Retinal, a purple pigmented molecule once dominated plant life on earth, enabling the growth of not-so-green foliage on the planet.

A NASA-funded research suggests that the earliest life on earth might have been purple as it is green today, owing to the presence of chlorophyll. Ancient microbes might have used a molecule other than chlorophyll to harness the ray of the sun. This molecule imparted a violet hue to the organisms. Keep reading to find out more about the Purple Earth Theory.

The hypothesis

A new study argues that 2.4 to 3.5 billion years ago, retinal was the dominant molecule and it preceded chlorophyll as the sunlight-absorbing molecule. The hypothesis has been propounded by Dr. Edward Schwieterman, an astrobiologist, and Shiladitya DasSarma, Professor of molecular biology at the University of Maryland. This study indicates that the earth may have been very purple in the beginning.

Retinal and purpleness

There is another form of life known as archaea that sometimes harnesses photosynthesis. A form of archaea is a halobacterium (it is not a bacteria and the word is misleading here), that uses a different chemical called retinal to enable the process of photosynthesis. Retinal absorbs the green light and reflects the rest, making it look purple rather than green.

Why is the earth green today and not purple?

The research further suggests that the earth is green and not purple now because chlorophyll may use an efficient part of the light spectrum, and is more efficient than retinal at releasing energy. In short, retinal may have failed to adapt the way chlorophyll did. Thus, the earth is now green and not purple.

Is the theory confirmed?

The purple earth hypothesis has not received confirmation yet, but when and if it does, the approach of scientists to seek life in the rest of the universe will need to change. The biologists believe that if retinal is simpler than chlorophyll, it could be a common source of life on the other planets of the universe.

Implications for astrobiology

DasSarma also suggests that if a scientist is looking at a planet at its early stage of evolution, looking for chlorophyll must be avoided, as they might miss it for looking at the wrong wavelength. Plantlife on earth emits a “red edge” (a reflected light biomarker – as most red light is absorbed by plants and a meager amount is reflected at a specific wavelength). Instead, the scientists must look for another biomarker if the purple earth hypothesis receives confirmation.

Do you think the earth was purple before?