There's a great deal about humans that is poorly developed -- so far, and which could therefore be considered transitional. Our eyes are very nice, as far as they go, but we are virtually blind to the full range of the electromagnetic spectrum. If we could see radar, radio, cosmic rays, etc. we could have learned much more about the world much faster. And it would have been very useful if we could see bacteria. Think of all the lives that might have saved. Our defective eyes really held us back.
Then there's the digestive system. Humans are terribly subject to (gasp!) constipation, a disorder which rarely affects our dogs, for example. This is because our internal intestinal configuration hasn't really adjusted yet to our upright posture. Our toes are a joke, when you take the time to think about them. Of what use is the little toe? Is it there only so your mother could play "this little piggy went to market ..."? A physician could go on and on with a list of our physiological imperfections. But this little sample should indicate that we are hardly a perfectly-formed species. Not yet. Maybe not ever.
A follow-up to that is one of "Physicist's" favorite points to folks hung up on the "just so" arguments: our legs are so perfectly adapted they are EXACTLY the right length to reach the ground. What are the odds of that?