  The high reflection of land vegetation in the near-infrared, the vegetation
red edge (VRE), is often cited as a spectral biosignature for surface
vegetation on exoplanets. The VRE is only a few percent change in reflectivity
for a disk-integrated observation of present-day Earth. Here we show that the
strength of Earth's VRE has increased over the past ~500 million years of land
plant evolution and may continue to increase as solar luminosity increases and
the planet warms, until either vegetation coverage is reduced, or the planet's
atmosphere becomes opaque to light reflected off the surface. Early plants like
mosses and liverworts, which dominated on land 500-400 million years ago,
produce a weaker VRE, approximately half as strong as that of modern
vegetation. We explore how the changes in land plants, as well as geological
changes like ice coverage during ice-ages and interglacial periods, influence
the detectability of the VRE through Earth's geological past. Our results show
that the VRE has varied through the evolutionary history of land plants on
Earth, and could continue to change into the future if hotter climate
conditions became dominant, encouraging the spread of vegetation. Our findings
suggest that older and hotter Earth-like planets are good targets for the
search for a VRE signature. In addition, hot exoplanets and dry exoplanets with
some water could be the best targets for a successful vegetation biosignature
detection. As well as a strong red edge, lower cloud-fractions and low levels
of atmospheric water vapor on such planets could make it easier to detect
surface features in general.
