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Human Evolution
Hobbit or not: All in the wrists
As the debate rages over whether the 18,000-year-old hominin bones discovered in 2004 on the Indonesian island of Flores are those of a new species or a diseased modern human, a new study is focusing on the wrist instead of the skull. Detailed 3-D analyses of the wrist bones of the hominin, dubbed Homo floresiensis, or the “hobbit,” by those who consider it a new species, suggest that its wrists are shaped more like primitive hominins and African apes than like modern humans. This, the authors say, provides the long-looked-for “smoking gun” that proves the hobbit is a new species.
The research, led by Matthew Tocheri of the Smithsonian National Museum of Natural History in Washington, D.C., came about as an extension of dissertation work he had already been doing on the wrist bones of primitive hominins. At issue is the shape of several different wrist bones, particularly a central bone called the trapezoid, which in African apes and older hominins like Australopithecus appears more wedge-shaped and in modern humans and Neanderthals is shaped more like a boot, as well as the configuration of the other wrist bones around the trapezoid. In older hominins, including the stone-tool-making Homo habilis, the wrist configuration is more like that of an African ape, with the thumb more in the front of the hand, while in modern humans and Neanderthals the thumb is more to the side of the hand.
“What I’m showing here is nothing different from what anatomists have been saying for 50 years,” Tocheri says. Some anthropologists have considered wrist bones to be too complex to be used to indicate species status, and although empirical differences have been noted between different species’ wrists, “people who don’t study these bones never really take it that seriously,” he says. “They look at them and say they all look odd.”
To find a way to quantify those differences more precisely, therefore, Tocheri laser-scanned the wrist bones in 3-D, getting precise shapes and sizes of hundreds of bones from apes, modern humans and Neanderthals. From that, he says, he could obtain quantifiable data that would not only show the differences visually, but also “show how, statistically, there’s very little overlap in the trapezoid shape.”
Taking a look at the “hobbit” wrist bones to see where they fit into this picture, therefore, was a logical step to take, Tocheri says. After performing the same scanning of the wrists, Tocheri and his colleagues found that they closely resembled the more primitively shaped wrists of older hominins and African apes, they reported Sept. 21 in Science. “When I first saw this specimen, if no one had told me what it was I was looking at, I would have told them they had a small chimpanzee,” he says.
Some researchers, including Dean Falk, a paleoanthropologist at Florida State University in Tallahassee, find Tocheri’s study to be quite convincing. “I think they’ve demonstrated sufficiently and convincingly that the wrists have these three little bones that retain primitive features, and they’ve addressed the question of whether or not they could be pathological,” Falk says.
The new paper is hardly a smoking gun for the primatologists and evolutionary biologists who dispute that the hobbit is a new species, however. For one thing, says Robert Martin, a primatologist at the Field Museum of Chicago, the hobbit bones were found along with sophisticated stone tools that could not have been made by anything other than Homo sapiens. Another question centers around the hobbit’s small braincase, which Martin and others say is more likely to be the result of a genetic disorder causing microcephaly (abnormally small brain size) or some other disorder, rather than evidence that it is a separate species. “It simply clashes with everything else I know about the brain size of hominids,” Martin says. “That is unaffected by this article — nobody is responding to the point that the brain size falls off the graph.”
Tocheri and colleagues say that microcephaly would not affect wrist bones, which form very early during the embryonic state, at around seven to 10 weeks. Martin discounts this, however, noting that similar developmental syndromes in human children match many of the features of the hobbit, particularly below the neck.
What it will take to finally settle the debate is unclear, and both sides exhibit frustration. Finding multiple skulls and skeletons might help, researchers say. But even finding a second skull that exhibits a similar small braincase might not end the controversy, says Robert Eckhardt, an evolutionary biologist at Pennsylvania State University in University Park. For example, medical literature describes families of microcephalics, he says. Thus, he says, such a find would not even be definitive.
“Whenever there have been major discoveries — australopithecines, Neanderthals — inevitably, there’s a group of people that says, ‘Oh no, it’s not a missing link, it’s not a new species, it’s pathological, it’s an ape,’” Falk says. “And that’s happening again.”
