The global expansion from southern Africa by modern humans? Its signal may be shown more through the cranial shape of males than females

Abstract

As the geographical distance between populations increases, populations are known to become more distant (dissimilar) in biological measures such as cranial shape. Relationships between geographical distance and biological distance are thought to be explained by modern humans having expanded from Africa. Expansion from Africa is also thought to explain why biological diversities (e.g., cranial shape diversity) decrease as geographical distance from Africa goes up. Research has suggested that modern humans expanded through different routes. If there were different routes, perhaps the relationship between geographical distance and cranial shape distance declines as the population (that distances are measured from) gets further away from Africa. This study used cranial shape measurements derived from the Howells data, and found that such a decline was suggested for males, unlike for females. It has been reasoned that variables which indicate the global expansion should signal an area of origin which is only in Africa. The relationship between geographical and cranial shape distances did not signal an exclusively African area. However, for males, the area was primarily in Africa. Overall, the relationship seemed to represent the expansion better amongst males than females. Congruently, cranial shape diversity declined more strongly for males than females as geographical distance from Africa accrued. To estimate which area of Africa the global expansion originated in, previous research pooled an estimate from genetic diversities, and suggested a southern origin. This estimate was supported when adding cranially-derived variables to the pooling, including the relationship between geographical distance and male cranial shape distance.

Cranial sexual dimorphism levels in modern humans: Supporting a global expansion originating in southern Africa

“…Therefore, cranial size dimorphism may be particularly useful for deducing the segment of Africa in which began the global expansion. Indeed, a cross-variable estimate from biological variables backs the south (Cenac, 2023b), and cranial size dimorphism (in the estimate) is inclined to the south (e.g., Figure 5A), which supports the cross-variable estimate.…”

“…To reiterate, the expansion appears to be represented less for females than males in cranial shape (Cenac, 2023b), and it is not clear if this can also be said for cranial size (present study).…”

“…As in previous research (Pinhasi & von Cramon-Taubadel, 2009), RMET 5.0 (Relethford & Blangero, 1990) was employed to calculate squared cranial shape distances (D 2 ). Like before (Cenac, 2023b), the square root of D 2 was cranial shape distance (D); the measure of cranial sexual shape dimorphism was the within-population cranial shape distance between males and females. Similarly, RMET was used to calculate squared cranial form distances from which were found cranial form distances, and the cranial form distance between males and females was the measure of sexual dimorphism in cranial form.…”

“…Cranial shape dimorphism may only be a weak indicator of the expansion, or it may not even be an indicator (Figures 3 and 4). Therefore, when using different variables to construct an estimate for the geographical source of the expansion (e.g., Cenac, 2023b), it would seem pointless to include cranial shape dimorphism. Nevertheless, for cranial shape dimorphism, southern Africa appears to be where the numerically highest correlation coefficients are (Figure 5B).…”

“…Relationships with Distance from Africa: Cranial Size, Shape, and Form Dimorphisms Note. Figure 5A exhibits correlation coefficients calculated in Cenac (2023b) for relationships which are with respect to the cranial size dimorphism of 24 populations (cranial data being sourced from the Howells dataset)…”