A Critical Evaluation of the Down Syndrome Diagnosis for LB1, Type Specimen of Homo floresiensis
Karen L. Baab , Peter Brown, Dean Falk, Joan T. Richtsmeier, Charles F. Hildebolt, Kirk Smith, William Jungers
Published: June 8, 2016 http://dx.doi.org/10.1371/journal.pone.0155731
Abstract
The Liang Bua hominins from Flores, Indonesia, have been the subject of intense scrutiny and debate since their initial description and classification in 2004. These remains have been assigned to a new species, Homo floresiensis, with the partial skeleton LB1 as the type specimen. The Liang Bua hominins are notable for their short stature, small endocranial volume, and many features that appear phylogenetically primitive relative to modern humans, despite their late Pleistocene age. Recently, some workers suggested that the remains represent members of a small-bodied island population of modern Austro-Melanesian humans, with LB1 exhibiting clinical signs of Down syndrome. Many classic Down syndrome signs are soft tissue features that could not be assessed in skeletal remains. Moreover, a definitive diagnosis of Down syndrome can only be made by genetic analysis as the phenotypes associated with Down syndrome are variable. Most features that contribute to the Down syndrome phenotype are not restricted to Down syndrome but are seen in other chromosomal disorders and in the general population. Nevertheless, we re-evaluated the presence of those phenotypic features used to support this classification by comparing LB1 to samples of modern humans diagnosed with Down syndrome and euploid modern humans using comparative morphometric analyses. We present new data regarding neurocranial, brain, and symphyseal shape in Down syndrome, additional estimates of stature for LB1, and analyses of inter- and intralimb proportions. The presence of cranial sinuses is addressed using CT images of LB1. We found minimal congruence between the LB1 phenotype and clinical descriptions of Down syndrome. We present important differences between the phenotypes of LB1 and individuals with Down syndrome, and quantitative data that characterize LB1 as an outlier compared with Down syndrome and non-Down syndrome groups. Homo floresiensis remains a phenotypically unique, valid species with its roots in Plio-Pleistocene Homo taxa.
Citation: Baab KL, Brown P, Falk D, Richtsmeier JT, Hildebolt CF, Smith K, 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
Editor: John Hawks, University of Wisconsin, UNITED STATES
Received: October 6, 2015; Accepted: May 3, 2016; Published: June 8, 2016
Copyright: © 2016 Baab et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All data files pertaining to neurocranial shape, symphyseal shape, and endocast measurements are available from the Harvard Dataverse (https://dataverse.harvard.edu/dataverse/Baab_et_al_LB1_DS). Original CT data from LB1 was used with permission of the Indonesian Center for Archaeology [ARKENAS] in Jakarta, Indonesia.
Funding: Grant support was provided to KLB by the National Science Foundation (BCS 04-24262; http://www.nsf.gov/), the L.S.B. Leakey Foundation (http://www.leakeyfoundation.org/), and the Sigma Xi Foundation (https://www.sigmaxi.org/). JTR was supported in part by a National Institutes of Health grant (R01-HD038384; http://www.nih.gov/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
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