A new study contradicts the claim that LB1, the type specimen of Homo floresiensis, had Down syndrome, and further confirms its status as a valid and distinct species of the genus Homo.
Reconstruction of Homo floresiensis. Image credit: Elisabeth Daynes.
The name Homo floresiensis was invented for nine fragmentary skeletons of small-bodied hominins found in the Liang Bua Cave on the Indonesian island of Flores in 2003.
Among the specimens discovered were an almost complete skull, labeled LB1, and a partial skeleton.
LB1 was an adult of about 30, probably female, and had unusual anatomical characteristics – a cranial volume reported as only 380 cubic cm, suggesting a brain less than one third the size of an average modern human’s and short thigh bones, which were used to reconstruct a creature standing 109 cm tall.
In 2014 a team of scientists led by Prof. Robert Eckhardt of Pennsylvania State University diagnosed LB1 with Down syndrome.
A new analysis by Dr. Karen Baab of Midwestern University and co-authors demonstrates that LB1 did not have Down syndrome.
The team compared physical traits preserved in the skeleton of LB1 to those found in Down syndrome.
While people with Down syndrome are not identical to one another, it was nevertheless clear that LB1 was very distinct from all humans, including those with Down syndrome.
The study found that LB1’s brain was much smaller than that seen in Down syndrome individuals.
Likewise, the shape of the skull vault, which surrounds the brain, and chin anatomy were both outside the range seen in humans, with or without Down syndrome.
Moreover, the LB1 individual was well below the height range of comparable individuals with Down syndrome.
In fact, females with Down syndrome from Turkey reach a comparable height as the adult LB1 by 6.5 years of age and are considerably taller as adults (1.45 m).
The femur is disproportionately short in LB1 relative to the feet and arms compared to all humans, regardless of whether they have Down syndrome.
Importantly, this study indicated that LB1 not only differed from individuals with Down syndrome, but was more clearly aligned with more archaic human species.
Its small brain, low cranial vault shape, absence of a chin, smaller body size and limb proportions all point to a pre-Homo sapiens ancestry.
“The skeletal evidence overwhelmingly contradicts a diagnosis of Down syndrome,” Dr. Baab and co-authors said.
“Rather, our study is yet further evidence that Homo floresiensis was a distinct species with a fascinating, if somewhat nebulous, evolutionary history.”
The results are reported in a paper published online by the journal PLoS ONE on June 8.
_____
Baab K.L. et al. 2016. A Critical Evaluation of the Down Syndrome Diagnosis for LB1, Type Specimen of Homo floresiensis. PLoS ONE 11 (6): e0155731; doi: 10.1371/journal.pone.0155731
