The term animal comes from the Latin animalis, meaning “having breath.” Breath may be slight.
One nematode – Halicephalobus mephisto – thrives with scant oxygen, in deep fracture groundwater that is thousands of years old, up to a depth of 3.6 km or more. These little black roundworms, 0.5 mm long, withstand crushing pressure and searing heat to feast on the biofilm bacteria deep down. Hellish it may be, but there is a functioning biosystem there.
Other nematode species have been found 1.3 km below ground at depleted oxygen levels, warmed to 41 ºC. These nematodes may have made their way down to the depths from more surface-oriented ancestors.
Less of an exotic ode than deep-dwelling nematodes, the common breadcrumb sponge sips scant oxygen wherever it may be found in the North Atlantic Ocean or Mediterrean Sea.
Breath does not distinguish animals from plants. Plant pores (stomata) have a regulated cycle of breathing in carbon dioxide and exhaling water vapor.
Animals originated during the chilly Cryogenian period. The last common ancestor of animals arose nearly 800 MYA.