New analysis of a 15,000-year-old skull and thigh bone shows the original descriptions of the remains may have been skewed. Instead of the abnormalities marking a new species, the bones more likely indicate Down syndrome, researchers say.
In October 2004, excavation of fragmentary skeletal remains from the island of Flores in Indonesia yielded what was called “the most important find in human evolution for 100 years.” Its discoverers dubbed the find Homo floresiensis, a name suggesting a previously unknown species of human.
Now, detailed reanalysis by an international team of researchers including Robert B. Eckhardt, professor of developmental genetics and evolution at Penn State, suggests that the single specimen on which the new designation depends, known as LB1, does not represent a new species.
Instead, it is the skeleton of a developmentally abnormal human and, according to the researchers, contains important features most consistent with a diagnosis of Down syndrome.
“The skeletal sample from Liang Bua cave contains fragmentary remains of several individuals,” Eckhardt says. “LB1 has the only skull and thighbones in the entire sample.”
No substantial new bone discoveries have been made in the cave since the finding of LB1.
Initial descriptions of Homo floresiensis focused on LB1’s unusual anatomical characteristics: a cranial volume reported as only 380 milliliters (23.2 cubic inches), suggesting a brain less than one third the size of an average modern human’s and short thighbones, which were used to reconstruct a creature standing 1.06 meters (about 3.5 feet tall).
Although LB1 lived only 15,000 years ago, comparisons were made to earlier hominins, including Homo erectus and Australopithecus. Other traits were characterized as unique and therefore indicative of a new species.
A thorough reexamination of the available evidence in the context of clinical studies, the researchers says, suggests a different explanation.
Reevaluation of the bones
In the first place, they write, the original figures for cranial volume and stature are underestimates, “markedly lower than any later attempts to confirm them.” Eckhardt, Henneberg, and other researchers have consistently found a cranial volume of about 430 milliliters (26.2 cubic inches).
“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 says.
The original estimate of 3.5 feet for the creature’s height was based on extrapolation combining the short thighbone with a formula derived from an African pygmy population. But humans with Down syndrome also have diagnostically short thighbones, Eckhardt says.
‘Unusual does not equal unique’
Though these and other features are unusual, he acknowledged, “unusual does not equal unique. The originally reported traits are not so rare as to have required the invention of a new hominin species.”
Instead, the researchers build the case for an alternative diagnosis: Down syndrome, one of the most commonly occurring developmental disorders in modern humans.
“When we first saw these bones, several of us immediately spotted a developmental disturbance,” says Eckhardt, “but we did not assign a specific diagnosis because the bones were so fragmentary. Over the years, several lines of evidence have converged on Down syndrome.”
Evidence leading to diagnosis
The first indicator is craniofacial asymmetry, a left-right mismatch of the skull that is characteristic of this and other disorders. Eckhardt and colleagues noted this asymmetry in LB1 as early as 2006, but it had not been reported by the excavating team and was later dismissed as a result of the skull’s being long buried, he says.
A previously unpublished measurement of LB1’s occipital-frontal circumference—the circumference of the skull taken roughly above the tops of the ears—allowed the researchers to compare LB1 to clinical data routinely collected on patients with developmental disorders. Here too, the brain size they estimate is within the range expected for an Australomelanesian human with Down syndrome.
LB1’s short thighbones not only match the height reduction seen in Down syndrome, Eckhardt says, but when corrected statistically for normal growth, they would yield a stature of about 1.26 meters, or just over four feet, a figure matched by some humans now living on Flores and in surrounding regions.
These and other Down-like characteristics, the researchers state, are present only in LB1, and not in the other Liang Bua skeletal remains, further evidence of LB1’s abnormality.
“This work is not presented in the form of a fanciful story, but to test a hypothesis: are the skeletons from Liang Bua cave sufficiently unusual to require invention of a new human species?” Eckhardt says.
“Our reanalysis shows that they are not. The less strained explanation is a developmental disorder. Here the signs point rather clearly to Down syndrome, which occurs in more than one per thousand human births around the world.”
Source: Penn State