Fossil evidence reveals early primates evolved in cold climates, not tropical forests

Primates—the group of animals that includes monkeys, apes and humans—first evolved in cold, seasonal climates around 66 million years ago, not in the warm tropical forests scientists previously believed.

Researchers from the University of Reading used statistical modeling and fossil data to reconstruct ancient environments and trace where the common ancestors of all modern primates lived.

The study, published in the journal PNAS, says these first primates most likely lived in North America in a cold climate with hot summers and freezing winters, overturning the long-held “warm tropical forest hypothesis” that has long influenced evolutionary biology.

Jorge Avaria-Llautureo, lead author at the University of Reading, said, “For decades, the idea that primates evolved in warm, tropical forests has gone unquestioned. Our findings flip that narrative entirely. It turns out primates didn’t emerge from lush jungles—they came from cold, seasonal environments in the northern hemisphere.

“Understanding how ancient primates survived climate change helps us think about how living species might respond to modern climate change and environmental changes.”

Moving to survive

Primates that could travel far when their local weather changed quickly were better at surviving and having babies that lived to become new species.

When primates moved to completely different, more stable climates, they traveled much further distances—about 561 kilometers on average compared to just 137 kilometers for those staying in similar, unstable climates. Early primates may have survived freezing winters by hibernating like bears do today—slowing down their heart rate and sleeping through the coldest months to save energy. Some small primates still do this—dwarf lemurs in Madagascar dig themselves underground and sleep for several months when it gets too cold, protecting themselves from freezing temperatures under layers of roots and leaves.

Primates didn’t reach tropical forests until millions of years later. They started in cold places, then moved to mild climates, then to dry desert-like areas, and finally made it to the hot, wet jungles where we find them today. When local temperatures or rainfall changed quickly in any direction, primates were forced to find new homes, which helped create new species.