What contributed the most to human migration more than ten thousand years ago?

Sturdy boats were invented.
Another Ice Age occurred.
A major volcano erupted in Africa.
A severe drought occurred.

Question

What contributed the most to human migration more than ten thousand years ago?

Sturdy boats were invented. 
Another Ice Age occurred.
A major volcano erupted in Africa.
A severe drought occurred.

anonymous · Answer

I'm going to guess that boats were finally perfected enough to permit decent passage through waterways?

anonymous · Answer

yes! it makes so much more sense when someone explains it to me. @Khirro