Shiladitya DasSarma, a microbiologist and professor at the University of Maryland, has explored and proposed the “Purple Earth hypothesis.” According to this research, our beautiful blue planet, which we call home, actually appeared purple during its early days.
This hypothesis doesn’t imply that there were purple trees, purple grass, or purple animals. This phenomenon occurred before multicellular life forms came into existence, during a time when single-celled microorganisms dominated the planet and created a purple hue that could be seen from outer space. These microorganisms thrived abundantly, densely populating the Earth during that era.
DasSarma’s research focuses on extremophiles, microorganisms that thrive in high-salt environments, and such microorganisms still exist in today’s environment where salt concentrations are high. In Senegal, Africa, there is a world-famous pink lake known as “Rose Lake.” The reason why Rose Lake is pink, and sometimes even shades of purple, in certain months is due to a type of ancient bacteria called Halobacterium.
Halobacterium can use sunlight for photosynthesis, but instead of using chlorophyll, they use retinal. Retinal has a simple molecular structure and can absorb the highest-energy blue light from sunlight. It does not absorb red and violet light, which is why it reflects red and violet wavelengths to our eyes, resulting in the pink and purple appearance of the lake.
DasSarma believes that the first photosynthetic organisms on Earth might not have used chlorophyll but rather retinal-like compounds. This is because retinal is an ancient and simpler molecule that existed before chlorophyll.
These early photosynthetic organisms dominated until the emergence of “chlorophyll,” the green pigment essential for photosynthesis in plants, which is responsible for converting atmospheric oxygen. Oxygen is a highly reactive gas that can oxidize organic compounds. The earliest life forms on Earth were mostly anaerobic bacteria and could not survive in an oxygen-rich environment. As a result, these microorganisms went extinct.
Like every mass extinction event, the arrival of oxygen not only disrupted Earth’s biological chain but also reshaped its ecological makeup.
Despite oxygen nearly wiping out life on Earth, there were still a few adaptable organisms that emerged victorious after the great oxidation event. These are the organisms that use chlorophyll for photosynthesis, while retinal-using organisms exited mainstream history, only to be found in a few extreme environments.
Another hypothesis is that our planet was once covered in ancient purple bacteria, ancestors of which can still be found in places that do not require extremely high salt concentrations.
These bodies of water contain a dense mat of purple bacteria, but they live deep underwater, making them invisible from the surface, according to Jennifer Glass, Assistant Professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. “However, in the past, before the accumulation of oxygen in the atmosphere, these purple bacteria grew to levels that could be seen covering the Earth.”
So, while Earth is green today, it’s fascinating to contemplate that it might have been purple in the distant past, offering us a glimpse into the colorful history of our planet.