Ever since 2003, when researchers found the skeletal remains of a diminutive, human-like creature--dubbed the Hobbit--on an island in Indonesia, a debate has raged over whether the find represents a new species or a just deformed population of an existing species. Two papers appearing in Nature today--one addressing the shape of its feet and the other the size of its head--confirm that Homo floresiensis is in fact a separate species, but each posits slightly different evolutionary origins for this latest addition to the human family tree.

"Both of these papers show things that could not have evolved or been a plastic response within our own species," George Washington University paleoanthropologist Bernard Wood told The Scientist. Wood, who was not involved with either study, added that the papers raise important questions regarding the evolutionary origins of H. floresiensis that only further research can answer.

H. floresiensis was a hominin--the taxonomic group that includes genus Homo, extant chimp species, and their ancestors--that inhabited the island of Flores more than 10,000 years ago and likely stood about a meter tall. The current debate over the Hobbit's evolutionary origins centers around whether or not the species could have evolved its peculiar body proportions from a Homo erectus ancestor or if some more primitive human ancestor was necessary to explain its mixture of modern and primitive features. Researchers had previously taken the lack of a mechanistic explanation for the relative smallness of H. floresiensis's brain as support for the idea that the find represented a deformed, extant species.

A nearly 3000 year old dwarf hippopotamus skull from the central highlands of Madagascar. Image: Courtesy of the Natural History Museum, London

Such a mechanistic explanation is provided by the first paper, from mammalian paleontologists Eleanor Weston and Adrian Lister of London's Natural History Museum. Their study suggests that H. floresiensis did indeed result from the dwarfing of a population of H. erectus individuals. The two researchers studied the cranial morphology of two recently-extinct Malagasy hippopotamus species and presented a model that overturns the notion that dwarf mammals, stunted by evolutionary pressures on confined island habitats, cannot develop brains as proportionately small as those seen in H. floresiensis. (Hobbits had a 417 cm2 brain in a body that was about 30 kg.)

The fact that the two pygmy hippopotamus species experienced dwarfing that left them with significantly smaller brains than their normal-sized ancestors on the mainland shows these morphological changes could have occurred in some Homo erectus specimens to result in the brain-body size ratio seen in H. floresiensis.

"It's possible that humans have been exposed to the same evolutionary pressures that have caused these changes in other mammals," Weston told The Scientist. "At the end of the day this doesn't prove that Homo floresiensis is a dwarf Homo erectus. It just suggests that it's a possibility." She added that Homo habilis could also be the Hobbit's ancestor with her island dwarfing model accounting for the reduced brain size.

In the other Nature paper, William Jungers, a paleoanthropologist at Stony Brook University in New York, and his coauthors compared the Hobbit foot to the few existing feet in the fossil record. "You just don't see complete feet until you get into Neanderthal," Jungers told The Scientist. "The fossil record of feet is surprisingly meager."

The left foot and right tibia of H. floresiensis Image: Courtesy of William Jungers

The group found that its proportions were more similar to feet found in human ancestors that roamed Africa more than three million years ago than they are to the feet of more modern hominin species, such as Homo erectus. "Homo erectus has a modern foot in comparison to Homo floresiensis," Jungers said. Fossil footprints recently discovered in Africa suggest that H. erectus evolved modern foot architecture some 1.5 million years ago.

Specifically, the Hobbit had long feet compared to the length of its leg bones, according to Jungers, and lacked arches; two decidedly primitive Homo features. If H. floresiensis was in fact a dwarfed H. erectus, the species would have had to amass primitive features after its ancestor had already evolved more modern skeletal characteristics. "It's asking a lot for evolution to backtrack like that," Jungers said. "Is it possible? I guess, but there's no precedent."

Instead, Jungers said he believes that H. floresiensis's ancestor could have been a species--possibly H. habilis--that left Africa before H. erectus did around 2 million years ago. "There were probably other dispersal events in human history that we don't have a handle on," Jungers noted.

Wood agreed. "I think [H. floresiensis's ancestor] is probably likely to be an early Homo taxon other than Homo erectus," he told The Scientist. Which species that might be isn't clear, he said; the Hobbit's ancestor could have been an ancient human species that has yet to be discovered or classified.

Richard Potts, a Smithsonian Institution paleoanthropologist not involved with either study, added that Weston's findings indicate that island dwarfing, a poorly understood phenomenon, could yield yet more surprises. "Junger's argument does not cast away the idea that [H. floresiensis's ancestor] could have been an early Homo erectus," he told The Scientist. "There are some unexpected things that occur with island dwarfing. It opens up the possibility that there could be ways that the foot or other body proportions could back track."


Related stories:

The Hobbit: Human after all?
[19th May 2006]

Return of the Return of the Hobbit
[13th October 2005]

No microcephaly for Hobbit
[4th March 2005]