(New Scientist) What a difference a day makes. Some 25 hours after a comprehensive genetic study suggested humans colonized North America in three successive migrational waves, a new study suggests there might have been a fourth.

Genetic evidence published on Wednesday by David Reich of Harvard Medical School in Boston suggests that there were at least three migrations into the Americas from Asia. The first, sometime before 13,000 years ago, was the most important, leaving descendants throughout the two continents. The other two came later, and remained in the far north (Nature, DOI: 10.1038/nature11258).

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