Subject: Why is biological evolution considered to work top down and not bottom up?

As far as i can tell, the first organisms to evolve in any given environment 
would be the simplest, unicellular ones. i believe that our biological 
evolution is driven by our interaction with these microorganisms, and not 
directly by our own human biological adaption. Thus any given cell type within 
our bodies would be evolved by infection from these constant & rapidly evolving 
organisms. We would then be simply the sum total of these, viable or not. We 
may have stalled our evolution to a point with modern medicine, but not stopped 
it completely.

Re: Why is biological evolution considered to work top down and not bottom up?