The Hidden Brain Connections Between Our Hands and Tongues: Sticking out your tongue while doing delicate work with your hands reveals a history of evolutionary relationships.

From the article:

• Where did this coordination come from? It likely originated in our ancient ancestors’ hand-to-mouth feeding movements and their development of language, because spoken language is typically accompanied by automatic hand movements. Presumably, hand gestures were the first type of communication to evolve, and they gradually blended with appropriate syllabic utterances — mouth sounds — that allowed for language. Indeed, functional brain imaging studies show that specific tongue and hand movements activate the same region of the brain in the premotor cortex (the F5 region). Furthermore, the same neurons in the premotor area fire when a monkey grasps an object with its mouth or its hand. Electrical stimulation of this same area triggers a monkey’s hand to make a grip motion while its mouth opens, and its hand moves to its mouth.

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