science

At least some of them took the kids down to the creek every 6 months or so, and threw the babies in the water to see who would swim. The ones that didn't swim stayed back at the camp and fixed pottery, cleaned, cooked, etc. The swimmers became the hunters and gatherers. Several of the Native American Nations in the Eastern US did this when white man came over and invaded. According to their oral histories, they had been doing this for a few tens of thousands of years, which seems to match up to the archaeological evidence we've found in the last couple decades.