Inert gas asphyxiation lacks oxygen and consists of an asphyxiant gas (one or more of argon, nitrogen or carbon dioxide), causing the animals to lose consciousness. Argon and nitrogen are important components of a gassing process which seem to cause no pain, and for this reason many consider some types of controlled atmosphere killing more humane than other methods of killing. However, "stunning" is often done using carbon dioxide. If carbon dioxide is used, controlled atmosphere killing is not the same as inert gas asphyxia, because carbon dioxide at high concentrations (above 5%) is not biologically inert, but rather is toxic and also
Reducing atmosphere had a reducing atmosphere, along with Mars and Venus. This would have proven to be a good environment for "Cyanobacteria" to evolve the first photosynthetic metabolic pathways which gradually increased the oxygen portion of the atmosphere, changing it to what is known as an oxidizing atmosphere. With increased levels of oxygen, the evolution of the more efficient aerobic respiration might have been enabled, allowing animal life to evolve and thrive. Though most scientists conceive of the early atmosphere as reducing, a 2011 article in "Nature" found that cerium oxidation in zircon—which comprises the oldest rocks on Earth at roughly 4.4