If we had to color a picture of the Earth, we would color it green and blue, right? Because it’s full of trees, plants and oceans. What if I told you our planet was purple billions of years ago? A group of scientists propose a surprising idea: in the past, before the existence of plants and forests, our planet could have a completely different color.

This theory is known as the Purple Earth Hypothesis and it explains that primitive life on Earth used a different molecule to chlorophile, which is responsible for the current green color, to capture the sunlight. This molecule would have given the planet a purple tone. So, let’s learn more about this theory and why some experts believe the Earth could become purple again.

Purple Earth

This theory was studied by doctor Edward Schwieterman, astrobiologist from the University of California in Riverside, and professor Schiladyta DasSarma, from the University of Maryland. Both are related to NASA and wanted to understand how Earth was before it had oxygen in the atmosphere.

The investigation focused on a molecule called retina, which is simpler than chlorophyll but also can capture the sunlight and transform it into energy. It’s something similar to plants’ photosynthesis.

However, the retinal behaves in a different way: it absorbs the green light and reflects the red and blue light. So, when these colors are reflected, the result is purple. This is why scientists believe Earth could have been covered with purple tones instead of green billions of years ago.

Life on purple Earth

At that time there weren’t any plants, animals or trees; life was very simple and was formed by tiny microorganisms called archaea. One of the examples of these beings is the halobacterium, a tiny organism that still exists today in very salty places like the Dead Sea. These microorganisms used the retinal to obtain energy from the sun like modern plants use chlorophyll.

Researchers think that, before there was any oxygen in the air, these organisms could have covered large parts of the planet making the Earth be seen with a purple color from space.

Great Oxygenation Event

The purple era didn’t last forever. About 2,000 million years ago, a huge change happened called the Great Oxygenation Event (GOE).  Before this moment, oxygen was almost nonexistent in the atmosphere, but new organisms emerged and started to use chlorophyll instead of retinal. Chlorophyll absorbed other types of light and released oxygen as a result of the process of photosynthesis.

With the passage of time, these organisms multiplied and filled the air with oxygen which allowed more complex ways of life like: plants, animals and human beings. This is how Earth left behind that purple color and became the green planet we know today.

Purple again?

It seems impossible, but scientists say that it could happen in the very far future. If at some point the level of oxygen in the atmosphere decreased a lot (due to climate change, serious natural disasters, or alterations in the ecosystem), the Earth could repeat similar conditions to those from billions of years ago. In that case, ways of life based on the retinal could expand again and Earth could recover its old purple color.

For now, this idea is still just a scientific hypothesis. Yet, this helps them to understand how life could have formed and how it could evolve in the future.

Purple beyond Earth

Professor DasSarma and other NASA experts are working along with astrobiologists and physicists to look for life on other planets. Thanks to advances in telescopes, today they can observe exoplanets (planets that orbit other stars) and analyze the colors of the light they reflect. If they get to detect purple tones on a far away planet, it could be a sign that there is life based on the retinal like the one that could have existed on ancient Earth.

Even though this is a hypothesis, it reminds us that our planet’s appearance and life itself have deeply changed with time and this could happen again some day.