'Purple Earth Hypothesis' is gaining momentum in the scientific community

Strange as it sounds, some scientists think the earliest life forms on Earth may have painted our planet in shades of 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.

NASA-supported work points to retinal as that pivotal molecule, which lent these microbes a vivid violet color.

The concept has been investigated by astrobiologist Dr. Edward Schwieterman of the University of California, Riverside, and professor Shiladitya DasSarma of the University of Maryland.

Early color from a different molecule

Chlorophyll is the green pigment modern plants rely on, but it may not have been Earth’s first choice for photosynthesis. Retinal is simpler and was likely present on Earth when atmospheric oxygen was scarce.

During that period, known for low oxygen and a hazy sky, scientists believe sunlight was still abundant enough to power these purple microbes. This scenario points to a very different Earth from the lush, leafy version we see now.

Many of these primeval organisms fell under the archaea umbrella, a group that thrives in environments hostile to most other life.

One notable example is often called halobacterium, a bright purple microbe that survives in salty spots like the Great Salt Lake.

Despite its name, halobacterium is not a bacterium but an archaeon that uses photosynthesis in a less common way. It absorbs green wavelengths through retinal and reflects red and blue, which produces its striking purple appearance.

How purple Earth turned green 

Over time, other organisms evolved a more efficient pigment called chlorophyll, enabling them to harvest sunlight at stronger wavelengths.

This shift eventually overshadowed the retinal-based approach and helped fuel the Great Oxygenation Event, when oxygen levels in our atmosphere rose dramatically.

Retinal-based life did not disappear, but it was no longer the dominant force shaping the planet’s surface color. Chlorophyll-using organisms thrived, turning Earth’s general look from purple to green.

The search for colorful life

Astrobiologists suspect exoplanets might host creatures that still rely on retinal.

“If the Purple Earth hypothesis was correct and there was a dominance of purple organisms in the early Earth, then we might be able to find another planet that’s at an earlier stage of evolution,” said DasSarma.

Color signals from these far-off worlds could reveal whether simple purple life is out there. Early retinal-based photosynthesis might offer a stepping stone before more advanced pigments evolve.

Purple pockets on modern Earth

Although Earth is now mostly green, places like the Dead Sea still shine with violet hues thanks to halobacterium. These hardy microbes thrive in extreme salt concentrations that deter many other forms of life.

By studying these vibrant spots, researchers gain insights into how purple life could survive on extraterrestrial terrains with similarly tough conditions. Observing them also helps scientists refine the signals they might seek on exoplanets.

Today’s vegetation has a recognizable red edge, where leaves strongly absorb red light but reflect a specific infrared wavelength.

Retinal, on the other hand, would likely create a different type of spectral fingerprint that peaks in the green region.

Scientists are calling for instruments that can track a wider range of wavelengths to detect such signatures. This approach broadens our view of what life might look like on planets circling distant stars.

What the future holds

Advances in telescope technology will soon allow more detailed observations of exoplanet atmospheres and surfaces.

Analysts hope to find color patterns that reflect biological processes, whether they resemble chlorophyll or something entirely different. A confirmed Purple Earth phase would be a key reminder that life can take unexpected paths. 

“This is another reference point in a library of potential biosignatures that we can search for elsewhere,” said Schwieterman.

Why Earth’s purple past matters

The Purple Earth hypothesis remains unproven, yet it encourages new ways of thinking about our planet’s past and the search for life beyond.

If Earth’s earliest successful photosynthesizers truly glowed purple, then that color might reappear wherever organisms adopt retinal as their main solar sponge.

This imaginative view highlights life’s capacity to adapt in surprising ways. It also urges us to look for signals we might otherwise miss if we focus only on what we see today.

The study is published in the journal Astrobiology.

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