It would be a simpler story with fewer ancestral species. Early, diverse fossils — those currently recognized as coming from distinct species like Homo habilis, Homo erectus and others — may actually 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 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 Thursday in the journal Science.
The key to this revelation was a cranium excavated in 2005 and known simply 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.
Dr. 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.
Although the Dmanisi finds look quite different from one another, Dr. Zollikofer said, the hominids who left them were living at the same time and place, and “so could, in principle, represent a single population of a single species.” He and his Zurich colleague, Marcia Ponce de León, conducted the comparative analysis of the Dmanisi specimens.
“Since we see a similar pattern and range of variation in the African fossil record,” Dr. Zollikofer continued, “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.”
But what species? Some team members simply call their finds “early Homo.” Others emphasized the strong similarities to Homo erectus, which lived between two million and less than one million years ago. Tim D. White, a paleoanthropologist at the University of California, Berkeley, called it “the most primitive H. erectus yet known,” noting that “it is more similar than any other yet found to early Homo from eastern Africa,” a group of hominids estimated to have lived 2.3 million years ago.
All five of the skulls and skeletal bones were found in underground dens, suggesting grisly scenes from the perilous lives these early Homos led. They resided among carnivores, including saber-toothed cats and an extinct giant cheetah. All five of the individuals had probably been attacked and killed by the carnivores, their carcasses dragged into the dens for the after-hunt feast, with nothing left but dinner scraps for curious fossil hunters.
Dr. White and other scientists not involved in the research hailed the importance of the skull discovery and its implications for understanding early Homo evolution. In an article analyzing the report, Science quoted Ian Tattersall of the American Museum of Natural History in New York as saying that the skull “is undoubtedly one of the most important ever discovered.”
A few scientists quibbled that the skull looks more like Homo habilis or questioned the idea that fossils in Africa all belong to Homo erectus, but there was broad recognition that the new findings were a watershed in the study of evolution. “As the most complete early Homo skull ever found,” Dr. White wrote in an e-mail, “it will become iconic for Dmanisi, for earliest Homo erectus and more broadly for how we became human.”
Dr. White, who has excavated hominid fossils in Ethiopia for years, said he was impressed with “the total evidentiary package from the site that is the really good news story here.” Further, he said, he hoped the discovery would “now focus the debate on evolutionary biology beyond the boring ‘lumpers vs. splitters' ” — a reference to the tendencies of fossil hunters to either lump new finds into existing species or split them off into new species.
In their report, the Dmanisi researchers said the Skull 5 individual “provides 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 that their limbs enabled them to walk long distances as fully upright bipeds. The shape of the small braincase distinguished them from the more primitive Australopithecus genus, which preceded Homo and lived for many centuries with Homo in Africa.