great question! it's a big concept, if only because of the massive time spans. I'd check out the library for some age appropriate evolution books. Best to go with evidence-based information.

you've inspired me - my 5 year old hasn't asked this yet, but I found this site http://learnthingsweb.hubpages.com/h...on-to-Children that looks good. (has some kids books listed too).

Have fun, sounds like you have a smart kid! Love the big questions!