The bones of "small-bodied people" who lived on a remote island in the Pacific Ocean have been discovered by scientists who believe they may represent an unusual branch of the human family that grew progressively smaller.
A team of palaeontologists discovered the remains - including a diminutive skull - in rock caves on the Micronesian island of Palau, about 600km from the Philippines to the west and Papua New Guinea to the south.
One theory is that the exceptionally short humans - who grew no taller than 1.2m - had undergone an evolutionary adaptation known as "island dwarfism" in response to living on a remote island where there were no predators but little to eat.
Scientists said bones belonging to 26 individuals had been excavated from two caves 14.5km apart and that radio-carbon dating suggested they lived on the island between 1000 and 3000 years ago.
However, the scientists believe the bones could belong to the descendants of the first colonisers of the islands, who could have arrived 4000 years ago. These colonisers may have been smaller than average individuals who, as a result of inbreeding and other evolutionary factors, produced an even smaller population.
Lee Berger, of the University of Witwatersrand in Johannesburg, who led the research team, said the bones, which included pelvis, teeth and a nearly complete skull, suggested they were from a rare group.
"They weren't very typical, very small in fact ... I felt right away this was a minute human being," said Dr Berger, whose study is published in the online journal Public Library of Science.
The scientists say in their report that it was difficult to assess the brain size of the people because the only complete skull was heavily encased in flowstone, a rock-like substance that forms inside the caves.
"Nevertheless, it is clear that the brain size is small, possibly at the very low end or below that typically observed in modern small-bodied humans," it said. Preliminary work suggests the brains will almost certainly fall below the size of 147 modern human specimens used for comparison.
The find is reminiscent of the discovery in 2004 of a group of humans, nicknamed "hobbits", in Indonesia. Their most unusual feature was a small head with a grapefruit-sized brain, which was in perfect proportion to a tiny body about 91cm tall.
Many scientists who studied these bones believe they are so unusual that they should be classed as a distinct human species called Homo floresiensis, or "hobbit". Others suggested the skull belonged to someone suffering from microcephaly, a congenital disorder where the braincase does not grow normally.
Dr Berger believes the diminutive people of Palau belong to the Homo sapien species, even though they have undergone some evolutionary change. "We feel that the most parsimonious, and most reasonable, interpretation ... is that they derive from a small-bodied population of Homo sapiens," the researchers said.
Other experts are sceptical of the dwarf theory and say modern Polynesian islanders show no such trends towards becoming smaller. They say the bones may belong to children buried together - a practice seen on nearby islands.