So I was reading about polydactyl kittens and I got to thinking about what it must have been like when the first ever human baby was born with opposable thumbs. All the other mothers must have been like, "Ew! That's disgusting! You should get those funny-looking fingers removed right away, they make your kid look like some kind of mutant!" And the baby's mum would be all, "I know, right? But I don't know any really good surgeons, first of all because we're in the stone age or whatever and surgery hasn't been invented yet, but more importantly none of them seem to able to wield a scalpel properly and I'm not letting any unqualified half-assed dinosaur butcher anywhere near my kid's hands - OMG! My son could be a doctor! I'm going to enroll him in medical school just as soon as I can figure out how to hold a pen to fill in the form. I'm so proud of little Johnny Two-Thumbs. He won't need to write with his mouth or his toes or whatever we're using right now, he can wrap his thumb right around the pen and hold it in his hand. Get me Fox News, we're gonna be rich!"
I'm pretty sure that's almost exactly how it went. Assuming pens had been invented.