I love the roller-coaster rush I get when I read numbers like this:

The pace of human history is a very fast one. Homo Erectus left Africa 90,000 generations ago. Early homo sapiens developed in Africa 30,000 generations ago, and only left the continent 30,000 years ago, or 1500 generations - an amazingly short period of time. ...

from Ethan Zuckerman's first post from the current TED conference.

Another one I like is that humans started farming about 8,000 years ago.