Purple Earth

Mother Earth is the “Green Planet”, because of its green vegetation. Chlorophyll and the photosynthesis it makes possible are the basis for the earth's more complex forms of life. Even the animal life, because we, as animals, would not be able to live, and would not have developed, without free atmospheric oxygen, and that is a residue of photosynthesis.

But there was something before that. Free atmospheric oxygen did not always exist. In my e-book “From Fungi to Cancer” (2012), I wrote:

"Very long ago, Mother Earth had an atmosphere very different from today's. It did not contain free oxygen. The single-celled organisms, which dominated then, had to produce energy from fermentation and glycolysis - not very effective methods. An organism (a cyanobacterium) developed the chlorophyll and the photosynthesis, a more effective system which is still generally used in the plant kingdom. But there was a side effect, a complication: it left free oxygen as a residue, and that residue ended up in the atmosphere. Oxygen, however, is strongly toxic, so when it had built up to a certain level, most organisms of that period died out. Those that did not, were adapted to the new conditions. They applied a new form of metabolism, one using oxygen, which was much more effective than fermentation."

The use of green puzzles some scientists, because the most energy-rich part of the sunlight is the green part of the spectrum, and that is the colour we see, and thus the green light is reflected and not used. Perhaps it is too rich in energy, so it is a protective measure not to absorb the green light. The chlorophyll absorbs mainly blue and red light.

It is also possible that the ozone layer filters away too much of the green (and blue) light, while letting through the red, which is what chlorophyll absorbs best.

There is, however, another theory: The Purple Earth Hypothesis.

There is another photosensitive pigment in nature, retinal (not to be confused with the form of vitamin A that is also called retinal), which absorbs green and reflects red and blue light. It has a simpler structure than chlorophyll, is its “opposite” colour-wise, and it looks purple. Today this pigment is found in the membrane of halobacteria, which are photosensitive. They belong to the archea, organisms which can be traced back to before the chlorophyll era. Theory suggests that retinal-using microbes dominated Earth before the development of chlorophyll, and that the green light was already used by them, so the chlorophyll-using microbes had to manage with the leftovers of the light, the less energy-rich red and blue light.

According to this theory, the “Green Planet” once must have been the “Purple Planet”, although chlorophyll gradually came to outcompete retinal, because it is more efficient when it comes to utilising the energy of the light it absorbs.

(This article is based on material previously published in Meriondho Leo and in my e-booklet “Purple”, 2018.)

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