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We propose that retinal-based phototrophy arose early in the evolution oflife on Earth, profoundly impacting the development of photosynthesis andcreating implications for the search for life beyond our planet. While theearly evolutionary history of phototrophy is largely in the realm of theunknown, the onset of oxygenic photosynthesis in primitive cyanobacteriasignificantly altered the Earth's atmosphere by contributing to the rise ofoxygen ~2.3 billion years ago. However, photosynthetic chlorophyll andbacteriochlorophyll pigments lack appreciable absorption at wavelengths about500-600 nm, an energy-rich region of the solar spectrum. By contrast, simplerretinal-based light-harvesting systems such as the haloarchaeal purple membraneprotein bacteriorhodopsin show a strong well-defined peak of absorbancecentered at 568 nm, which is complementary to that of chlorophyll pigments. Wepropose a scenario where simple retinal-based light-harvesting systems likethat of the purple chromoprotein bacteriorhodopsin, originally discovered inhalophilic Archaea, may have dominated prior to the development ofphotosynthesis. We explore this hypothesis, termed the 'Purple Earth,' anddiscuss how retinal photopigments may serve as remote biosignatures forexoplanet research.

Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures