Earth Once Had Purple Oceans, Scientists Say

When most people picture Earth from space, they imagine a blue planet streaked with white clouds and edged with green continents. Yet some researchers argue that early Earth may have looked very different. Long before forests and algae dominated the surface, vast microbial communities could have tinted shallow seas purple or magenta. This idea, known as the Purple Earth Hypothesis, is based on peer-reviewed research into the pigments used by some of the planet’s earliest life forms.

The Origins of the Purple Earth Hypothesis

The Purple Earth Hypothesis was formally articulated in 2018 by microbiologist Shiladitya DasSarma and astrobiologist Edward W. Schwieterman in the International Journal of Astrobiology. Their work proposed that, before chlorophyll-based photosynthesis became widespread, early microorganisms may have relied on a simpler light-harvesting molecule, retinal.

Retinal is a small chromophore that absorbs light in the green to yellow portion of the visible spectrum. Unlike chlorophyll, which absorbs red and blue wavelengths and reflects green, retinal absorbs green light and reflects red and blue. This combination gives retinal-containing organisms a purple or magenta appearance. The hypothesis suggests that if retinal-based microbes were abundant in shallow marine environments billions of years ago, they could have imparted a purple hue to large regions of Earth’s surface waters.

Pigments and Early Energy Capture

Chlorophyll is central to modern photosynthesis and underpins most ecosystems today. However, chlorophyll is structurally complex and requires intricate biochemical pathways for its synthesis. Retinal, by contrast, is chemically simpler and forms part of light-driven proton pumps such as bacteriorhodopsin. Bacteriorhodopsin is found in certain halophilic Archaea that thrive in highly saline environments today. These organisms often form bright purple blooms in salt ponds and coastal lagoons. Their colour provides a modern analogue for what retinal-dominated ecosystems might have looked like in Earth’s distant past.

DasSarma and Schwieterman argue that retinal-based phototrophy may have evolved early in life’s history, possibly more than 3 billion years ago. Chlorophyll-based oxygenic photosynthesis likely emerged later, prior to the Great Oxygenation Event around 2.4 billion years ago. When oxygen-producing cyanobacteria became widespread, they transformed the atmosphere and gradually shifted the planet’s dominant biological pigment from purple to green.

Modern Evidence and Biological Plausibility

The Purple Earth Hypothesis rests on multiple strands of indirect evidence. First, retinal-based phototrophy is widespread in modern oceans. Studies show that proteorhodopsins, light-sensitive retinal proteins, are common in marine bacteria and contribute significantly to microbial energy budgets.

Second, the spectral properties of retinal complement those of chlorophyll. Retinal absorbs green wavelengths that chlorophyll reflects, which suggests that these pigments could have coexisted or sequentially dominated without complete overlap in resource use. This spectral complementarity strengthens the argument that retinal-based systems could have thrived before chlorophyll became dominant. In interviews discussing the hypothesis, DasSarma has emphasised that retinal-based phototrophic metabolisms remain important in present-day marine ecosystems. Their persistence suggests that such systems are robust and evolutionarily ancient.

Implications for the Great Oxygenation Event

The transition from retinal-based phototrophy to chlorophyll-driven oxygenic photosynthesis had profound consequences. Cyanobacteria using chlorophyll began splitting water molecules and releasing oxygen as a byproduct. Over millions of years, oxygen accumulated in the atmosphere during the Great Oxygenation Event, fundamentally altering Earth’s chemistry and enabling the evolution of complex multicellular life.

If retinal-based organisms once dominated the biosphere, then the shift to chlorophyll represents not only a metabolic transition but also a planetary colour change. The familiar green of modern vegetation may therefore be a relatively recent development in Earth’s 4.5-billion-year history.

Astrobiological Significance

The hypothesis has implications beyond Earth’s past. Astrobiologists searching for life on exoplanets often focus on detecting chlorophyll-like spectral signatures. However, Schwieterman and colleagues have argued that retinal-based life would produce a different reflectance pattern, sometimes referred to as a purple edge instead of the vegetation red edge associated with chlorophyll.

Recognising alternative biosignatures broadens the search for extraterrestrial life. If early Earth supported retinal-dominated ecosystems, similar pigment systems might evolve on other worlds with different atmospheric compositions or stellar radiation environments.

Limitations and Ongoing Research

It is important to note that the Purple Earth Hypothesis remains a scientific proposal rather than confirmed history. Direct fossil evidence of ancient pigments is scarce because molecular structures degrade over geological timescales. Most support for the hypothesis comes from evolutionary reasoning, comparative biochemistry, and ecological modelling.

Researchers continue to investigate early metabolic pathways, pigment evolution, and the spectral properties of microbial communities. Improved models of early Earth’s atmosphere and ocean chemistry may clarify whether conditions favoured widespread retinal phototrophy.

Rethinking Earth’s Early Appearance

The Purple Earth Hypothesis challenges the assumption that green is the default colour of life. Instead, it suggests that Earth’s earliest biosphere may have reflected sunlight in shades of purple before oxygen and chlorophyll reshaped the planet. While definitive proof remains elusive, the hypothesis expands our understanding of life’s adaptability and reminds us that Earth’s appearance has not always matched the blue and green world we know today.