And just where do you think that genetic diversity came from?
If you go back 100,000 years or so, you'll find that there were only a handful of our species around. Modern Humans arose in a very small geographic area. It wasn't until they migrated in small groups and became geographically isolated from one another that genetic diversity developed among different populations. This is basic Darwinian evolution 101. Evolution requires genetic bottlenecks in order to establish new traits within a population, otherwise new mutations are simply flushed out of the pool like small crustaceans at high tide.