Purple Earth hypothesis

The purple sulfur bacteria provide strong support for the purple Earth hypothesis.

In short, early Earth was largely anoxic. Since the sun's energy reaches Earth's surface most efficiently in the green region of the visible spectrum, the most energy-efficient way for early photosynthesizers would have been to absorb green light while reflecting red and blue—resulting in a purple color.

To compete, a new group of organisms evolved that reflected green light and absorbed red and blue to make sugars. This group also produced oxygen, which was toxic to the anoxic purple sulfur bacteria. As the Earth became increasingly oxygenated, these green photosynthesizers outcompeted the purple ones.

Purple sulfur bacteria still exist today but are limited to oxygen-free environments.