*shrugs* I can't see this being more of a problem to them than evolution? Not starting a discussion on that, my point is just:
For instance, they could just as well ask: If , by chance, the first two cats happened to be white, why are there so many colours of cats? and even if they were two different colours, why is there all of todays variey? ... Because it changed over time. Well, why can't creations of God change over time then?
Whether the first two people on earth were black, brown, white or blue, humans today look different.
I personally think God is perfectly able to create diversity by his own choice