Recently I was struck by the fact that some people have an internal monologue and others don't, and some people can see vivid images in their mind and some can't see anything at all. These seem like very dramatic differences to me.

I'm skeptical that we are less diverse than other species. In a herd in nature, exposed to the wild, exposed to predators and food scarcity, there is not much room for diversity: you must be able to survive. In human society, on the other hand, we live in an artificial environment insulated from those risks, and where any number of skills are sufficient for survival: you can be funny, musical, logical, artistic, patient and caring, mathematical, strong, good at fighting etc.