The upshot to all these changes was a turnover of species. The Eocene–Oligocene (E–O) extinction event ensued. It was relatively modest, occurring over 10 million years.
Species generally became more cold-tolerant or suffered loss of habitat range. The marine food web crashed during the early Oligocene, as temperate zone species faltered from changing deep-sea circulation patterns. By 36 MYA, tropical forests were restricted to the equatorial belt.
From 34 MYA into the Oligocene ushered a gradual change at the higher latitudes to drier climates and cooler temperatures. A feedback loop led to rapid Antarctic glaciation over 300,000 years, with a dramatic 30 °C drop in sea temperatures and concomitant cooling on land.
The cooling culminated in a mass extinction 32 MYA. As tropical forests became open woodlands and grasslands, primates vanished in North America and Europe.
Monkeys originated when Africa and Arabia were joined as an island continent. The animals there evolved in isolation until docking with Eurasia 24–20 MYA. It was only after the continents connected that the mammals of African origin – antelope, pigs, lions, rhinos – entered Eurasia. Monkeys colonized South America via island hopping on vegetative rafts: whence New World monkeys.
More global cooling during the Miocene diminished tropical forest in Africa. It was replaced by savanna and open woodlands.
Waves of adaptive radiation resulted in diverse primate species in Africa, Asia, and southern Europe.
Volcanism entered the evolutionary picture again 29.5 MYA with eruptions in Ethiopia and elsewhere. Lava fields were extensive.
Large bolide strikes 35 MYA – at Chesapeake, Bay Toms Canyon (160 km east of Atlantic City, New Jersey), and Popigai (Siberia) – may have been instrumental in stirring volcanic activity.
The scoured East African landscape recovered, with new ecosystem opportunities. Not coincidentally, apes evolved from Old World monkeys 25 MYA in that region, becoming the dominant primates during the Miocene.
Various lineages of apes descended. Several took hominoid form.