Re:chimps & sign language (Score 2, Interesting)

Speaking from experience, you're wrong about "Babies don't usually get any tangible reward simply for saying a word or two". Babies are HEAPED with attention for their first nonsense syllables - and it's definitely not the same kind of attention they get by crying. It's eye-contact, smiles, laughs, high-pitched repetitive dialog. When the parent suspects that a sound is being used meaningfully, the attention is heaped even higher.