Analyses of the genomes of Neanderthals and Denisovans, the closest evolutionary relatives of present-day humans, suggest that our ancestors were part of a web of now-extinct populations linked by limited, but intermittent or sometimes perhaps even persistent, gene flow.
For decades, paleontologists intensely debated how ‘modern humans’ — that is, the direct ancestors of all humans alive today — originated. One view was that modern humans emerged recently in Africa from which they spread worldwide and replaced all other forms of humans, or ‘hominins’, without mixing with them — a process that ended some 30–40 thousand years ago. Another view favoured regional continuity under which hominins in different parts of the world evolved more or less independently over hundreds of thousands or millions of years into present-day humans, with some gene flow between them. This debate was so intense because it bears on the fundamental question of when and where modern humans originated, but it is also of importance for guiding how we should think about the genetic variation in the current human gene pool — does it all go back to variation that accumulated exclusively in Africa, or does it have deep roots elsewhere as well?
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