why did humans bother to leave Africa at all and go to such far away places as Australia and Americas?

why did humans bother to leave Africa at all and go to such far away places as Australia and Americas?
In Guns, Germs, and Steel, why did humans bother to leave Africa at all and go to such far away places as Australia and Americas? Did they decide it was time to colonize the whole world? Were they driven by curiosity about the unknown? Did the simply stumble and drift mindlessly from place to place? None of the above?