A groundbreaking study has revealed that early primates evolved in cold climates, not in the tropical jungles long believed to be their birthplace. Instead of warm, lush forests, these ancestors of monkeys, apes, and humans likely adapted to seasonal environments with freezing winters and hot summers in what is now North America.

The findings, published Tuesday in the journal PNAS, challenge a long-standing theory in evolutionary biology. For decades, scientists assumed primates emerged in tropical regions, but new evidence shows their origins lie in much harsher, colder conditions around 66 million years ago.

Researchers at the University of Reading reached this conclusion after combining fossil records with advanced statistical models to reconstruct ancient habitats.

They found that the earliest primates thrived in areas that experienced dramatic seasonal changes—overturning the so-called “tropical forest hypothesis” that has shaped scientific thinking for generations.