Expansion of modern humans over the world: The origin when considering non-linearity

Abstract

Types of diversity have been known to decline linearly with the rise of geographical distance from Africa. Declines have helped to suggest the area in Africa which holds the origin for the global expansion of modern humans. Research has, at times, explored if there is a non-linear relationship between diversity and distance from Africa. A previous suggestion was that non-linearity could affect where the expansion appears to have originated. Linear analysis with Y-chromosomal microsatellite heterozygosity has been contrary to the expansion from Africa, instead indicating an origin involving Asia; could this be attributable to non-linearity? The present study looked into whether there are non-linear relationships between distance and diversities, and approximated where the expansion began. This study used diversities from previous research - genetic (autosomal, X-chromosomal, Y-chromosomal, and mitochondrial) and cranial shape. The Bayesian information criterion was the statistic for comparing linear and non-linear (quadratic) models to indicate if there is a non-linear relationship. This criterion was also used to estimate where the expansion launched from. Autosomal microsatellite heterozygosity favoured a non-linear relationship. This may be due to South American populations. Mitochondrial diversity suggested non-linearity too, but not when minimum temperature was controlled for. Whilst non-linear relationships indicated that the expansion had its start in Africa, for autosomal microsatellite heterozygosity, the area of origin appeared to be rather affected by the type of model (linear or non-linear). Other diversities (e.g., Y-chromosomal) supported linear relationships. Therefore, non-linearity does not seem to explain Y-chromosomal microsatellite heterozygosity being unexpressive of the global expansion.