It took humans a long time to reach South America. But once they got there, they spread like weeds—literally. In a new study, researchers tallied up 1147 archaeological sites that had been radiocarbon-dated to between 14,000 years ago (right around the earliest known settlements in South America) and 2000 years ago. By mapping those sites, the scientists can see where people lived and when. The density of settlements increased rapidly and steadily from 13,000 to 9000 years ago, as humans spread to every corner of the unoccupied continent and learned to take advantage of its resources. (Way better than the 6000 years their ancestors spent stranded in the Beringian tundra.) But about 9000 years ago, South American population growth appears to level off, the researchers report online today in Nature. That’s a pattern followed by many an invasive species that finds itself in a new, hospitable habitat: It spreads quickly but then maxes out its resources and levels off. Eventually, however, people in South America found a way around this natural limit. About 5000 years ago, human populations started expanding once again, at this time, they grew even more rapidly. So what changed? People all over South America transitioned from hunting and gathering to farming. Agriculture and sedentary lifestyles gave them more time and resources, making it easier to have lots of babies.