Skull find points to single, evolving human species
Revolutionary discovery of skull 1.8 million years old, most complete yet seen, suggests man is one species with individual differences
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 the human genus known as Homo.
The research suggests that early man was a single, evolving species with a wide range of appearances, and not a wide range of different species as currently thought.
Diverse fossils currently recognised as coming from distinct species like Homo habilis, Homo erectus and others may represent variation among members of a single 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 palaeoanthropologist 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, ape-like 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 the physical traits of ancestors that apparently lived at the same location and around the same time.
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, 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 Leon, conducted the comparative analysis of the Dmanisi specimens.
"Since we see a similar pattern and range of variation in the African fossil record," 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".
But what species? Some team members simply call their finds "early Homo". Others emphasised the strong similarities to Homo erectus, which lived between two million and less than one million years ago. Tim White, a palaeoanthropologist at the University of California, Berkeley, called it "the most primitive H. erectus yet known".
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 sabre-toothed cats and an extinct giant cheetah. All five of the individuals had probably been attacked and killed by the carnivores.
A few scientists quibbled that Skull 5 looked more like Homo habilis or questioned the idea that fossils in Africa all belonged 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," White wrote, "it will become iconic for Dmanisi, for earliest Homo erectus and more broadly for how we became human."