Hominoid intraspecific cranial variation mirrors neutral genetic diversity

Natural selection, developmental constraint, and plasticity have all been invoked as explanations for intraspecific cranial variation in humans and apes. However, global patterns of human cranial variation are congruent with patterns of genetic variation, demonstrating that population history has influenced cranial variation in humans. Here we show that this finding is not unique to Homo sapiens but is also broadly evident across extant ape species. Specifically, taxa that exhibit greater intraspecific cranial shape variation also exhibit greater genetic diversity at neutral autosomal loci. Thus, cranial shape variation within hominoid taxa reflects the population history of each species. Our results suggest that neutral evolutionary processes such as mutation, gene flow, and genetic drift have played an important role in generating cranial variation within species. These findings are consistent with previous work on human cranial morphology and improve our understanding of the evolutionary processes that generate intraspecific cranial shape diversity within hominoids. This work has implications for the analysis of selective and developmental pressures on the cranium and for interpreting shape variation in fossil hominin crania

Remnants of an ancient forest provide ecological context for Early Miocene fossil apes

The lineage of apes and humans (Hominoidea) evolved and radiated across Afro-Arabia in the early Neogene during a time of global climatic changes and ongoing tectonic processes that formed the East African Rift. These changes probably created highly variable environments and introduced selective pressures influencing the diversification of early apes. However, interpreting the connection between environmental dynamics and adaptive evolution is hampered by difficulties in locating taxa within specific ecological contexts: time-averaged or reworked deposits may not faithfully represent individual palaeohabitats. Here we present multiproxy evidence from Early Miocene deposits on Rusinga Island, Kenya, which directly ties the early ape Proconsul to a widespread, dense, multistoried, closed-canopy tropical seasonal forest set in a warm and relatively wet, local climate. These results underscore the importance of forested environments in the evolution of early apes

Homo floresiensis contextualized: a geometric morphometric comparative analysis of fossil and pathological human samples.

The origin of hominins found on the remote Indonesian island of Flores remains highly contentious. These specimens may represent a new hominin species, Homo floresiensis, descended from a local population of Homo erectus or from an earlier (pre-H. erectus) migration of a small-bodied and small-brained hominin out of Africa. Alternatively, some workers suggest that some or all of the specimens recovered from Liang Bua are pathological members of a small-bodied modern human population. Pathological conditions proposed to explain their documented anatomical features include microcephaly, myxoedematous endemic hypothyroidism ("cretinism") and Laron syndrome (primary growth hormone insensitivity). This study evaluates evolutionary and pathological hypotheses through comparative analysis of cranial morphology. Geometric morphometric analyses of landmark data show that the sole Flores cranium (LB1) is clearly distinct from healthy modern humans and from those exhibiting hypothyroidism and Laron syndrome. Modern human microcephalic specimens converge, to some extent, on crania of extinct species of Homo. However in the features that distinguish these two groups, LB1 consistently groups with fossil hominins and is most similar to H. erectus. Our study provides further support for recognizing the Flores hominins as a distinct species, H. floresiensis, whose affinities lie with archaic Homo

Box-and-whisker plot of Procrustes distances between LB1 and each of the other specimens.

<p><b>Procrustes distances are calculated based on the entire set of neurocranial landmarks.</b> In median distance, LB1 is most similar to the <i>H. erectus</i> sample and most dissimilar to the Laron syndrome individual. LB1 has the shortest distance to the D2700 <i>H. erectus</i> fossil from Dmanisi, Georgia. Boxes are bounded by 25th and 75th percentiles, with medians indicated by the solid lines; whiskers denote minimum and maximum distances in the sample to LB1. LB1 is closest in shape space to a Georgian <i>H. erectus</i> specimen, D2700, pictured below LB1 in the inset photographs (not to scale).</p

Information about samples used in this study.

<p>1, NMK: National Museum of Kenya; AMNH: American Museum of Natural History; GMU: Gadja Mada University; NME: National Museum of Ethiopia; NHM: Natural History Museum (London); MH: Musee de l'Homme; PM: Peabody Museum (Harvard University); IPH: Institut de Paleontologie Humaine; UCT: University of Cape Town; DC: Duckworth Collection (Cambridge University); NMB: Naturhistorisches Museum Basel; MM: Mutter Museum (Philadelphia); TAU: Tel Aviv University; MLU: Meckelsche Sammlungen, Martin-Luther Universität of Halle-Wittenberg, scanned at the Paleoanthropology High Resolution Tomography Laboratory, University of Tübingen; UM: University of Michigan; UV: University of Vienna; WU: Washington University; INCA: Indonesian National Center for Archaeology.</p><p>2, ME: myxoedematous endemic.</p

Shape differences associated with the first four components of the PCA based on the full sample and illustrated in Figure 1.

<p>Wireframes are superimposed on warped surfaces to illustrate shape differences from the negative (left) to positive (right) ends of (A) PC 1, (B) PC 2, (C) PC 3 and (D) PC 4.</p

Summary statistics for Procrustes distances between LB1 and each group, with individual distances included for each fossil hominin.

<p>1, Excluding the juvenile specimen:  = 0.154, <i>s</i> = 0.017.</p><p>2, Excluding the juvenile specimen:  = 0.154, <i>s</i> = 0.015.</p><p>3, Excluding the juvenile/subadult specimens did not affect or <i>s</i>.</p><p>4, Excluding the juvenile/subadult specimens:  = 0.139, <i>s</i> = 0.019.</p

Principal component analysis of neurocranial shape with minimum convex polygons drawn as shaded regions around each group.

<p>(A) The shape of the LB1 neurocranium is distinct from that of healthy humans and humans with hypothyroidism or Laron syndrome on PC 1, and within the <i>Homo erectus</i> distribution on PC 2. (B) LB1 overlaps both fossil <i>Homo</i> and microcephalic humans on PCs 1 and 3, but (C) again groups with <i>H. erectus</i> on the fourth component. Figure legend: LB1: green star; <i>H. habilis</i>: brown target; <i>H. erectus</i>: yellow squares; Mid-Pleistocene <i>Homo</i>: purple crosses; Neanderthals: purple Xs; Primary microcephaly: red triangles; Secondary microcephaly: pink triangles; ME hypothyroidism: blue circles; Sporadic hypothyroidism: light blue circles; Laron syndrome: dark aqua dash; “Pituitary dwarf”: light aqua dash. For clarity, only the gray convex polygon is shown for the healthy human sample rather than individual data points. Light blue and light pink polygons extend the hypothyroid and microcephaly distributions to include the sporadic hypothyroid and secondary microcephaly specimens, respectively.</p

Landmarks used in this study.

<p>Landmarks used in this study.</p