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Wrist bones bolster hobbit status

September 20, 2007 By Michael Hopkin This article courtesy of Nature News.

Ape-like wrists suggest that was a distinct species.

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More evidence has emerged supporting the theory that the 'hobbits', which lived on the remote Indonesian island of Flores tens of thousands of years ago, were indeed a unique species.

Painstaking study of Homo floresiensis wrist bones shows that their wrists were far more primitive than ours — suggesting that they were evolutionarily distinct from modern humans. The hobbits' wrists are so primitive-looking, say the researchers, that tracing our shared heritage would involve going back millions of years, perhaps to very birth of the genus Homo in Africa.

Since H. floresiensis was first shown to the world in 2004, the debate has raged over whether the hobbits were a genuinely distinct species or merely dwarf humans.

Sceptics argue that the hobbits' grapefruit-sized skulls are simply too small to have accommodated the tool-making intelligence attributed to them. They claim the tools must have been made by normal Homo sapiens, and the 'hobbit' bones are from individuals suffering from a condition called microcephaly (see 'Old tools shed light on hobbit origins' ).

All in the hand

Palaeontologists led by Matthew Tocheri of the National Museum of Natural History in Washington DC decided to take a different tack by studying the wrist bones of LB1, the first and most complete skeleton to be unearthed when the fossils were discovered in Liang Bua cave in 2003.

They made detailed measurements of three types of wrist bone — called the trapezoid, scaphoid and capitate — and compared them with those of modern humans, Neanderthals, living great apes such as chimps and gorillas, and more ancient human ancestors of the Australopithecus genus.

The hobbit wrists were far more similar to those of apes and primitive human ancestors than to those of modern humans and our recent relatives such as Neanderthals, Tocheri and his colleagues report in Science1. The wrist bones also do not resemble bones from modern humans affected with dwarfism, they claim.

The ape-like trapezoid bones of the hobbits, for example, are tapered and wedge-shaped, in contrast to the 'boot-shaped' trapezoids of modern humans, Tocheri and his colleagues explain (see picture).

But wrist bones might only be able to tell us a limited amount about the hobbits, says Daniel Lieberman, a biological anthropologist at Harvard University in Cambridge, Massachusetts. "I think [the researchers] do a fine job of showing that the wrist bones were more primitive," he says. "The $64,000 question is whether or not this gives information about what the hobbit is."

Lieberman, who says he remains "truly agnostic" about whether the hobbits were a unique species or microcephalic human, adds that there are many different forms of microcephaly that could potentially produce a range of different effects on different bones.

Robert Martin, an anthropologist at the Field Museum in Chicago, who is sceptical that the hobbits were a unique species, adds that the analysis did not include data from true microcephalics or from modern human pygmies. "I'm surprised the reviewers [of the research manuscript] didn't insist on that," he says.

More generally, Lieberman says that the investigation of the hobbit question has been hampered by the factionalization that has divided the field into two increasingly hostile camps. Some of the Liang Bua fossils have been the subject of bizarre episodes in which researchers with opposing views have jealously withheld fossils from one another and refused to release them for study. "It's no way to do science," Lieberman reflects.

Early ancestor

The discovery suggests that the most recent ancestor we share with the hobbits lived more than 800,000 years ago — the earliest known incidence of the 'modern' style of wrist bones, which seem to allow for greater control when making and manipulating tools. This innovation is thought to have happened between 800,000 and 1.8 million years ago, Tocheri says.

If so, that means the common ancestor of H. sapiens and H. floresiensis likely existed well over a million years ago, says Tocheri, and almost certainly lived in Africa.

It is difficult to draw any firmer conclusions than that, says Tocheri, because the African fossil record from this time lacks the wrist bones necessary to investigate the theory further. The prime candidate for this common ancestor is Homo erectus — but despite being an otherwise well-documented species, no wrist bones have been found.

The first H. erectus wrist bone to be found will provide a crucial test for the theory, says Lieberman. If Tocheri's theory is correct, it will be expected to be primitive. If it turned out to look more advanced, then adherents to the wrist theory would be forced to assume that the lineages diverged even earlier than the time of H. erectus.

That would be an unlikely-sounding assumption, given that other aspects of the hobbits' skeletons look so similar to ours. "You would have to bend over backwards and make all sorts of contrived arguments," Lieberman says.

References

  1. Tocheri, M.W., et al. Science 317, 1743 - 1745 (2007).

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