Many of the most interesting and complex relationships in nature can be found right in front of our noses, we just do not notice them. The study of symbiosis in many cases is research into things that have often been taken for granted.
Further study is needed into relationships that seem obvious.
Take the human body, for example, where ten percent of the dry body weight is bacteria. Some are producing nutrients that humans cannot produce for themselves, vitamin B12, for example. The relationships can occur within cells, within organs, and on the surface of cells and tissues. There are a number of theories that try to explain this process. What is interesting is the fact that multicellular organisms are somehow establishing relationships with single-celled ones.