What defines humanity?
The human opposable thumb is longer, compared to finger length, than any other primate thumb. This long thumb and its ability to easily touch the other fingers allow humans to firmly grasp and manipulate objects of many different shapes.
That is an abstract definition
No it isn't.
not a particularly specific one you are able to measure.
Your question was what defines humanity, not how do I measure what defines humanity. Besides, if you were interested in measurements you'd have measured the human thumb before challenging its uniqueness.