The broad face, large teeth, and snout-like skull of an exquisitely preserved 1.8-million-year-old skull discovered near a medieval town at the crossroads of Europe and Asia, challenges our fundamental understanding of modern human origins, according to a detailed analysis of the fossil unveiled Thursday by an international team of scientists.
The prevailing view of early human evolution holds that we had a panoply of primitive predecessors, which scientists identified by examining fossil remnants with varied features: Homo erectus, Homo rudolfensis, Homo habilis. The new early human skull, unprecedented in how remarkably intact and old it is, blurs those boundaries, said the team, which includes a Harvard researcher. The skull’s mosaic of features challenges the idea that those previous specimens are each a different species.
The skull was found in Dmanisi, Georgia, near four others from roughly the same time period that incorporate a broad spectrum of bone structure, bolstering the argument that scholars may have underestimated the natural diversity that existed within a single species of early humans.
“This calls into question what has been in paleonathropology a standard model for a long time,” said G. Philip Rightmire, a paleoanthropologist at Harvard University and a member of the team that published the provocative finding in the journal Science. “Perhaps that thinking that has become so standard over the last 25 to 30 years is, really, to some extent a house of cards that is now going to collapse.”
Rightmire began traveling to the country of Georgia more than a decade ago, where some of the oldest human fossils outside of Africa have been dug up. He worked in the field and examined specimens in person at a museum where excavated fossils are housed. He and colleagues based their provocative argument on an analysis that incorporates sophisticated medical imaging and statistical analysis of the skull’s shape, but he said his own study was largely informed by more traditional methods.
“Nothing quite beats sitting with the original fossil on a table top in front of you, being able to move it or turn it over, and check over the fine morphological detail,” Rightmire said in an interview.
Scientists not involved in the work said the find was extremely important and the mixture of characteristics in its face and brain case was fascinating. The find demonstrates that while species are usually seen as textbook categories, each with different hallmark features, evolution is more nuanced and changes occur gradually. But many scholars remain unpersuaded that there was just one species of early human.
“This sounds plausible, and it would be good because it’s kind of spring cleaning—all these people who gave a new name to every new fossil, that’s something of the last century,” Fred Spoor, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, wrote in an e-mail. “Nevertheless, I must admit that I’m not convinced by their arguments ... because the methods they used are not adequate to address the question they ask.”
Rightmire and his colleagues do not believe the five individuals at Dmanisi represent a family; the fossils were found along with many carnivore bones, so one possibility is that the early humans were just unlucky, found together because they were prey. But the scientists believe the bones represent a single species and were deposited around the same time—perhaps within centuries, a blink of an eye in evolutionary time scales.
What intrigued them about the last skull, called Skull 5, were very primitive characteristics of prehuman ancestors, combined with distinct hallmarks of Homo erectus, an early human that emerged about 2 million years ago whose remains have been found in Europe and Asia. The face, Rightmire said, is strikingly large, with a wide nose and flaring, thickened cheek bones. Its mouth protrudes, almost like a muzzle, quite unlike modern humans. Its brain is also small—about a third of the size of a modern human brain—but the skull and the shape of the back of the skull resemble that of Homo erectus.
Jeremy DeSilva, a paleoanthropologist at Boston University, said that classifying fossils is often difficult because of the enormous differences that can exist between individuals—something he refers to in his classes as the “Shaquille O’Neal/Danny DeVito effect.”
DeSilva said the strength of the new find is the number of individuals from the same time and species found together; at other sites, fossils have been found in isolation. He said that he leans toward agreeing with the authors’ interpretation, but that more evidence will be needed.
“There is no question that this interpretation will be challenged by those who suspect there might be two different species of early Homo co-existing,” DeSilva wrote in an e-mail. “We’ll see as more fossils are unearthed.”
Adam Van Arsdale, a paleoanthropologist from Wellesley College who has worked at the Dmanisi site, argued in a paper published earlier this year in the journal Evolution that it was time to rethink whether early Homo species were really different.
He argues that there may be less variation among humans today because of our ability to use change and adapt the environment using technology, which may bias our expectation of how much variation to expect when trying to understand a fragmentary fossil record from two million years ago.
“If you look around Boston today and look at the way people differ, the two biggest ways they differ is how old they are and if they’re male or female—so what we have in Dmanisi in the five relatively complete skulls is individuals who vary in age from late adolescent to older adult, and we have individuals who vary in sex,” Van Arsdale said. “I agree that we’ve been naming too many species in East Africa, because I don’t think we’ve been appreciating the full spectrum.”Carolyn Y. Johnson can be reached at firstname.lastname@example.org. Follow her on Twitter @carolynyjohnson.