South America was the last habitable continent to be colonized by humans. Researchers have just discovered two distinct phases in the demographic history of South Americans: rapid spread with low population sizes, followed by sedentary lifestyles with exponential growth. The study, published in Nature this week, compared the population growth of prehistoric South Americans to that of invasive species. 

The first Americans were descendants of Siberians that crossed the ancient land bridge that connected Asia with Alaska some 15,000 to 30,000 years ago. They reached southern South America pretty rapidly, by at least 14,500 years ago. Unlike most other continents, South America was peopled during a single wave of invasion over a narrow time frame. However, the human population dynamics during colonization, subsequent expansions, and domestication remain unclear. 

To reconstruct spatiotemporal patterns of human population growth in South America, a Stanford team led by Amy Goldberg compiled a database of 5,464 radiocarbon dates from 1,147 archaeological sites. These various areas of human occupation span 14,000 to 2,000 years ago. Rather than a steady expansion, the team found that the demographic history of South Americans is characterized by two distinct phases. 

The first phase occurred between 14,000 and 5,500 years ago. Initial growth was rapid, with the population doubling every 19 to 21 generations. (Generation time was about 26.5 years.) And the earliest and highest population growth patterns occurred along the Pacific coastline in Peru, Chile, and Ecuador. Though they spread rapidly throughout the continent, they stayed at low population sizes. Population sizes stayed relatively constant from 9,000 to 5,500 years ago. Even with boom-and-bust cycles, the population saw no net growth. 

The combination of the rapid spread with low population sizes is similar to the population size dynamics of a typical invasive species: rapid initial expansion and resource-limited growth over time. Supplementing hunting with domesticated animals and crops (like squash, peppers, and maize) had little impact on the carrying capacity of the population. 

The second demographic phase began around 5,500 years ago, with the shift to a sedentary and agricultural subsistence. Stable food sources and the need for human labor in one spot meant that parents were more available, the interval between births became shorter, and populations became larger. 

During this time, the population increased threefold, and by 2,000 years ago, there were as many as a million people in South America.