Journal of a Cosmic Anthropologist

Götherström thinks there are a few possible explanations. One is that after humans left Africa, some flourished by becoming farmers and exploiting the fertile lands they found in the Near East. Others ventured further north, where they continued to rely on the wild animals they killed and the plants and berries they collected from the landscape. These groups were split into small populations that survived in isolation between the ice sheets.

The Swedish data shows some mixing between the two groups, suggesting that as the farmers later swept north they interbred with the hunter-gatherers they encountered and assimilated them into their cultures. The exchange seems to have been unidirectional: there is very little if any evidence that farmers were assimilated into the hunter-gatherers.