Migration dynamics of modern humans from Africa into Eurasia are sufficient to explain the replacement of Neanderthals by modern humans, suggests a model published in Nature Communications this week. Previously, it had been argued that the extinction of Neanderthals was caused by environmental pressure, out-competition by modern humans, or both.

Oren Kolodny and Marcus Feldman developed a model of Neanderthal replacement based on species drift, in which changes in frequencies of species are driven by random events rather than by a selective advantage. Even under the assumption that Neanderthals and modern humans were equal except in migration pattern and initial population size, they found that modern humans were certain to replace Neanderthals due to the repeated migration of modern humans from Africa into Eurasia. The model suggests that this replacement was gradual: that Neanderthals were replaced by the offspring of one or more small groups of modern humans that migrated out of Africa, established in Eurasia, and increased in numbers until eventually reaching 100% of the hominin population.

Although the extinction of Neanderthals may still have been hastened by environmental factors or an advantage for modern humans, the new model suggests that the replacement of Neanderthals was inevitable. The model also provides a baseline for further testing of the relative roles of other causal factors.