San Diego Museum of Man - Footsteps Through Time

Timestone 1
65 million years ago
Purgatorius unio
Fossil tooth

This is the earliest known primate-like mammal. This species is not considered a true primate. Its dental pattern suggests it was a primate ancestor-a transitional form between insectivores (insect eaters) and primates.

Purgatorius unio lived when Earth's climate was moist and warm and when mammals were shrew-like. With pointed molars that were taller at the rear, it likely ate insects, fruits, or seeds. Mammals like this survived the event that caused the dinosaurs' extinction, then diversified. True primates can be identified in the fossil record about 10 million years later.

Purgatorius unio, fossil tooth
Found in Montana in 1965 by Leigh Van Valen and Robert Sloan
Age: around 65 million years

Continental Drift
What is geology's role in primate evolution?

Earth's crust is like a shifting puzzle
Earth's crust is like a puzzle whose pieces float on a layer of molten rock. Over time, the pieces, or plates, slowly shift position, taking the continents with them. About 250 million years ago all the continents were joined together. Since then they have separated. Some, like Africa and South America, are drifting apart. Others, like India and Asia, are colliding.

Environments change as Earth's plates drift
The changing position of landmasses altered the course of primate evolution. As the continents drifted, ocean currents shifted and climates changed. Ice caps formed, higher latitudes cycled through cool and warm periods, and dry grasslands replaced lush tropical forests. Some primates evolved because favorable variations enabled them to survive the environmental changes. Others became extinct.

Earth's climate was warm and wet.

Sixty-five million years ago Earth's climate was warm from pole to pole, and getting warmer. Dense, swampy forests grew worldwide. Most were tropical, while those at the poles were subtropical. In hot, swampy forests on the North American-European continent small mammals, including the ancestors of primates, were thriving.

Skull of modern tree shrew