Over the past two decades, enormous advances in the detection of
exoplanets have taken place. Currently, we have discovered hundreds of
Earth-sized planets, several of them within the habitable zone of their
star. In the coming years, the efforts will concentrate in the
characterization of these planets and their atmospheres to try to detect
the presence of biosignatures. However, even if we discovered a second
Earth, it is very unlikely that it would present a stage of evolution
similar to the present-day Earth. Our planet has been far from static
since its formation about 4.5 Ga ago; on the contrary, during this time,
it has undergone multiple changes in its atmospheric composition, its
temperature structure, its continental distribution, and even changes in
the forms of life that inhabit it. All these changes have affected the
global properties of Earth as seen from an astronomical distance. Thus,
it is of interest not only to characterize the observables of the Earth
as it is today but also at different epochs. Here we review the
detectability of the Earth's globally averaged properties over time.
This includes atmospheric composition and biosignatures and surface
properties that can be interpreted as signs of habitability (bioclues).
The resulting picture is that truly unambiguous biosignatures are only
detectable for about 1/4 of the Earth's history. For the rest of the
time, we rely on detectable bioclues that can only establish an
statistical likelihood for the presence of life on a given planet.