five Dmanisi skulls show that these hominids were short in stature, but their limbs enabled them to walk long distances as fully upright bipeds. The shape of the braincase distinguished them from the more primitive Australopithecus genus, which preceded Homo and lived for many centuries with Homo in Africa. — New York Times News Service