Scientists studying early life on Earth believe the planet may have appeared purple around 2.4 billion years ago, long before the rise of oxygen-rich atmospheres and green vegetation. The theory is based on research into ancient microbes that likely dominated Earth’s surface and oceans during this period, known as the Archean and early Proterozoic eons.

According to researchers, primitive microorganisms may have relied on a molecule called retinal, rather than chlorophyll, to capture sunlight for energy. Retinal absorbs green light and reflects red and violet hues, which could have given large areas of the planet a purple appearance. These microbes are thought to have thrived in low-oxygen environments, well before photosynthetic organisms began producing oxygen at scale.

This phase is believed to have preceded the Great Oxidation Event, when cyanobacteria using chlorophyll started releasing significant amounts of oxygen into the atmosphere. Over time, oxygen-producing life forms outcompeted retinal-based organisms, leading to the dominance of green plants and algae and fundamentally transforming Earth’s atmosphere and surface colour.

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Scientists say the idea of a “purple Earth” has important implications beyond understanding our planet’s past. It offers new clues for the search for extraterrestrial life, particularly in how astronomers interpret light signatures from distant exoplanets. If life elsewhere relies on pigments other than chlorophyll, alien worlds may not appear green at all.

By expanding the range of biological signatures scientists look for, the theory challenges long-standing assumptions about what habitable planets might look like. Telescopes searching for life beyond Earth often focus on signs associated with oxygen and chlorophyll, potentially overlooking worlds dominated by different biochemistries.

Researchers stress that Earth’s evolutionary history shows life can thrive under vastly different conditions. Understanding these early, alternative forms of energy capture not only reshapes our view of the past but also broadens expectations for the future, both in terms of planetary evolution and the possibility of life elsewhere in the universe.

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