Humans have opposable thumbs – meaning we can touch the surface of the thumb (the side with the finger print) to the surface of each of the fingers on the same hand. ‘New World monkeys’ like marmosets can’t do this, but I think most if not all ‘Old World monkeys’ have opposable thumbs like humans though. I really don’t know why that this is though. Clearly there is an evolutionary advantage to humans having a thumb joint that allows us to grasp very small things in our fingers, but I’m not sure why the same thing isn’t an advantage for ‘Old World monkeys’. Marmosets have claws rather than nails, so I guess it wouldn’t be much use trying to hold things between two claws. It may be that their claws are evolutionarily more important to for keeping them safe by making them good climbers than grasping small objects with their fingers would be.