There are many forms of "life", both natural and artificial, within the Terragen sphere. These include various self-replicating machines, with or without nanotech components, sometimes entirely artificial or sometimes, in the case of the botworlds, naturally evolved from abandoned or escaped originals. There are also many forms of life that appear to have a natural origin but do not employ biochemistry as the term is normally used (the Rheolithoids and the Hildemar's Knots are good examples of these). There are even a number of neogens with novel artificial biochemistries, such as the famous fluorine breathers of the Chen Zhuo 1101 habs. However, the known naturally evolved biochemistries discovered so far can be divided into just a few broad categories: methane based, ammonia based, and water based. This might be predicted on the basis that these are all combinations of hydrogen, the most common element in the universe, with the next three most common elements that are capable of forming compounds with it. There are many subtypes of these basic biochemistries, of course, and there is huge biochemical variety between and within biospheres, but there are also broad similarities. In all cases there is considerable variation around the mean temperature at which these life forms can operate. The broadest range is the Terragen type, since water is liquid over a larger range than are the other common solvents. The other two major types of life are active over a much smaller temperature range, but they can typically survive much greater dips in temperature in inactive, frozen form. This is because the solutions they depend on do not expand when they solidify the way that frozen water does. An extremely rare fourth type of life that uses sulphuric acid is included by most but not all experts. Agreement that this relatively uncommon form evolved naturally is not universal.
Some experts include a fifth classification called Jovian, or Jovic. However, such life forms typically use one of the above styles of biochemistry, according to the temperature and pressure regime of the local atmosphere. In a few celebrated cases the planet's biosphere includes all three of the other major types in variant forms. However, these are believed to be the result of seeding by sophonts, Terragen or otherwise.
Type I (Terragen/Water-based)
Type I life typically operates at about 20° to 30° C, with variant forms that thrive at temperatures as low as 0° C or as high as 125° C; all life forms of this type use H2O as a solvent. A well developed planetary ecosystem eventually evolves forms capable of photosynthesis with water as the hydrogen donor and produces an oxygen/nitrogen/carbon dioxide atmosphere, with anything from trace amounts of carbon dioxide (for planets like Old Earth that are near the inner edge of the Type I life zone) to much larger amounts (for planets that are further out from their primary and maintain type I life by a strong greenhouse effect). Typical chemicals are proteins, fats, carbohydrates, and nucleic acids. This is by far the most common type, since oxygen is the third most common element in the universe and water is liquid over such a broad range of temperatures and pressures. It is generally associated with the Gaian planetary type, but any environment that allows liquid water may give rise to type I life forms. Examples are numerous: Old Earth, Trees, Kemmerer, Ridgewell, Darwin, Euripedes Mey, Whisper, Orwell, Elmo, the Auld Limner worlds, and the original homeworlds of the Daharrans, the Cthonids, the Paulans, the Meistersingers, the Singers, and the Silent Ones to name only a few.
Type Ia (To'u'ulhian/Hot Water-based)
Type Ia is like type I in most respects, but operates at 125° to 150° C. The extremely high surface pressures (up to 100 or more bars) allow the use of water as a solvent since at such pressures it remains a liquid. There is a much larger fraction of carbon dioxide in the atmosphere, usually 50 to 90 percent of the total, but oxygen is also present in a fully evolved biosphere of this type. This biochemistry features the addition of a number of minor elements that are volatile at these temperatures, such as mercury and tin. Sulphur is also more frequently employed in the local biochemistry than in standard Type I biology. Usually the ecology includes some high altitude cloud dwelling life forms that are more typically type I, and are extremophiles in the context of To'ul'h worlds. Some experts regard types I and Ia as simply two extremes on a continuum. This is a less common type, than regular type I biochemistry, since the strong insolation that leads to To'ul'hian life more often leads to a runaway greenhouse effect and desiccation of the planetary atmosphere resulting in a lifeless world of the true Cytherean type. The associated Planetary Classification type is Cytherean/Gaian, (To'ul'hese subtype). The To'ul'h homeworld is the archetype and the richest natural example.
Type Ib (Halogenic/Water-hydrochloric acid based)
Type Ib is like the above two, but the solvent mixture includes some HCl in addition to water and the atmosphere includes a fraction of chlorine gas as well as oxygen. The associated planetary type is Gaian (Halogenic). Examples include Chorus, Doreen, Tytalus, and Outland. Such biospheres are vanishingly rare due to the relative scarcity of chlorine. There is only one known origin to this type of life, and the entire group may ultimately prove to be artificial in origin, despite hundreds of millions of years of subsequent independent evolution on a large number of planets.
Type II (Softoneian/Water-ammonia based)
Type II life operates at about -50° C. The solvent is an H2O and NH4 mix, and the biochemistry involves extensive use of fatty acids. The associated planetary classification type is a Europan variant. There are only a few natural occurrences. The Soft Ones are a famous example of a life form from such a world.
Type III (Muuhian/Methane-ethane based)
Type III life operates at approximately -160° C. The primary solvent is a mixture of methane, CH4 , and ethane, C2H6. Methane alone has an extremely narrow range over which it is a liquid, but together the two substances make for a fluid with a range more useful to life. As the primary solvent is nonpolar, the biochemistry of such life forms has few analogues with typical Terragen life. The typical atmosphere is composed of nitrogen and methane with heavier hydrocarbon gases as contaminants. It is very rare. The associated planetary classification type is Titanian (Titan in Solsys is a chilly and barren version). Known examples are Muuhome and other Muuh worlds.
Type IV (Vitriolic/Sulphuric acid based)
Type IV life exists at temperatures between 250 and 330°C, and in conditions where liquid sulphuric acid is present but water is not. Organic molecules in these hot biospheres are generally silicone-based. The biospheres associated with Vitriolic Worlds are all causally connected, having been seeded more than half a billion years ago by a xenosophont species known today as the Thiogens. Vitriolic vegetation tends to have a reddish tint, but hundreds of millions of years of divergence means this is not always the case.