Everyone has seen pictures of Earth from space and recognized that it is primarily blue with some parts that are green or brown, accented with white clouds in the sky.

Without even being taught, most people can figure out that this is largely because the Earth is covered with water and most plants are green.

According to a hypothesis by Professor Shiladitya DasSarma, who is a molecular biologist from the University of Maryland, and Dr. Edward Schwieterman, an astrobiologist from the University of California at Riverside, however, that may not always have been the case.

Their idea, which is called the Purple Earth Hypothesis, suggests, as the name implies, that the Earth would have once appeared a shade of purple.

If correct, the color would have been caused by plants using a different type of photosynthesis that reflects purple shades rather than green.

Today, most plants get their energy by absorbing energy from light at a wide range of wavelengths. The one wavelength that they have evolved to ignore, however, is green. This is somewhat odd since green is in the middle of the visible spectrum and contains lots of energy.

In the past, the theory says, the dominant organisms on Earth may have been microorganisms in the ocean that reflected purple light and absorbed the other colors, including green.

Billions of years ago, Earth had far less oxygen. In low-oxygen environments, plants can do well using a chemical called retinal rather than chlorophyll for photosynthesis. Even today, there are some microbes, including the Halobacteria, that thrive in these environments using this method.

If the oceans of the world were filled with microbes that used retinal, the oceans would have reflected much more light in the purple wavelength. Anyone who happened to be observing us from space may have called the planet a ‘purple marble’ rather than a blue one.

If this hypothesis is true, the color shift from purple to blue would have happened around 2.4 billion years ago. That is when the Earth went through its ‘Great Oxygenation Event’ that dramatically increased the amount of oxygen in the air.

Microbes that evolved to use chlorophyll began to thrive, and those that used retinal struggled to survive. The oxygen-rich environment also allowed larger and more complex plants and animals to evolve, many of which introduced the green color to land that we all recognize today.

So, while we all know and love our blue-green planet, it is fun to recognize that it may not have always looked this way, and who knows? Maybe someday it will shift to another color entirely.

If you thought that was interesting, you might like to read about 50 amazing finds on Google Earth.