Humans are more highly evolved as the human taxonomic group has undergone more evolutionary changes. If we consider that the first organisms were similar to bacteria, it would mean that existing bacteria have changed very little since life first began. The human lineage however has seen dramatic changes in life form from bacteria to multicellular organisms and so on to progressively more complex forms.
It would be a pain to write all out all the details, so this might help. There isn't that much reading involved. http://lsa.colorado.edu/essence/texts/lungs.html