  Planet Earth has evolved from an entirely anoxic planet with possibly a
different tectonic regime to the oxygenated world with horizontal plate
tectonics that we know today. For most of this time, Earth has been inhabited
by a purely microbial biosphere albeit with seemingly increasing complexity
over time. A rich record of this geobiological evolution over most of Earth's
history provides insights into the remote detectability of microbial life under
a variety of planetary conditions. We leverage Earth's geobiological record
with the aim of a) illustrating the current state of knowledge and key
knowledge gaps about the early Earth as a reference point in exoplanet science
research; b) compiling biotic and abiotic mechanisms that controlled the
evolution of the atmosphere over time; and c) reviewing current constraints on
the detectability of Earth's early biosphere with state-of-the-art telescope
technology. We highlight that life may have originated on a planet with a
different tectonic regime and strong hydrothermal activity, and under these
conditions, biogenic CH$_4$ gas was perhaps the most detectable atmospheric
biosignature. Oxygenic photosynthesis, which is responsible for essentially all
O$_2$ gas in the modern atmosphere, appears to have emerged concurrently with
the establishment of modern plate tectonics and the continental crust, but
O$_2$ accumulation to modern levels only occurred late in Earth's history,
perhaps tied to the rise of land plants. Nutrient limitation in anoxic oceans,
promoted by hydrothermal Fe = fluxes, may have limited biological productivity
and O$_2$ production. N$_2$O is an alternative biosignature that was perhaps
significant on the redox-stratified Proterozoic Earth. We conclude that the
detectability of atmospheric biosignatures on Earth was not only dependent on
biological evolution but also strongly controlled by the evolving tectonic
context.
