Prof Stephen Hawking: man faces nuclear armageddon and must colonise space

[txlibdem]

12. With 5 mass extinctions plus one that nearly wiped out the humans I count 6

Here's a link that shows what caused each and you can drill down to see specifics of the percent of species killed off.
http://dsc.discovery.com/earth/wide-angle/mass-extinctions-timeline.html

Now that we are sentient and have the capability, it's just plain crazy not to try to colonize space. Hawking gave us a hundred years to start. There' s no telling where our technology will be at that time. But since we can do it with today's technology (admittedly at great expense), I see no reason why we should not start planning for it starting in 2100.