http://www.plantphysiol.org/content/154/2/434.full plantphysiol.org reference to full recent article Early Evolution of Photosynthesis http://www.plantphysiol.org/content/154/2/434.full 3.4 billion years ago plantphysiol.org over 3 billion years ref http://www.plantphysiol.org/content/154/2/434.full http://www.plantphysiol.org/content/154/2/434.full