Different Cranial Ontogeny in Europeans and Southern Africans

Modern human populations differ in developmental processes and in several phenotypic traits. However, the link between ontogenetic variation and human diversification has not been frequently addressed. Here, we analysed craniofacial ontogenies by means of geometric-morphometrics of Europeans and Southern Africans, according to dental and chronological ages. Results suggest that different adult cranial morphologies between Southern Africans and Europeans arise by a combination of processes that involve traits modified during the prenatal life and others that diverge during early postnatal ontogeny. Main craniofacial changes indicate that Europeans differ from Southern Africans by increasing facial developmental rates and extending the attainment of adult size and shape. Since other studies have suggested that native subsaharan populations attain adulthood earlier than Europeans, it is probable that facial ontogeny is linked with other developmental mechanisms that control the timing of maturation in other variables. Southern Africans appear as retaining young features in adulthood. Facial ontogeny in Europeans produces taller and narrower noses, which seems as an adaptation to colder environments. The lack of these morphological traits in Neanderthals, who lived in cold environments, seems a paradox, but it is probably the consequence of a warm-adapted faces together with precocious maturation. When modern Homo sapiens migrated into Asia and Europe, colder environments might establish pressures that constrained facial growth and development in order to depart from the warm-adapted morphology. Our results provide some answers about how cranial growth and development occur in two human populations and when developmental shifts take place providing a better adaptation to environmental constraints

Diversity among African Pygmies

Although dissimilarities in cranial and post-cranial morphology among African pygmies groups have been recognized, comparative studies on skull morphology usually pull all pygmies together assuming that morphological characters are similar among them and different with respect to other populations. The main aim of this study is to compare cranial morphology between African pygmies and non-pygmies populations from Equatorial Africa derived from both the Eastern and the Western regions in order to test if the greatest morphological difference is obtained in the comparison between pygmies and non-pygmies. Thirty three-dimensional (3D) landmarks registered with Microscribe in four cranial samples (Western and Eastern pygmies and non-pygmies) were obtained. Multivariate analysis (generalized Procrustes analysis, Mahalanobis distances, multivariate regression) and complementary dimensions of size were evaluated with ANOVA and post hoc LSD. Results suggest that important cranial shape differentiation does occur between pygmies and non-pygmies but also between Eastern and Western populations and that size changes and allometries do not affect similarly Eastern and Western pygmies. Therefore, our findings raise serious doubt about the fact to consider African pygmies as a homogenous group in studies on skull morphology. Differences in cranial morphology among pygmies would suggest differentiation after divergence. Although not directly related to skull differentiation, the diversity among pygmies would probably suggest that the process responsible for reduced stature occurred after the split of the ancestors of modern Eastern and Western pygmies

Tooth dimensions and body size in a Pygmy population

Background: The relationship between tooth size and stature has been analysed extensively at the interspecies level but has received less attention at the intraspecies level. The relationship between these two parameters does not seem to be the same among modern human populations. Aim: The aim of this study is to analyse the relationship between tooth dimensions and body measurements in the Baka Pygmies. Subjects and methods: Height, weight, and tooth dimensions were obtained for 45 adult Baka females and 17 males from Le Bosquet (Cameroon). Correlations were obtained between the variables and compared to results for other human populations. Results: The Baka population is distinctive in the small number of significant correlations. Only two buccolingual diameters among Baka females show any significant correlation with height. The lack of significant correlations between tooth dimensions and body dimensions among the Baka means that changes in body size are accompanied by random variations in tooth dimensions. Conclusion: The absence of correlations may be accounted for by the impact of environmental effects on the somatic growth of the Baka producing a Pygmy phenotype adapted to live in the forest. It is worth noting that many correlations become significant when sexes are pooled.This research was funded by PICS 07492 MAMIBAKA, Wenner-Gren Foundation [grant No. 7819], National Geographic Society [grant No. 8863-10], and Agence National de la Recherche [ANR-11-BSV7-0011] to F.V.R.R. and Spanish Ministerio de Economía y Competitividad [grant Nos. CGL2011-22999 and CGL2014-52611-C2-1-P] to A.R

GPA/PCA results for neurocranial PC2.

<p>(a) PC2 scores vs chronological age. Smoothing splines accounted for 10.5 and 13% of variation in Europeans and Southern Africans, respectively. (b) PC2 scores vs dental age. (c) PC2 scores vs CS. (d) Lateral view of neurocranial shape, considering extreme negative values as the reference and extreme positive values as the target (other views do not show deformation). (e) Mean and 95% standard error for PC2 scores vs post-pubertal stages. Green: Europeans. Red: Southern Africans. Horizontal lines in a represent adult means: Europeans, solid line; Southern Africans, dotted line.</p

Ranking of dental maturation and sample distribution.

<p>Ranking of dental maturation and sample distribution.</p

GPA/PCA results for neurocranial PC1.

<p>(a) PC1 scores vs chronological age. Smoothing splines accounted for 74% of variation in both distributions. (b) PC1 scores vs dental age. (c) PC1 socres vs CS. (d) Lateral view of neurocranial shape in extreme positive values (newborns = target), considering extreme negative values as the reference. (e) Lateral view of neurocranial shape in extreme negative values (adults = target), considering extreme positive values as the reference. (f) Mean and 95% standard error for PC1 scores vs post-pubertal stages. Green: Europeans. Red: Southern Africans. Horizontal lines in a represent adult means: Europeans, solid line; Southern Africans, dotted line.</p

Landmarks registered with Microscribe on the left side of the skull.

<p>Landmarks registered with Microscribe on the left side of the skull.</p

Regression equations and ANCOVA results.

<p>Numbers in bold indicate probability under 0.05.</p

ANOVA for dental stages 7 and 8.

<p> <b>Numbers in bold indicate probability under 0.05.</b></p

Neurocranial and facial size variation.

<p>(a) Neurocranial centroid size values vs chronological age. Smoothing splines accounted for 91.7 and 84.8% of variation for Europeans and Southern-Africans, respectively. (b) Neurocranial centroid size values vs dental age. (c) Mean and 95% standard error of neurocranial centroid size vs post-pubertal stages. (d) Plot of facial centroid size values vs chronological age. Smoothing splines accounted for 92 and 90.4% of variation for Europeans and Southern-Africans, respectively. (e) Facial centroid size values vs dental age. (f) Mean and 95% standard error of facial centroid size vs post-pubertal stages. Green: Europeans. Red: Southern Africans. Horizontal lines in a and b represent adult means: Europeans, solid line; Southern Africans, dotted line.</p