Thumbs, at least opposable ones, are held to be the one remaining thing that makes us humans different from the other animals.
But think about it. Are thumbs really that useful? They get caught in car doors all the time. They are sliced open while opening cans of food for your cat overlords. Most of us except for the person sitting next to you can't pick our noses with our thumbs. And how many times have you stuck your thumb in your eye by mistake even if you haven't seen one Three Stooges short?
Of course, without a thumb, you couldn't hitch a ride. Instead of giving someone a thumbs up, you'd have to give them a finger, which is bound to lead to a lot of problems. Okay, so you can pick up and hold small objects with your thumb and forefinger. But they're probably from [[pizza] or something and you will end up launching an expensive and essential part of something to god-knows-where.
So citing thumbs as being important is just a last-ditch attempt to say we're somehow different from animals. So go ahead. Be you inner animal and hang by your feet from a tree branch eating a banana like your instincts tell you to do.