from two simple gases, hydrogen and carbon dioxide. The endosymbiont (the future mitochondrion) was a versatile bacterium (perfectly normal for bacteria), which provided its host cell with the hydrogen it needed to grow. The details of this relationship, worked out step by step on a logical basis, explain why a cell that started out living from simple gases would end up scavenging organics (food) to supply its own endosymbionts. But
