If, like me, you have been left a little confused by the explosion in studies of human migration based on inheritance of the Y chromosome, be prepared to be sorted out. The Bradshaw Foundation has a wonderful animated map that shows the various migration routes, their timing, and what was happening on the global weather front. This was, for me, a really useful summary.
Thanks to Luigi for the link. There's some other good stuff on the foundation's home page, though I do wish it wouldn't click at me as I mouse over the tabs.
2022-07-27: The map is currently being updated, and the site is now called Journey of Humankind.