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Why Neanderthals Sported Arms Like Popeye | LiveScience
"Neanderthals have really interesting upper bodies," researcher Colin Shaw, a biological anthropologist at the University of Cambridge in England, told LiveScience. "If you and I are both right-handed, you'd expect 4 to 13 percent asymmetry between our arms. Neanderthals have up to 50 percent or more asymmetry. They were doing something with their dominant arms that were either more intense or repetitive or both than we do today. The only population of modern people that we see who are similar are tennis players, who hit tennis balls many, many years aggressively."
Edit to add - My partner suggested maybe they were fapping to cave art