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

I think the "can communicate with poor vision" is the most important as its dark in about 50% of the day when sign language doesn't really work. Not to mention that you first need to call the other's attention to start communicating with signs, possibly by making some sort of sound so they look at you. The second most important benefit is (IMHO) that you can speak while you are focusing on something, e.g. waiting for the prey to come closer while everyone holds their weapons.

I would hypothesize that vocal speech is much faster and relatively effortless. Hunters still use sign language, so it has its place. For private speech people whisper or lipread.

If you try to getva child to talk, it takes about two years of arduous effort. To get a primate to talk, you would need to raise as your child, spending a few years in the process.

That parrot that talked, was raised in such a way.