Longish article, follow link for more.

http://www.newscientist.com/article/dn22277-climate-change-determined...

Humans may have conquered the world, but not without a big
helping hand from climate change. A major study of the last
120,000 years of history reminds us that, while we are
adaptable, our species is ultimately at the mercy of the
climate.

Homo sapiens evolved in Africa around 200,000 years ago, but
only left the continent about 70,000 years ago. After that
our species rapidly went global, colonising first Europe and
Asia, and then Australasia and the Americas.

But why did early humans linger so long in Africa, and what
spurred them to finally move? Several theories have been
proposed, but according to a large effort to reconstruct
the last 120,000 years of human history – including the
climate we lived in and the vegetation we fed on – the
current population spread around the planet would not be
as it is without key changes in the climate.
...