'Hobbit' wrist bones suggest a distinct species

Source: Bob Holmes, New Scientist

The tiny, human-like creature living and using tools in Indonesia just 18,000 years ago really was a distinct species, not just a malformed modern human.

That is the clear implication of a new study of the so-called "hobbit". It states that the creature had wrist bones almost identical to those found in early hominids and modern chimpanzees, and so must have diverged from the human lineage well before the origin of modern humans and Neanderthals.

Palaeontologists have battled bitterly over the diminutive skeleton ever since its discoverers described it as a new species, Homo floresiensis, three years ago.

Its exceptionally tiny brain simply did not fit with the current understanding of human evolution, particularly since no other hominid fossils in the last 3 million years, and none outside of Africa, had such a small brain.

This dissonance led sceptics to argue that the hobbit must be merely a modern human with some form of microcephaly a disorder which causes an abnormally small head and brain.

Most of the argument has centered on the skull, even though much of the rest of the skeleton was excavated at the same time. When Matthew Tocheri, a palaeoanthropologist at the Smithsonian Institution in Washington, DC, happened by chance to see casts of the specimen's wrist bones, he knew they told an important story.

Ancient structure

Tocheri, an expert in the evolution of the human wrist, could see immediately that the hobbit's wrist bones looked just like those of a chimpanzee, or an early hominid such as Australopithecus, and had none of the specialisations for grasping that are seen in the wrist bones of modern humans. A careful statistical comparison gave the same conclusion.

"The modern human wrist hasn't looked like this for at least 800,000 years, and maybe much longer," says Tocheri. "It was immediately apparent to me that the hobbit is the real deal."

Developmental abnormalities such as microcephaly are unlikely to change the shape of the wrist bones, he adds, because the shape of these bones is laid down very early in development, long before the genes controlling growth rates become active.

Other soon-to-be-published studies of the hobbit's foot bones, upper arm and shoulder also suggest that its anatomy differed from that of modern humans. "As you add these up, you do certainly get a picture of something distinct and not human," says Chris Stringer, an authority on human evolution at the Natural History Museum in London, UK.

However, Robert Martin, a primatologist at the Field Museum in Chicago, US, who is the leading advocate of the microcephaly explanation, remains unconvinced. No one has studied the wrist bones of microcephalic humans, he notes, so it is pure conjecture to say they would not look like the hobbit's bones.

Journal reference: Science (DOI: 10.1126/science.1147143)