We live in a green world, but the ancient Earth could have been wrapped in a purple mantle.

The eldest microbes might have employed another molecule than chlorophyll to get advantage of the Sun's energy. Chlorophyll absorbs mainly blue and red wavelengths of the sunlight and as it reflects green ones, the plants appear as such for us.

But the green waves contain the highest energy level of the visible spectrum. "Why would chlorophyll have this dip in the area that has the most energy?" said Shil DasSarma, a microbial geneticist at the University of Maryland.

The human retina is the most sensitive to green light. DasSarma thinks that retinal, another light-sensitive molecule, preceded the chlorophyll emergence. Retinal, still found in the plum-colored photosynthetic halobacteria, uses the green waves and reflects the red and violet light, whose combination looks purple. "Primitive microbes that used retinal to harness the sun's energy might have dominated early Earth. Being latecomers, microbes that used chlorophyll could not compete directly with those utilizing retinal, but they survived by evolving the ability to absorb the very wavelengths retinal did not use," said DasSarma.

"Chlorophyll was forced to make use of the blue and red light, since all the green light was absorbed by the purple membrane-containing organisms," said William Sparks, an astronomer at the Space Telescope Science Institute (STScI) in Maryland.

Researchers believe that the chlorophyll won because it is more efficient than the retinal. "Chlorophyll may not sample the peak of the solar spectrum, but it makes better use of the light that it does absorb," explained Sparks.

The new theory could match with many of the retinal's traits and early Earth conditions. "For example, retinal has a simpler structure than chlorophyll, and would have been easier to produce in the low-oxygen environement of Early Earth," said DasSarma.

Also, halobacteria are a type of archaebacteria, organisms much older than classical bacteria, emerged in a period when Earth lacked an oxygen atmosphere. "Taken together, these different lines of evidence suggest retinal formed earlier than chlorophyll," said DasSarma. Others do not embrace the new theory so easily. "I'm a little cautious about looking at who's using which wavelengths of light and making conclusions about how things were like 3 or 4 billion years ago," said David Des Marais, a geochemist at NASA's Ames Research Center in California. "An alternative explanation for why chlorophyll doesn't absorb green light is that doing so might actually harm plants. That energy comes screaming in. It's a two-edged sword ... you can get too much of a good thing", said Des Marais.

For example, cyanobacteria, photosynthesizing bacteria that employ the green light, screen themselves beneath the sea surface to avoid the full impact of the Sun light. But the purple Earth hypothesis could offer new clues for those looking for life on space.

One biomarker on which astrobiology focuses is the "red edge" generated by plants on Earth: they absorb most (but not all) of the red light in the visible spectrum. The small percentage of reflected red light would be in this case an indicator of life on other planets. "I think when most people think about remote sensing, they're focused on chlorophyll-based life," signaled DasSarma. "If you happen to see a planet that is at this early stage of evolution, and you're looking for chlorophyll, you might miss."