CO2 can only accept electrons in pairs. Addition of two electrons gives formate (HCOO–); two more give formaldehyde (CH2O); another two give methanol (CH3OH); and a final pair gives the fully reduced methane (CH4). Life, of course, is not made of methane, but it is only partially reduced carbon, roughly equivalent in its redox state to a mixture of formaldehyde and methanol. This means there are two important kinetic barriers relating to the origins of life from CO2 and H2. The first needs to be overcome, to get to formaldehyde or methanol. The second must not be overcome!
