Interesting thought, but how do we know they couldn't talk in the first place? Maybe it used to be a skill we were born with...a natural skill for the first humans who did not have conversations to observe and copy. Who knows?! :)
Probably started with grunts and squawk. Grunt for lion. Squawk for leopard. Then they had to start coming up with new sounds for HIDE, RUN, or CLIMB TREE. Eventually a few words. BUT words could only come into play until after the mutation that caused our adam's apple to move down into our throat where it is now. Before that it was so high in our throat that we couldn't use our tongues and lips to vary the sounds we made. Infants have this "high adam's apple" for a while; that's how they can nurse without stopping for breath.