Human evolution theory utilizing concepts of neoteny & female sexual selection
An etiology of neuropsychological disorders such as autism and dyslexia, and the origin of left handedness.
"Contrasting the Herculean task of synthesizing (creating) syntax from words spoken alone, the cognitive act of focusing on part of the manual gesture to stand for part of the pattern does not seem difficult. A manual-brachial gesture understood as representing a raptor seizing prey could be taken, in proper context, to stand for the raptor, or the prey, or the act of catching. Taking apart the manual gesture by focusing on the active hand in one situation, on the inactive target or object hand in the another, and in still another on the action itself, would have resulted in an explosive multiplication of the the lexicon of gestural words, and because of the syntactic pattern in the gesture, the visible words in it would already be effectively divided into nouns and verbs. This brain-eye-limb activity, as was noted above, could have been equally as effective as speech in providing a selective factor for the rapid increase in human brain size and complexity in the last two million years. The social advantages such communication afforded would have contributed greatly to fitness of the population." (Armstrong DF, Stokoe WC, Wilcox SE (1995) Gesture and the Nature of Language. Cambridge Univ. Press: Cambridge pp. 185-186