I walked past a small twig hanging off a tree today, tried to push it out of my way, and it fwapped me on the lip. It's not even the first time in my life I've done that, so I'm obviously not a genius to observe it.

From there maybe I'd make it a toy if I were a bored cave man. Maybe I'd tie some rope my cave dad taught me how to make on one end and flick it around. It's really not so much of a stretch that upon observing some boucy wood and rope that you could use that rope to propel something else. Maybe a rock. Wait. Spears work pretty well on those animals we like. What if I made a small spear and flung that with it?

Of course, it's easy to say it's easy 50k years later, but still. The logic isn't too hard to believe.