I used to wonder what gas filled these bladders that kelp use to lift themselves toward the surface. Now I am convinced that it is oxygen, which these algae, like all plants, produce as a waste product. "Your Inner Fish" says that it was the increase in oxygen in the Earth's atmosphere that allowed multicelled organisms to develop. Like any answer to "why" questions, this seems slightly incomplete: why was there suddenly enough oxygen around? Why weren't there lots of plants earlier?