Skeletal remains found nearly a decade ago on the Indonesian island of Flores are not those of a previously undiscovered species of "hobbit"-like human, according to researchers at Pennsylvania State University.
A reanalysis of the remains has suggested they are actually indicative of Down syndrome, one of the most commonly-occurring developmental disorders in humans, the researchers said.
The fragmentary skeletons of several people, estimated to have lived as recently as 12,000 years ago, were found in October 2004 during an excavation of the Liang Bua Cave.
Only one set of remains, called LB1, had a skull and thighbones.
Researchers at the time found the skull's cranial volume suggested a brain size of less than one-third that of an average modern human. From the length of the thighbones, they estimated the creature would have stood just 3.5 feet tall.
They hypothesized the skeleton belonged to a newly found species of diminutive, stone-tool-using humans that evolved on Flores in isolation.
The researchers dubbed the species Homo floresiensis but informally referred to them as "hobbits," a reference to J.R.R. Tolkien trilogy The Lord of the Rings.
Scientists hailed the discovery as "the most important find in human evolution for 100 years" and described the Homo floresiensis population as "the most extreme human ever discovered."
But, Penn State developmental genetics and evolution professor Robert B. Eckhardt pointed out, "unusual does not equal unique."
"The originally reported traits are not so rare as to have required the invention of a new hominin species," Eckhardt said.
"When we first saw these bones, several of us immediately spotted a developmental disturbance, but we did not assign a specific diagnosis because the bones were so fragmentary," Eckhardt said. "Over the years, several lines of evidence have converged on Down syndrome."
The team found the cranial volume and stature of the LB1 specimen were initially underestimated.
When corrected statistically for normal growth, the thighbones of LB1 suggest a stature of just over four feet, a figure matched by some humans living now on Flores and in surrounding regions, researchers said.
They also pointed to a previously unpublished skull-circumference measurement, which they claim suggests a brain size in line with that of a modern, developmentally disordered human.
"The difference is significant, and the revised figure falls in the range predicted for a modern human with Down syndrome from the same geographic region," Eckhardt said.
Researchers further noted the LB1 skull's craniofacial asymmetry, a left-right mismatch that is characteristic of developmental disorders but was not reported by the excavating team and was later dismissed as a result of the skull's being long buried, according to the study's authors.
Finally, the team said, the "unusual" characteristics first interpreted to suggest a new species were present only in LB1, not in the other skeletal remains found in Liang Bua.
Eckhardt said the findings led them to conclude that the presence of a developmental disorder is a "less-strained explanation" for the differences than the discovery of an entirely new species.
"Here the signs point rather clearly to Down syndrome, which occurs in more than one per thousand human births around the world," Eckhardt said.