Five million years ago, the warm Miocene gradually descended into the cool Pliocene before dropping into the frigid Pleistocene. Such a change in temperature was so gradual that life went on without a global, life-changing catastrophe like what happened at the end of the Ordovician or the Devonian or the Permian or even the Cretaceous.
In this alternate scenario, no such in-between existed, and five million years ago, the warm Miocene dropped immediately into the frigid Pleistocene. The reasons for such an early change are as follows:
- North America has been connected to Asia for 45 million years via Beringia, barring the Arctic Ocean from the Pacific.
- The Himalayas had been rising for 65 million years to a current height of 33,500 feet above sea level. Tibet had been rising with them.
- At the time in question, two things happened simultaneously. One, Antarctica had now become 100% ice. Two, tectonic forces created Panama, connecting North to South America, barring the Atlantic from the Pacific.
In OTL, warm had become cool had become frigid. Here, warm had immediately dropped to frigid. Could such a sudden drop in global temperature result in a mass extinction?