The Purple Earth Hypothesis
Between 3 and 3.8 billion years ago life on Earth may have been purple, just the way it is green today. It is suggested that ancient microbes might have used a purple-pigmented molecule called retinal for their metabolic activities instead of chlorophyll. Retinal absorbs green light and reflects back red and blue, the combination of which appears purple. This is completely opposite to what chlorophyll does.
Microbes that came later had to make use of the blue and red light as all the green light was absorbed by the organisms containing purple membrane and so they used chlorophyll. Scientists speculate that both chlorophyll and retinal based organisms coexisted for a while. But because retinal is a simpler molecule than chlorophyll, and would have been easier to produce in the low-oxygen atmosphere of early Earth, it likely came first.
If further research confirms this hypothesis, it would help scientists looking for life on other worlds.