I just learned the following.

• We have had periodic ice ages for only about the last 2.5M years. I had assumed that they have been going on for billions of years.
  
• Ice ages can occur suddenly, in perhaps 10 or 20 years.
  
• North and South American were disconnected until about 2.5M years ago, and ocean currents flowed thru what is now Panama. Closing this (natural) Panama canal may have been what made the Earth's climate unstable.
  
• Hominids (human ancestory) just started to make stone tools and acquire intelligence about 2.5M years ago. Some people think that this is not a coincidence.