Terrestrial organic life is much less common than Gas Giant life. This is largely because terrestrial life is limited by the geological presence of liquid solvents. In practice, this solvent is almost always water. The vast majority terrestrial sapient life releases energy from hydrocarbons through chemical reaction with oxygen (hence the common practice of referring to the entire group as O-2 life). However, at any given time "O-2 civilization" contains a few species with anaerobic metabolisms (usually based on hydrogen sulfide) as well as a tiny number of Halogen Breathing species. Anaerobic life is --of course --rather common. However, multi-cellular anaerobic species with uplift potential are very rare. Halogen (Fluorine, Chlorine, and Iodine) based biospheres are incredibly rare. Those known to exist are very carefully managed by the Galactic Institute for Migration. Such Anaerobic and Halogen species as exist usually participate actively in wider O-2 civilization.
One other kind of terrestrial life was once common throughout the Galaxies. These were Methane and/or Ammonia life-forms that lived in Titan-like habitats. Though complex Methane-Ammonia ecologies are rare, they do occur, and historical sources clearly indicate that Titan-forming was relatively easy and economical. At the time of Contact, Terran scientists were surprised to learn that no methane or ammonia life existed in the galaxies. It took some time to discover that Methane-Ammonia intelligence was simply extinct. H-2 civilization considered the Methane-Ammonia sub-civilization a partial competitor and immediate security threat to both existing H-2 species and to fallow H-2 ecologies. Some of the wars between H-2 and Terrestrial civilizations were genocidal in intent. In one of these H-2 civilization was partly successful. They exterminated most Methane-Ammonia species, and the terms of a peace treaty ended further Methane-Ammonia uplift.
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