Bacteria helped early animals to breathe:
Microbial mats might have functioned as oxygen oases for primitive multicellular life.
http://www.nature.com/news/2011/1105....2011.290.html
Gingras, M. et al. (2011). Possible evolution of mobile animals in association with microbial mats.
Nature Geoscience, 4: 372-375.