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Like EA mentions, spear throwing comes to mind. Also boomerangs, although I'm not sure if those are thrown more sidearm or not. And there are numerous beating/hacking type motions that are pretty similar to throwing, like swinging an axe to split wood, or hack up a freshly killed mastodon carcass. : )
No doubt, pitching pushes the physical limits and a lotta pitchers hurt their arms because of it. But I never pitched, and when I walk along the beach, I can't help but grab some rocks and chuck 'em in the ocean. Seems pretty natural to me.