Scholars of human evolution often don’t have much to go on. It’s rare that prehistoric bones are preserved as fossils, and much rarer still that paleo-anthropologists find anything approaching the complete skeleton of any individual. Newly discovered specimens are often categorized based only on a single surviving bone—and if that sounds like a sketchy exercise, it is. You can easily imagine a scientist of the future digging up the arm bone of a jockey and the leg bone of an NBA center and deciding they belong to two entirely different species.
That’s why a new discovery reported in the latest Science is so important. A team of investigators has uncovered the exquisitely preserved skull of a hominid dating back some 1.8 million years at a site called Dimanisi, in the Republic of Georgia, in Central Asia. And that artifact raises doubts about the generally accepted idea that several human species populated the Earth at that time.
“We have one global human species today,” said Christoph Zollikofer, of the Anthropological Institute and Museum in Zurich, Switzerland, a co-author of the Science report, at a press conference. “And what we can infer from our study is that 1.8 million years ago there was another [single] global human species.”
This flies in the face of conventional wisdom in the field of human evolution. Experts know that at various points in our history, multiple human species have walked the Earth together, including as recently as 30,000 years ago, when modern humans and Neanderthals co-existed—and maybe even cuddled—in parts of Europe. (There might even have been a third species hiding out at the time in parts of Indonesia.) Back in 1.8 million B.C., there may have been multiple species as well, Homo erectus, Homo ergaster and Homo rudolfensis among them.
Or maybe not: the Dimanisi site has proven so rich that Zollikofer and his co-authors had not the newly announced skull to work with, but four others as well. What’s more, the five skulls were found close together physically, and they were all deposited within a few hundred years of each other in what had been a cave, now collapsed. “It’s the most complete collection of hominid fossils from any site of this age,” said lead author David Lordkipanidze, of the Georgian National Museum in Tbilisi. Along with thousands of other plant and animal remains found at the site, “it’s a real snapshot in time; it preserves the whole ecosystem.
The proximity in space and time makes it very likely that all five individuals belonged to the same population, although not necessarily the same generation. (“It would be a beautiful story if they all were from one family,” said Lordkipanidze, “but maybe too beautiful.”) And that in turn allowed the scientists to measure variations from one individual to the next—particularly since they weren’t limited to fragments that would leave them studying the jawbone of one skull and the braincase of another. In fact, the lower jaw of the new skeleton was found first, back in 2000; the rest of the skull emerged in 2005. “We are convinced,” said Zollikofer, “that if the pieces had been found in different places they would have been attributed to different species.”
That’s no surprise, said co-author Marcia Ponce de Leon, also of Zurich’s Anthropological Institute. “The braincase is very unexpectedly small, around a third of that of modern humans. At the same time, the face is quite large and the jaws are quite massive. It’s a fresh combination of features we didn’t know before in early Homo.” But the jaw fits the skull perfectly, and when a variety of characteristics from all five individuals were compared, the result, said Zollikofer, was that “they’re no more different from each other than five randomly chosen humans would be, or five chimps, or five bonbos.”
This has implications for other parts of the fossil record. The differences among the specimens at Dimanisi are no greater than those among some collections of African fossils that were assumed to have come from different species. But if the new remains represent a single species, the earlier ones might too. Not everyone agrees: paleo-anthropologist Ian Tattersall, for example, of New York’s American Museum of Natural History, told Science for an accompanying news story that he remains unconvinced of the conclusions the authors draw about the Dimanisi fossils, and others are equally wary.
But everyone agrees that the new, complete skull is an extraordinary find. It is, Tattersall told Science, “undoubtedly one of the most important ever discovered,” while Tim White, of the University of California, Berkeley called it “iconic.” And there’s likely more to come: the Dimanisi site covers some 540,000 sq. ft. (50,000 sq. m), only a fraction of which has yet been excavated. By the time Lordkipanidze and his colleagues are done, at least one chapter in the story of human evolution could be completely rewritten.