[duaneb]

If my college biology understanding of evolution is at all accurate, a functional oxygen pathway would provide an edge in the ecosystem (read: high rates of population growth) and may have, in fact, been a catalyst for multi-cellular growth.

[ethbro]

The most recent discussion I heard seemed to be the theory that a functional oxygen pathway enabled multicellular life by providing enough surplus energy to power the entire organism in an evolutionarily efficient manner. (Add in the fact that by that point the planet was also teeming with single celled biomass to consume)