Warren, for example primates don’t have the physiological means to control the phonemic contrasts of human languages, but it is well established that they can learn sign language, even from other primates. For speech, you control contrasts between vocal sound configurations. In primates, the vocal folds cannot close completely, they are said to lack sufficiently fine motor control of the jaw and tongue, and as Phil Lieberman found, the root of the tongue is too far forward in the oral-pharyngeal cavety. However, they do have the necessary dexterity to control contrasts between configurations of the hands and fingers for signing.
The learning of signing I would guess requires not only developing perceptual input functions for the specific configurations but also repurposing and reorganizing some of those ‘mirror neuron’ systems that we were reading about 15 years ago. Rick suggested at the time that the neural firing that was called ‘mirroring’ is the firing of reference signals. The idea is that on observing another doing X, the reference signals for doing X fire, but presumably controlling in imagination since the observing animal does not actually do X. But the ‘mirror neurons’ were observed not in the motor area but in Broca’s area, which in humans is involved with language. Bill thought this might localize the proposed Category level. The thread initiated in 2005 by Dick Robertson is Mirror Neurons ?