The hand

Man’s hand is what gives him speech.

Extract text echoing the film

... it is above all for the language that nature has added hands to our body. If man had no hands, the parts of the face would have been formed at home, like those of the quadrupeds, to allow him to feed himself: his face would have had an elongated shape, thinner in the region of the nostrils, with prominent, calloused, hard and thick lips to pull out the grass [...].

If the body had no hands, how would the articulated voice be formed in it? The constitution of the parts surrounding the mouth would not conform to the needs of language. The man, in this case, would have had to bleat, cry out, bark, neigh...

Thus, it is thanks to this organization that the mind, like a musician, produces language in us and that we become capable of speaking.

Gregory of Nyssa, “Treatise on the Creation of Man”, 379 AD.
Quoted by André Leroi-Gourhan in “Gestures and speech. 1. Technology and language”, 1964.