In 2008, workers at an animal sanctuary on the island of Kaja in Borneo noticed the apes hanging from tree branches and attempting to spearfish with harpoons, just like the locals do. In other words, the orangutans saw us using weapons to cause harm to other species and quickly realized how powerful and awesome that makes you feel. Look at that majestic bastard up there. He even seems proficient, dangling precariously above water that we assume is full of deadly piranhas, steadying himself for a lethal strike of laser-like precision.
Luckily for human civilization, physical evolution has not yet caught up with the orangutans' desire, and their lack of dexterity makes it almost impossible for them to actually spear any fish. But they are able to use the sticks to spear fruit that's fallen from the trees to float tauntingly out of reach in the river, because before you learn to walk, you have to learn to crawl.
Jay Ullal/H.F. Ullmann
"Walk" and "crawl" being polite euphemisms for "conquer humanity" and "it'll happen any day now."
That said, the orangutans have figured out how to use the spears to steal fish out of the natives' fishing nets, which means they have already discovered the primary reason humans developed tools -- to fuck with other people.