  The next generation of ground- and space-based telescopes will be able to
observe rocky Earth-like planets in the near future, transiting their host
star. We explore how the transmission spectrum of Earth changed through its
geological history. These transmission spectra provide a template for how to
characterize an Earth-like exoplanet - from a young prebiotic world to a modern
Earth. They also allow us to explore at what point in its evolution a distant
observer could identify life on our Pale Blue Dot and other worlds like it. We
chose atmosphere models representative of five geological epochs of Earth's
history, corresponding to a prebiotic high CO2-world 3.9 billion years ago
(Ga), an anoxic world around 3.5 Ga, and 3 epochs through the rise of oxygen
from 0.2 percent to present atmospheric levels of 21 percent.
  Our transmission spectra show atmospheric spectral features, which would show
a remote observer that Earth had a biosphere since about 2 billion years ago.
These high-resolution transmission spectral database of Earth through
geological time from the VIS to the IR is available online and can be used as a
tool to optimize our observation strategy, train retrieval methods, and
interpret upcoming observations with JWST, the Extremely Large Telescopes and
future mission concepts like Origins, HabEx, and LUOVIR.
