the human body as evolutionary assembly
consider unicellular organisms. over the course of evolution, these organisms evolved a differentiated set of skills. to better survive, each strand of organism would "team up" with a different, complementary strand where, together, they would stand a better chance for survival.
these organisms fused together, giving up their individuality to become a super-organism. of these unions and their progenies, some strands survived and some perished. again, these superorganisms fused into yet-higher organisms, and so on.
one of the outcomes of these fusions is the human body - along with the bodies of all the animals and plants on this planet. each distinguishable, functional area of the body can be found in a lower-level organism. in the higher level, the "organ" is developed and integrated within a greater whole.