Abstract
Modern humans have gone through varied processes of genetic adaptations when their ancestors left Africa about 100,000 years ago. The environmental stresses and the social transitions (e.g., emergence of the Neolithic culture) have been acting as the major selective forces reshaping the genetic make-up of human populations. Genetic adaptations have occurred in many aspects of human life, including the adaptation to cold climate and high-altitude hypoxia, the improved ability of defending infectious diseases, and the polished strategy of utilizing new diet with the advent of agriculture. At the same time, the adaptations once developed during evolution may sometimes generate deleterious effects (e.g., susceptibility to diseases) when facing new environmental and social changes. The molecular (especially the genome-wide screening of genetic variations) studies in recent years have detected many genetic variants that show signals of Darwinian positive selection in modern human populations, which will not only provide a better understanding of human evolutionary history, but also help dissecting the genetic basis of human complex diseases.
Highlights
Two schools holding different opinions about where and when modern humans originated have been debating for more than two decades
The “Out-of-Africa” hypothesis of modern human origin was initially proposed in the mid1980s [1, 2] by studying the genetic variations of mitochondrial DNA, and it was suggested that modern humans first originated in Africa about 200 kya expanded to the other parts of the world and completely replaced the local archaic populations [3]
The most recent common ancestor of modern humans emerged only about 100–200 kya ago [8, 10, 13, 16,17,18,19], which is consistent with the fossil and culture records of anatomical modern humans firstly appearing in Africa about 150–200 kya ago [17, 20]
Summary
Two schools holding different opinions about where and when modern humans originated have been debating for more than two decades. The “Out-of-Africa” hypothesis of modern human origin was initially proposed in the mid1980s [1, 2] by studying the genetic variations of mitochondrial DNA (mtDNA), and it was suggested that modern humans first originated in Africa about 200 kya (thousand years ago) expanded to the other parts of the world and completely replaced the local archaic populations [3]. The genetic data revealed the prehistoric migratory routes from Africa to East Asia. They first reached the southern part of East Asia (mainland Southeast Asia) [38, 39] migrated northward to mainland China, Siberia, and eventually to America. There was an early southward migration from mainland Southeast Asia to Australia and Papua New Guinea [37, 39, 40]
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