Looking for unique signals of human expansion from Africa: Beyond diversity

Abstract

There are known to be different views on which portion of Africa modern humans globally spread from. Biological diversities have been employed to estimate the origin of our global expansion. These diversities vary with geographical distance from Africa, thereby expressing the signal of the expansion. In preprints, the signal supposedly appeared beyond diversity - in cranial sexual size dimorphism and a cranial shape distance-based measure. Compared to when diversity is used alone, the addition to analysis of variables which are beyond diversity could improve recovery of the signal, therefore improving origin identification. I explored this through cranial and genetic measures which had been calculated in prior studies. Various analyses were used, e.g., ridge regression and Mantel tests. Amongst cranial variables (shape diversity, sexual size dimorphism, and a shape distance-based measure), only dimorphism had a unique portion of the expansion signal. In comparison to when diversity was utilised alone, the additional use of dimorphism and the distance-based measure did not substantially impact signal recovery. However, their addition possibly improved origin identification, reducing by 46% the size of the geographical area which may have the origin. This smaller area approximately matched southern Africa, however, it was not only in the south. It was questionable if the signal was present in a genetic distance-based measure, which called into question whether the expansion signal is truly present in the cranial shape distance-based measure. Analysis suggested that the apparent presence of the signal in distance-based measures is affected by the representation of Oceanian populations. This study supports cranial sexual size dimorphism being a helpful indicator of the expansion whilst calling into question whether biological distance-based measures are indicators. Clarity remains missing on which African region was the origin.