  We propose that retinal-based phototrophy arose early in the evolution of
life on Earth, profoundly impacting the development of photosynthesis and
creating implications for the search for life beyond our planet. While the
early evolutionary history of phototrophy is largely in the realm of the
unknown, the onset of oxygenic photosynthesis in primitive cyanobacteria
significantly altered the Earth's atmosphere by contributing to the rise of
oxygen ~2.3 billion years ago. However, photosynthetic chlorophyll and
bacteriochlorophyll pigments lack appreciable absorption at wavelengths about
500-600 nm, an energy-rich region of the solar spectrum. By contrast, simpler
retinal-based light-harvesting systems such as the haloarchaeal purple membrane
protein bacteriorhodopsin show a strong well-defined peak of absorbance
centered at 568 nm, which is complementary to that of chlorophyll pigments. We
propose a scenario where simple retinal-based light-harvesting systems like
that of the purple chromoprotein bacteriorhodopsin, originally discovered in
halophilic Archaea, may have dominated prior to the development of
photosynthesis. We explore this hypothesis, termed the 'Purple Earth,' and
discuss how retinal photopigments may serve as remote biosignatures for
exoplanet research.
