  Essential insights on the characterization and quality of a detectable
biosphere are gained by analyzing the effects of its environmental parameters.
We compiled environmental and biological properties of the Phanerozoic Eon from
various published data sets and conducted a correlation analysis to assess
variations in parameters relevant to the habitability of Earth's biosphere. We
showed that environmental parameters such as oxygen, global average surface
temperatures, runoff rates and carbon dioxide are interrelated and play a key
role in the changes of biomass and biodiversity. We showed that there were
several periods with a highly thriving biosphere, with one even surpassing
present day biodiversity and biomass. Those periods were characterized by
increased oxygen levels and global runoff rates, as well as moderate global
average surface temperatures, as long as no large or rapid positive and/or
negative temperature excursions occurred. High oxygen contents are diagnostic
of biomass production by continental plant life. We find that exceptionally
high oxygen levels can at least in one instance compensate for decreased
relative humidities, providing an even more habitable environment compared to
today. Beyond Earth, these results will help us to understand how environmental
parameters affect biospheres on extrasolar planets and guide us in our search
for extraterrestrial life.
