The prehistoric peopling of South America - the last habitable continent colonized by humans - occurred in two distinct phases, reports a paper published online this week in Nature. The study finds that the earliest and highest population growth patterns occurred along the Pacific coastline in Peru, Chile and Ecuador.

Amy Goldberg and colleagues reconstructed the history of human population growth in South America using a newly assembled database of radiocarbon dating from 1,147 archaeological sites. They find that a two-phase model of the long-term human population size in South America best fits the data. The first phase, between about 14,000 and 5,500 years ago, consisted of a rapid spread of humans throughout the continent and low population sizes, similar to population growth of an invasive species. A more recent phase of exponential growth then occurred from about 5,500 to 2,000 years ago, during which the population increased threefold. This pattern is distinct from those seen in North America, Europe and Australia.

The authors conclude that the phase of exponential population growth was made possible by the more recent rise of sedentary and agricultural societies, rather than the rise of domestication thousands of years earlier.