Want to know how many times ancient humans migrated out of Africa? Check their genes

We know that tens of thousands of years ago, humans migrated out of Africa — but the question is, when, and how many times? By sequencing the genomes of nearly 800 people from more than 270 population groups around the world, three massive teams of scientists have collected the most comprehensive genetic evidence yet of a single wave of people out of Africa less than 100,000 years ago. But the three teams disagree somewhat about how ancient human populations shifted and changed before and after this exodus.

There were probably multiple groups that ventured north into Europe and east to Asia and beyond. We know that the ancient humans and Neanderthals — a human cousin that had already successfully taken hold outside of Africa — were...