Our Ancestors Were Apparently Fine With Freezing

The story of primate evolution has always started the same way: a warm jungle, dense canopy, year-round fruit. Turns out that story is wrong. The order was reversed: the primate lineage began roughly 66 million years ago in cold, seasonally frozen terrain, not the lush tropical canopy the field had pointed to for decades.

A University of Reading team built statistical models from fossil records to map the climates early primates actually inhabited, working backward to locate the probable origin of all modern primate lineages. Their conclusion: most likely North America, in a climate with hot summers and freezing winters, overturning what the field had called the "warm tropical forest hypothesis" for decades. Early primates may have survived those winters by hibernating, slowing their heart rates and sleeping through the coldest months. Some small primates carry that same reflex today: Madagascar's dwarf lemurs burrow underground and go dormant for months each year as winter arrives.

The dispersal pattern that followed is the part that rearranges your assumptions. Tropical forests came much later, millions of years after primate origins. They started cold, moved to mild climates, then dry desert-like areas, and finally arrived in the hot, wet jungles where we find most of them today. Mobility was the mechanism: those that relocated to very different, more stable climates moved about 561 kilometers (349 miles) on average, compared to just 137 kilometers (85 miles) for primates that stayed in similar but unstable habitats. Climate instability didn't eliminate them; it scattered them into new species.

We've long assumed primates evolved in tropical forests because that's where most of them live now, which, as a chain of reasoning, is roughly as sound as assuming you were born in the hospital where you had your appendix out.

Read the full story at University of Reading, August 5, 2025

Hot Take: The "warm tropical forest hypothesis" held for decades not because the evidence was strong but because nobody checked. The uncomfortable thing about a field-defining assumption that goes unquestioned is that you can't tell it apart from a field-defining assumption that's simply correct. The pharmacology parallel writes itself.