Early life arose in an oxygen-free environment, but if any of these microbes had somehow come in contact with oxygen, the most likely effect would have been .

the effect of oxygen on these types of microbes is it will kill them.

Explanation:

When oxygen present in the environment come in contact with anaerobe bacteria it kill them because oxygen in air act as excited oxygen singlet molecule which will react with the water present in the cell of bacteria and convert it into hydrogen peroxides and bacteria do not have any defense system from hydrogen peroxide and ultimately it kill the bacteria.