Earth Used To Be Purple, Study Shows

Posted: Oct 18 2018, 3:54pm CDT | by , Updated: Oct 18 2018, 3:58pm CDT , in Latest Science News


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Earth Used to be Purple, Study Shows
Credit: NASA/Ames/JPL–Caltech

The results have implications for the search for life beyond our planet

Earliest life on Earth may have used a purple-pigmented molecule called retinal rather than chlorophyll to harness energy from sunlight. As a result, young Earth likely appeared purple rather than green. If earliest life forms relied on retinal-based photosynthesis, it has implications for finding distinctive biosignature outside our solar system.

Earth's atmosphere was not always filled with oxygen. It contained carbon dioxide and methane until the emergence of oxygenic photosynthetic microbes like cyanobacteria around 2.4 billion years ago. These microbes caused Great Oxygenation Event and significantly altered the Earth’s atmosphere with the rise of oxygen. Cyanobacteria converted sunlight into energy using a green pigment called chlorophyll.

Photosynthetic life forms are known to have existed before Great Oxygenation Event as far back as 3.5 billion years ago, but the origin and evolution of photosynthesis via chlorophyll remains largely unknown. Now, Dr. Edward Schwieterman, an astrobiologist at the University of California, Riverside suggests that both retinal and chlorophyll-based photosynthesis evolved in tandem, absorbing sunlight at complementary wavelengths.

Retinal can be easily produced in low-oxygen environments. It absorbs green light in the same way that vegetation on Earth absorbs red and blue light today. Therefore, retinal-based life would appear purple in color and this purple hue could serve as a possible indicator of habitable exoplanets.

“Retinal-based phototrophic metabolisms are still prevalent throughout the world, especially in the oceans, and represent one of the most important bioenergetic processes on Earth.” Shiladitya DasSarma, a Professor of molecular biology at the University of Maryland, said.

Chlorophyll, the main photosynthetic pigment of plants, absorbs light peaking at wavelengths of 465nm and 665nm and reflects green ones. This reflected light gives plants their green color. Many proteins like bacteriorhodopsin absorb light peaking at 568nm, close to the wavelength at which the Sun's light peaks (550nm). And chlorophyll does not absorb light at this range.

“This is exactly what got us thinking that the two pigments – retinal and chlorophyll – may have co-evolved.” DasSarma said.

Since retinal is the simpler molecule, it would have come first and could be more commonly found in life in the early Universe. Such a stage in Earth's history has been named 'Purple Earth'.

“We propose here that the biochemical simplicity of retinal-based phototrophy, the spectral complementarily of bacteriorhodopsin pigments with chlorophylls and the newly uncovered widespread diversity of microbial rhodopsins throughout aquatic and terrestrial ecosystems are suggestive of the fundamental role retinal may have played in the early history of life on Earth.” Authors wrote in the study. “We posit here that domination by retinal-based phototrophs in the early history of life may have created the first ‘Purple Earth’ that at some point gave way to modern photosynthesizers before the rise of atmospheric oxygen.”

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<a href="/latest_stories/all/all/47" rel="author">Hira Bashir</a>
The latest discoveries in science are the passion of Hira Bashir (). With years of experience, she is able to spot the most interesting new achievements of scientists around the world and cover them in easy to understand reporting.




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