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As strange as it may sound, some scientists believe that the earliest life forms on Earth may have colored our planet purple, rather than the green we see today. This idea, known as the Purple Earth hypothesis, suggests that single-celled organisms depended on a less complex molecule than chlorophyll to harness sunlight. The NASA-supported work points to retinal as the key molecule that gave these microbes their bright purple color. This concept was explored by astrobiologist Dr. Edward Schwitterman of the University of California, Riverside, and Professor Shiladitya DasSarma of the University of Maryland.
Early color from another molecule
Chlorophyll is the green pigment that modern plants rely on, but it may not have been Earth’s first choice for photosynthesis. Retinal is simpler and likely existed on Earth when there was little oxygen in the atmosphere. During that period, known for its low oxygen levels and hazy skies, scientists believe there was still enough sunlight to fuel these purple microbes. This scenario points to a very different Earth from the lush, leafy version we see today.
Many of these primitive organisms fell under the umbrella of archaea, a group that thrives in environments hostile to most other life forms. One of the most prominent examples is often cited as halobacteria, a bright purple microbe that survives in salty places like the Great Salt Lake. Despite its name, halobacteria is not a bacterium but an archaeon that uses photosynthesis in a less common way. It absorbs green wavelengths through the retina of the eye and reflects red and blue, which creates its striking purple appearance.
How the purple Earth became green
Over time, other organisms evolved a more efficient pigment called chlorophyll, which allows them to harvest sunlight with longer wavelengths.
This change eventually eclipsed the retinal-based approach and helped spark the Great Oxygenation Event, when oxygen levels in our atmosphere increased dramatically. Life on the retina didn't disappear, but it was no longer the dominant force shaping the color of the planet's surface. Organisms that use chlorophyll flourished, changing the overall appearance of Earth from purple to green.
The search for a colorful life
Astrobiologists suspect that there may be creatures on exoplanets that still rely on the retina of the eye.
"If the Purple Earth hypothesis were correct and the early Earth was dominated by purple organisms, then we might find another planet that is at an earlier stage of evolution," DasSarma said.
Color signals from these distant worlds could reveal whether simple purple life existed. Early retinal-based photosynthesis could be a stepping stone to more sophisticated pigments.
Purple pockets on modern Earth
Although Earth is now mostly green, places like the Dead Sea still glow with purple hues thanks to halobacteria. These hardy microbes thrive in extreme salt concentrations that would keep many other forms of life out. By studying these bright spots, researchers are gaining insight into how purple life might survive in extraterrestrial environments with similarly harsh conditions. Observing them also helps scientists refine signals they might look for on exoplanets.
Modern vegetation has a recognizable red edge, where the leaves strongly absorb red light but reflect certain infrared wavelengths. Retinal, on the other hand, is likely to produce a different type of spectral fingerprint, one that peaks in the green region. Scientists need instruments that can monitor a wider range of wavelengths to detect such signatures. This approach expands our understanding of what life might look like on planets orbiting distant stars.
What the future holds
Advances in telescopic technology will soon allow us to observe the atmospheres and surfaces of exoplanets in greater detail. Analysts hope to find color patterns that reflect biological processes, whether they resemble chlorophyll or something else entirely. The confirmed Purple Earth phase would be a key reminder that life can take unexpected paths.
“This is another starting point in a library of potential biosignatures that we can look for elsewhere,” Schwitterman said.
Why Earth's purple past matters
The Purple Earth hypothesis remains unproven, but it prompts new ways of thinking about our planet's past and the search for life beyond. If Earth's earliest successful photosynthesizers did indeed glow purple, then the color could reappear wherever organisms adopt the retina as their primary sun sponge. This imaginative view highlights life's ability to adapt in a remarkable way. It also prompts us to look for signals that we might miss if we focused only on what we see today. The study is published in the journal Astrobiolo gy.
