i think that last statement is the key to why the bow replaced the atlatl. the wildlife in the americas was quite a bit larger when man first appeared, and required a larger spear to kill them. once they were killed off, and nothing bigger than a bison was around, the bow was more convenient, and required less wood for it's little 'spears', for the game remaining (and for anti-personnel duties). the 3-5 oz. atlatl dart at 848 ft delivers quite a bit more punch than a puny lightweight 180 grain turkish flight arrow 'spear' at 900 yds.
Turkish flight arrows