![]() Taken together, these different lines of evidence suggest retinal formed earlier than chlorophyll, DasSarma said. It belongs to a group of organisms called archaea, whose lineage stretches back to a time before Earth had an oxygen atmosphere. Lastly, halobacteria, a microbe alive today that uses retinal, is not a bacterium at all. "Fatty acids were likely needed to form the membranes in the earliest cells," DasSarma said. "Chlorophyll may not sample the peak of the solar spectrum, but it makes better use of the light that it does absorb," Sparks explained.ĭasSarma admits his ideas are currently little more than speculation, but says they fit with other things scientists know about retinal and early Earth.įor example, retinal has a simpler structure than chlorophyll, and would have been easier to produce in the low-oxygen environment of early Earth, DasSarma said.Īlso, the process for making retinal is very similar to that of a fatty acid, which many scientists think was one of the key-ingredients for the development of cells. "You can imagine a situation where photosynthesis is going on just beneath a layer of purple membrane-containing organisms," DasSarma told LiveScience.īut after a while, the researchers say, the balance tipped in favor of chlorophyll because it is more efficient than retinal. The researchers speculate that chlorophyll- and retinal-based organisms coexisted for a time. "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, who helped DasSarma develop his idea. Primitive microbes that used retinal to harness the sun's energy might have dominated early Earth, DasSarma said, thus tinting some of the first biological hotspots on the planet a distinctive purple color.īeing 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, DasSarma said. ![]() ![]() Retinal, today found in the plum-colored membrane of a photosynthetic microbe called halobacteria, absorbs green light and reflects back red and violet light, the combination of which appears purple. DasSarma thinks it is because chlorophyll appeared after another light-sensitive molecule called retinal was already present on early Earth.
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