Actually it is much more complicated than that. You need the thumb to hold the spear but you also need the shoulder and back arranged in a way to throw it effectively. Poop and rocks are relatively easy to throw but to throw a spear you must rotate the hand 90 degrees. From one documentary I saw, humans are the only species on the planet to have the necessary muscles, skeleton and ligaments to throw a spear effectively. Pretty much other creatures with opposable thumbs can only effectively spike it.
Actually it is much more complicated than that. You need the thumb to hold the spear but you also need the shoulder and back arranged in a way to throw it effectively. Poop and rocks are relatively easy to throw but to throw a spear you must rotate the hand 90 degrees. From one documentary I saw, humans are the only species on the planet to have the necessary muscles, skeleton and ligaments to throw a spear effectively. Pretty much other creatures with opposable thumbs can only effectively spike it.