system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why did humans develop in Africa when there's so much more water/food in the Northern Hemisphere? 

Similarly, I understand why the Yangtze was the home of the asian cradle of civilization, but why the Tigris and Euphrates? Iraq seems really dusty and not as low in civilization costs as say, Turkey.
gpt
Evolution needs some sort of selective pressure.  Let me try to tone it down to ELI5:  If food is plentiful, a species may not evolve.  For example, bison have roamed the great plains of North America for thousands of years, and they have no reason to evolve any further (ignoring the problem of humans hunting them to near extinction).

Here is the key:  Africa was not always this dry.  It used to be more of a paradise for primates.  However, it gradually became drier and drier, and food gradually became harder to find.  This is a perfect scenario for evolution to do its work.  Our ancestors had to work harder and harder to find food over a long period of time.  If you were smarter, you would survive long enough to have children.  If you couldn't figure out how to communicate with others or how to fashion tools for hunting, your genes would die out.  Thus, Africa was the perfect place for humans to evolve.  

Once we were smart enough to live in Africa, we were smart enough to figure out how to leave Africa and find the bountiful paradises that awaited us.