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, 11 (6), e0155731
eCollection

A Critical Evaluation of the Down Syndrome Diagnosis for LB1, Type Specimen of Homo Floresiensis

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A Critical Evaluation of the Down Syndrome Diagnosis for LB1, Type Specimen of Homo Floresiensis

Karen L Baab et al. PLoS One.

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.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Photographs of LB1 cranium and LB1 and LB6 mandibles.
(A) The cranium is shown in right lateral and anterior views. (B) The LB1 (left) and LB6 (right) mandibles are shown in left lateral and occlusal views.
Fig 2
Fig 2. Box plot of endocranial volumes.
The DS and matched euploid clinical samples are from Aylward, Habbak [73]. The center line represents the average value while the box captures ± 2 SDs. The mean value for the Rampasasa population is from Henneberg, Eckhardt [25]. The average value of an individual with DS from the Rampasasa population (“Rampasasa DS”) was estimated as 87% of the euploid value, based on the relationship between the matched DS and euploid samples in Aylward, Habbak [73].
Fig 3
Fig 3. Principal components 1 and 2 of a PCA of neurocranial shape based on 3D landmarks (see Materials and Methods) in euploid and DS samples of humans and LB1.
The LB1 neurocranial shape is distinct from the two modern human samples, which themselves evince considerable overlap in shape. The solid blue squares are the adults with DS, the blue outlined squares are juvenile / subadults with DS, gray diamonds are euploid adults and the red asterisk is LB1. Surface renderings are single examples from each group and are for illustrative purposes only. The first two components accounted for 14.6% and 9.6% of the total variance, respectively (PC 1 eigenvalue = 0.0006; PC 2 eigenvalue = 0.0004).
Fig 4
Fig 4. “Cranial templates” for euploid and DS samples of males between the ages of 19 and 29 years and pseudo-lateral cephalogram tracing of LB1.
The euploid and DS cephalograms were based on average roentgencephalometric dimensions (modified from Kisling [49]). All three images have been scaled to approximately the same cranial length. Midfacial hypoplasia in the DS facial phenotype is apparent and contrasts strongly with the relatively long and prognathic maxilla and mandible of LB1. Other differences include the thicker cranial bones, shape of the mandible and the low neurocranial profile of LB1. Note that the LB1 cranium suffered damage to midline structures of the face, including the glabella, nasal bones and subnasal region; morphology of anterior maxilla was estimated based on surrounding morphology and indication of edge-to-edge occlusion of incisors by PB.
Fig 5
Fig 5. Comparison of symphyseal anatomy, shape and dimensions in a euploid modern human, LB1 and LB6.
Note presence of chin (mental protuberance or trigone), inverted T, incurvature and absence of internal buttressing in modern human. LB1 and LB6 are similar anatomically and distinct from H. sapiens. No mental protuberance, incurvature, inverted-T, or tubercles. LB1 and LB6 have inferior and superior transverse tori, with deep genioglossal pit.
Fig 6
Fig 6. Principal components 1 and 2 of a PCA of symphyseal shape based on Fourier shape variables.
Shape differences (anterior facing left) associated with the PCs are illustrated below (PC 1) and to the left (PC 2) of the ordination. The samples include regionally appropriate modern humans, individuals with DS, Pleistocene Homo, Australopithecus and one Paranthropus boisei fossil, as well as LB1 and LB6. LB1 and LB6 are quite similar in their symphyseal shape and most closely resemble A. afarensis. The DS sample overlaps the euploid modern humans and some Pleistocene Homo samples. The first two components accounted for 73.1% and 9.6% of the total variance, respectively (PC 1 eigenvalue = 0.01; PC 2 eigenvalue = 0.001).
Fig 7
Fig 7. Matrix-filled maxillary and sphenoid sinuses in LB1.
Arrows indicate the right (A) and left (B) maxillary sinuses as seen in parasagittal sections of the LB1 skull using medical CT imaging. The probable sphenoid sinus is illustrated in parasagittal (C), transverse (D) and coronal (E) sections based on higher resolution micro-CT scans of the cranium. The positions of the three sections are shown on the surface renderings in the top row. Micro-CT images (C-E) were provided courtesy of Yousuke Kaifu.
Fig 8
Fig 8. Observed stature for 18-year-old males (gray) and females (black) with DS from Turkey, comparable estimates for Javanese with DS, and estimates for LB1 based on different reference populations.
The Turkish data are from Tüysüz et al. (2012). Estimates for Javanese with DS are based on the relationship between euploid and DS Turkish populations and average stature for adult male and female (euploid) Javanese (see text for details). The three lines indicate the 97th, 50th and 3rd percentiles. The first three stature estimates for LB1 are from Henneberg et al. (2014) while the range on the left were generated for this study.
Fig 9
Fig 9. Plot of the foot:femur (or foot:thigh) ratio in recent humans of normal stature and short stature, LB1, and a DS and a matched euploid control sample.
The box-and-whisker plots include the mean and ±1 SD as well as the range. The asterisks for the LB1 value are the 95% confidence intervals based on the regression equation used to obtain the estimate [7]. The fleshy foot:thigh bars labeled “anthropometric” are the values based on the data from the DS and euploid control samples from Smith and Ulrich [115], while the bars labeled “adjusted” represent the raw values adjusted to make them more comparable to the skeletal ratios (see text for details).

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