Red Planet

Recent studies have suggested that photosynthesis may have begun about 3.4 billion years ago. But before photosynthetic organisms developed, the earth might have not been a blue planet at all. The unoxidized iron in the ocean might have given it a dark green or even black color. Then as photosynthesis ramped up, it would have turned the oceans red as the iron oxidized and the earth became a blue planet as the dissolved iron oxide precipitated to the bottom.