  When Carl Sagan observed the Earth during a Gallileo fly-by in 1993, he found
a widely distributed surface pigment with a sharp reflection edge in the red
part of the spectrum, which, together with the abundance of gaseous oxygen and
methane in extreme thermodynamic disequilibrium, were strongly suggestive of
the presence of life on Earth. This widespread pigmentation that could not be
explained by geological processes alone, is caused by the cellular structure of
vegetation - a mechanism for potentially limiting damage to chlorophyll and/or
limiting water loss. The distinctive increase in the red portion of Earth's
global reflectance spectrum is called the vegetation red edge in astrobiology
literature and is one of the proposed surface biosignatures to search for on
exoplanets and exomoons. Earth's surface vegetation has only been widespread
for about half a billion years, providing a surface biosignature for
approximately 1/9th our planet's lifetime. However, as chlorophyll is present
in many forms of life on Earth, like cyanobacteria, algae, lichen, corals, as
well as leafy vegetation, such a spectral red edge feature could indicate a
wide range of life, expanding its use for the search for surface biosignatures
beyond vegetation alone to a time long before vegetation became widespread on
Earth. We show how lichens could extend the presence of Earth's red edge
surface biofeature to 1.2 Gyr ago, while ocean surface algae and cyanobacteria
could extend it to over 2 Gyr ago, expanding the use of a photosynthetic red
edge to earlier times in Earth's history.
