1Laboratory for the Comparative Study of Morphology, Mechanics and Molecules, Department of Kinesiology, Pennsylvania State University, 2Biological Anthropology and Comparative Anatomy Unit, University of Adelaide
Friday 12, Park Concourse
Reification of a novel hominin taxon for Liang Bua Cave, Flores, skeletons embodies several leitmotivs: 1. descriptions of morphologies that are abnormal (e.g. rotated bilateral upper premolars) or pathological (marked craniofacial asymmetry of the LB1 skull with endocranial volume several standard deviations below relevant reference populations, disproportionate limb segment ratios, etc.); 2. insistence that such features are normative for the hypothetical taxon; 3. contention that clinical signs of abnormality cannot be accepted without diagnosis of some specific disorder.
Logically, documentation of abnormality precedes inferences about causation. Salient for LB1, short stature, microcephaly, plus skull asymmetry occur jointly in 100 to 200 syndromes. Only three were considered before hypothesizing the new species; subsequently, fewer than ten have been evaluated.
Here we explore another possible diagnosis: Down Syndrome (DS), which occurs worldwide 1/<700 births. Signs of DS include reduced brain size, brachycephaly, shortened extremities, hypotonia, etc., all documented in LB1. Notably, abnormally low femur:foot ratios are so common in DS that they are used in prenatal screening. Jungers, et al. (2009 Nature 459) estimated foot:femur ratio of 70.0 (67.5–72.9, 95% prediction interval) are compared here to data on 8 preadolescents (foot:thigh ratio 67.9, 62.6–73.6) and 12 adults (foot:thigh ratio 61.1, 57.0–67.1).
Further testing of the DS hypothesis requires access to the original specimens, denied to us since 2007. Should further work eliminate DS for LB1, observations of abnormality persist.