We owe our existence to predation.

First, there were the Blue-Green algae producing tons of oxygen as a result of the splitting of carbon dioxide and water mostly for their carbon and hydrogen content to make hydrocarbons. The excess oxygen can not exceed certain levels or else it would become toxic. At first, I thought that organisms would evolve that consumed decayed matter using up oxygen in the process but the problem I see here is that the decayed matter may have been too oxidized to have been of any real value. The best solution would have been the creation of a predator that kept the population of photosynthesizing organisms in check and, consequently, the oxygen levels.

After predation, multicellular organisms arose as a defense against the predator, and here we are. Organisms would have stayed unicellular because there is little advantage to grouping together and may be detrimental if your aim is to maximize exposure to light.