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.
Highlights
Interpretation of the Late Pleistocene Flores hominins has been controversial since their initial description in 2004 as a distinct species, Homo floresiensis [1]
Workers have confirmed that cranial and endocranial shape and many cranial characteristics of LB1 correspond to classic H. erectus characters [2,3,4,5], while some postcranial and mandibular features, such as the interlimb proportions and symphyseal shape are more primitive than H. erectus, and more closely resemble early Homo or even australopiths [6,7,8]
A concrete diagnosis of Down syndrome (DS) can only be accomplished through genetic testing, we critically evaluated whether the skeletal anatomy of LB1 is compatible with the DS phenotype as proposed by Henneberg et al [25]
Summary
Interpretation of the Late Pleistocene Flores hominins has been controversial since their initial description in 2004 as a distinct species, Homo floresiensis [1]. The many phylogenetically primitive features observed in the Flores specimens relative to modern humans suggest that their ancestry is rooted in Plio-Pleistocene Homo, perhaps H. erectus or H. habilis. A minority of workers maintains that the Flores hominins are small-bodied modern humans, and, at least in the case of the most complete specimen, LB1, pathologically altered. Several clinical signs (e.g., microcephaly) and specific pathologies (endemic hypothyroidism, Laron syndrome) have been proposed [13,14,15,16,17,18] and subsequently rejected [19,20,21,22,23,24]. Henneberg, Eckhardt [25] reported that LB1 manifested many clinical signs of Down syndrome (DS). We provide an overview of the DS phenotype, and re-evaluate the evidence presented by Henneberg, Eckhardt [25] in support of this diagnosis for LB1
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