Contributed by Kenneth J. Hsü, May 14, 2014 (sent for review November 21, 2013; reviewed by Alan G. Fix, Robert G. Bednarik, and Judith Hall)
The population that has become known as Homo floresiensis has been described as “the most extreme human ever discovered.” Specimen LB1 from Liang Bua Cave is unusual, but craniofacial and postcranial characteristics originally said to be diagnostic of the new species are not evident in the other more fragmentary skeletons in the sample that resemble other recent small-bodied human populations in the region (including the Andaman Islands, Palau, and Flores itself). Here we demonstrate that the facial asymmetry, small endocranial volume, brachycephaly, disproportionately short femora, flat feet, and numerous other characteristics of LB1 are highly diagnostic of Down syndrome, one of the most commonly occurring developmental disorders in humans and also documented in related hominoids such as chimpanzees and orangutans.
Human skeletons from Liang Bua Cave, Flores, Indonesia, are coeval with only Homo sapiens populations worldwide and no other previously known hominins. We report here for the first time to our knowledge the occipitofrontal circumference of specimen LB1. This datum makes it possible to link the 430-mL endocranial volume of LB1 reported by us previously, later confirmed independently by other investigators, not only with other human skeletal samples past and present but also with a large body of clinical data routinely collected on patients with developmental disorders. Our analyses show that the brain size of LB1 is in the range predicted for an individual with Down syndrome (DS) in a normal small-bodied population from the geographic region that includes Flores. Among additional diagnostic signs of DS and other skeletal dysplasiae are abnormally short femora combined with disproportionate flat feet. Liang Bua Cave femora, known only for LB1, match interlimb proportions for DS. Predictions based on corrected LB1 femur lengths show a stature normal for other H. sapiens populations in the region.
Author contributions: M.H., R.B.E., and K.J.H. designed research; M.H., R.B.E., S.C., and K.J.H. performed research; M.H., R.B.E., and K.J.H. analyzed data; and M.H., R.B.E., and K.J.H. wrote the paper.
Reviewers: A.G.F., University of California, Riverside; R.G.B., International Federation of Rock Art Organisations; and J.H., University of British Columbia.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1407382111/-/DCSupplemental.
Freely available online through the PNAS open access option.
