Other primates already have the anatomy required for natural language more or less, namely arms and hands. Signed languages are as complete and expressive as spoken languages, and there is speculation [1] that spoken and signed language use the same underlying grammatical facility or "language organ".
Kinda OT, but this got me thinking about why human evolution has preferred spoken over signed language - why did spoken language evolve as the favourite mode when we already had perfectly good anatomy for signed language? Spoken language has some obvious disadvantages that I can think of:
* limited iconicity compared to sign
* not good for communicating while hunting
* less private
* speech production requires fine articulations and motor control, takes longer to learn, extends adolescence?
* "The positioning of the larynx deep in the throat, and the tongue far enough low and back to articulate a range of vowels, also compromised breathing and chewing. Presumably the communicative benefits outweighed the physiological costs." [2]
Spoken has some advantages over sign though:
* communication over distance
* sound-based iconicity/ possible
* can communicate with poor vision
* can communicate while performing manual tasks (this is probably the biggest one)
Maybe early humans used a combination of both depending on the situation, and as the abstraction capability of the brain increased, spoken became the (hearing) norm. Either way, it seems like anatomical explanations for language evolution aren't very useful. Female Cosmetic Coalitions[3] is an interesting theory from anthropology which relates evolution of natural language with in-group trust.
[1] http://www.lscp.net/persons/peperkamp/Peperkamp_Mehler_(1999...
[2] The Language Instinct, Steven Pinker
[3] https://en.wikipedia.org/wiki/Female_cosmetic_coalitions