THE INDEFATIGABLE TONGUE. “A recent study by Sanders et al. (2013) showed that human tongue musculature has a greater percentage of slow-twitch fibers than tongue musculature from chimpanzees. Authors of that study correlated this evolutionary innovation in muscle physiology of the human tongue with the ability of the human tongue to slow down and produce more specific and longer contractions during speech, relative to how the tongue behaves in chimpanzees during vocalizations.”
http://onlinelibrary.wiley.com/doi/10.1111/joa.12440/full