A genetic analysis of people from Papua New Guinea (PNG) reveals a sharp genetic divide between those residing in the highlands and lowlands, beginning 10,000 to 20,000 years ago. The divide emerged around the same time that people began cultivating plants on the island, suggesting that population structure was affected by the transition into a Neolithic lifestyle. Due to the fact that Papua New Guinea was a likely stepping stone for human migration from Asia to Australia, the population structure is of great interest from both a population genetic and archaeological perspective. However, samples from present day populations have generally been limited. Here, Anders Bergström and colleagues genotyped 381 individuals from 85 language groups across PNG, also analyzing 39 previously generated high coverage whole-genome sequences. The data suggest that highlanders and Sepik lowlanders separated 10,000 to 20,000 years ago, with all splits within the highland populations appearing to have occurred within the last 10,000 years. Human innovation and migration seen in events like the Bronze Age and the Iron Age, in west Eurasia and Africa respectively, drove genetic diversity characterized by rapid expansions in Y-chromosome lineages - yet the authors find no evidence for this in the PNG data. Regardless, both the high- and lowland populations exhibit a surprisingly strong level of genetic differentiation. Thus, the authors propose that, in PNG, the differentiation observed may be a result of the genetic, linguistic, and cultural diversity that sedentary human societies can achieve in the absence of massive technology-driven expansions.