This reduction is usually effected using a reducing atmosphere consisting of some mixture of natural gas, hydrogen (H2), and carbon monoxide.
However, other common reducing atmospheres in the metal processing industries consist of dissociated ammonia, vacuum, and direct mixing of appropriately pure gases of N2, Ar, and H2.
[3][4] Some hypotheses for the origin of life invoke a reducing atmosphere consisting of hydrogen cyanide (HCN).
Experiments show that HCN can polymerize in the presence of ammonia to give various products including amino acids.
Some aerobic bacteria eventually became endosymbiont within other anaerobes (likely archaea), and the resultant symbiogenesis led to the evolution of an entirely new lineage of life — the eukaryotes, who took advantage of mitochondrial aerobic respiration to power their cellular activities, allowing life to thrive and evolve into ever more complex forms.
The results quantify the nature of gas molecules containing carbon, hydrogen, and sulphur in the earliest atmosphere, but they shed no light on the much later rise of free oxygen in the air.