Researchers have discovered the bones of tiny Homo sapiens on a small island in the Micronesian nation of Palau. The find shows that humans can shrink to small sizes on islands, the discoverers claim, and strengthens the possibility that so-called "hobbits" found in Indonesia (ScienceNOW, 5 March 2008) may be members of our own species. But some experts are skeptical, saying that the bones represent nothing more than a small-sized clan of Palauans.
Paleoanthropologist Lee Berger of the University of Witwatersrand in Johannesburg discovered the bones while on vacation in Palau in 2006. (His wife had deliberately chosen a remote, young island because it was unlikely to have fossils on it). While kayaking among small rocky islands off the coast of the island of Babeldaob, a guide took them to a cave full of bones. Astonished by seeing a very small partial hominid face, Berger returned with a team and excavated in two caves.
The researchers found rich bone beds, apparently burial grounds, with thousands of fragments of human bones jumbled together. "In the one small geological trench, we recovered more than 1200 human fragments and some more complete pieces in a meter square that was only 50 centimeters deep," says Berger. Radiocarbon dates put the small bones at about 1400 to 3000 years old, although other, larger bones were younger, the team reports in this week's issue of PLoS ONE.
Details of the dentition, jaw, and shape of the skull help place the bones within our own species, H. sapiens. Yet the Palauans were much smaller than most of us: Berger estimated from part of the pelvis and femur bones that they weighed between 30 kilograms (for females) and 47 kilograms (for males), about the same size or smaller than the weights of pygmies in the region. Hobbits from the Indonesian island of Flores--which were placed into a distinct species, H. floresiensis--were apparently even smaller, however: They were estimated at only 16 to 36 kilograms, using other methods.
Berger hypothesizes that the Palauans underwent a process known from other animals called island dwarfing, in which species isolated on small islands in time evolve a diminutive size in order to accommodate reduced resources. The only other putative example of island dwarfing in humans is that of the hobbits, dated from 18,000 to 95,000 years ago. Human pygmies are found on mainlands as well as on some islands and aren't generally considered an example of island dwarfing.
The Palauan bones also have traits usually considered primitive for our species, including a reduced chin, large teeth, a ridge of bone at the eyebrow, and small orbits, says Berger. The Flores bones have similar traits. Finding these same traits in small H. sapiens on Palau "reduces the importance of these characters in describing Flores as unique," says Berger. He can't estimate the Palauans' brain size yet--the best specimen is still encased in sediment--but he guesses that it will be smaller than that of most humans but not as tiny as the chimp-sized brain of H. floresiensis. A diseased member of the Palauan population might look as anomalous as the single skull of H. floresiensis, he says. If the analyses are confirmed, the Palau sample may be the smallest population of H. sapiens ever described, says paleoanthropologist Christoph Zollikofer of the University of Zurich, Switzerland. "They are dealing with what appears to be a population, not just some 'freaks,' as is often argued in the Flores debate," he says.
But archaeologist Scott Fitzpatrick of North Carolina State University in Raleigh, who has worked in Palau for a decade, says he doesn't think the bone beds represent a true population. In a site only 4 kilometers from Berger's caves, he has excavated the burials of Palauans of similar age--and normal stature. That would seem to rule out isolation and island dwarfing, he says. "It would be very unusual to have a group of people living in close contact with a normal size population who evolved to be smaller." Instead, "the most parsimonious explanation is that they were Palauans with a genetic anomaly leading to small people who were buried in a clan or family plot."
Those studying H. floresiensis say the find is not relevant to their work. For example, relatively big teeth are known from living pygmies and are not considered diagnostic in H. floresiensis, says William Jungers of Stony Brook University School of Medicine in New York state. He adds that for some of the bones, such as an ankle bone, Berger hasn't ruled out that it is a child's bone. And he says there are living Filipino pygmies in the region, so that small people in Palau are not surprising and need not be explained by island dwarfing. "There's nothing new about these little guys; … this offers zero window into Homo floresiensis," he says.