By studying fossils from southern China, scientists have gained insights into how primates in Asia evolved to resemble the array seen today. The results suggest that a distinct period marked by cooler weather served as a filter of sorts in Asia, altering the makeup of primates there to reflect fewer anthropoids (monkeys and apes) and more strepsirrhines, a suborder of primates that includes lemurs.

Primates are sensitive to shifts in temperature, and thus, to climate change. Accordingly, changes in the distribution of primate fossils throughout time reflect this, with intervals of extreme warmth corresponding to expansions for these species, as reflected in the fossil record, and cooler periods (such as the Eocene-Oligocene transition, or EOT), resulting in primates' extinction on large scales.

During the EOT in North America and Europe, primates were lost completely, but in Africa, where climate shifted less dramatically, primates continued to flourish.

In Asia meanwhile, primates in tropical regions weathered the EOT, although with some measure of turnover. Assessing the exact impact of EOT environmental changes on Asian primate evolution has been difficult given the lack of primate fossils from the region during that time.

Here, to help to fill this gap, Xijun Ni and colleagues studied a diverse array of fossils from Yunnan Province in southern China dated to the early Oligocene, the time just after the cooler transition, discovering that surviving the EOT required primates to adapt to lemur-like strepsirrhine primates.

A similar comparison of Afro-Arabia fossils during this time shows a very different pattern of adaptation in response to the EOT: very few strepsirrhine primates survived, whereas anthropoids diversified.

The results suggest that the EOT functioned as a critical filtering episode during the evolutionary history of Asian primates, one that ultimately impacted the course of primate evolution across the Old World.