The correct answer is biological anthropology, or physical anthropology.
It is centered around human beings only, with respect to their common or different features that they either share or don't share with their related non-human primates and other ancestors. It deals with human evolution, the study of their behavior, reproduction, etc.
Light is to find the distance between planets called light years.
A single fundamental similarity and or unity among all living organisms, is that all living organisms are composed of the basic units is life known as cells.
