The problem, according to Martin, is that while the body size of a large species shrinks considerably in cases of island dwarfism, brain size shrinks moderately.
Based on models of dwarfing, the Flores skull is too small for its 3.3-foot-tall (1-meter-tall) skeleton, he says.
For hobbits to be dwarves of H. erectus, they would have to have stood just a foot (a third of a meter) tall and weighed only four pounds (two kilograms).
Instead, Martin and colleagues suggest that the small brain resulted from microcephaly.
Since the disease is genetic, it runs in families. "So it wouldn't be too surprising in a small, isolated island population to find a number of cases cropping up," Martin said.
In addition, Martin says, the hobbit remains were found near advanced stone tools thought to have been made by modern humans only.
"So there is a mismatch between tiny brains and sophisticated stone tools," he said.
Martin and colleagues direct their argument to a paper published in the March 4, 2005, issue of Science by Dean Falk, an anthropologist at Florida State University in Tallahassee, and colleagues. (Read "'Hobbit' Brains Were Small but Smart, Study Says.")
Martin's team says that Falk's team made a mistake in the way they ruled out microcephaly.
The disease has dozens of different forms, Martin says. But Falk and colleagues only compare the Flores fossil to one poorly matched microcephalic skull of a modern human.
Martin's team, by contrast, identified other microcephalic skulls that more closely resemble the Flores fossil skulls, he says.
Falk acknowledges that her team only examined one skull. But she adds that the evidence that Martin's team's skulls are better matched is poorly illustrated in Martin's paper.
Regardless, Falk adds, her team is finishing up an in-depth analysis on microcephaly.
"We're confident that [the hobbit skull] is not a microcephalic," she said.
Also, Falk and her colleagues noted in their original paper that normal dwarfing of H. erectus could not explain the Flores fossils.
Rather, they suggested the hobbits resulted from dwarfing of apes or australopithecines, earlier human ancestors.
"I don't understand why they argue about it," she said. "We were the first to point out" that the hobbit cannot be an H. erectus dwarf.
As for the stone tools, Falk says she is unqualified to comment, though she agrees with the suggestions of other scientists that the hobbits could have developed sophisticated tools.
Richard Potts is the director of the Human Origins Program at the Smithsonian Institution in Washington, D.C. He was not involved with either of the teams.
Potts says Martin and colleagues are primarily reacting to the original interpretation of the hobbit find, published in 2004 in the journal Nature.
That study said that the Flores fossils represent island dwarfing in H. erectus and not dwarfing of an ape or australopithecine.
"So what would island dwarfing in an ape look like?" Potts asked. "We don't knowthat's one of the big gaps of this whole thing."
In addition, Potts says, Martin and colleagues' suggestion that the Flores skull represents a microcephalic modern human is unsupported by recent studies on leg and shoulder fossils from Flores that suggest similarities to earlier human ancestors.
"We're dealing with something unprecedented in modern humans," Potts said.
"[The hobbit is] either a representative of a unique and unreported range of variation in a modern human, or it's a new species that seems to be derived from an earlier ancestor.
"That second idea is more in line with the original interpretation and probably the safest at this stage," he continued.
"But it's a wonderful mystery."
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