Tracing Out the Paths Modern Humans Took From Africa

Tracing the paths of modern humans from Africa

Author:

Weaver

Extract:

In the 1980s, genetic and fossil evidence began to call attention to Africa’s preeminence in the origins of modern human populations (1), but this evidence could be interpreted in two fundamentally different ways (2). Was Africa’s role greater than other continents because it always harbored a larger human population (size) or because modern humans arose in Africa first and subsequently expanded their range across the world (time)? In the 2000s, improvements in DNA sequencing technology and genetic sampling of more present day human groups made it possible to accurately characterize the genetic diversity of groups from different regions of the world, and it became clear that within-group genetic diversity decreased predictably with increased geographic distance from subSaharan Africa (3, 4). Subsequently, similar, albeit weaker, relationships were found between within-group variation in aspects of skeletal morphology (cranial, dental, and pelvic measurements) and distance from subSaharan Africa (5–8). These results effectively settled the size vs. time debate, because it was hard to imagine how the observed geographic distribution of within-group diversity could have arisen without a recent range expansion of modern humans from Africa. However, how exactly did the spread out of Africa happen? In PNAS, Reyes-Centeno et al. (9) are, to my knowledge, the first to use both morphological data (cranial measurements) and genetic data (single nucleotide polymorphisms) to explicitly evaluate competing models for the expansion of modern humans from Africa. They conclude that after modern humans left Africa, they first went south, only later heading north in a second expansion wave.