Procaryotes have simpler structures, but are more diverse in their chemistry than are eucaryotes.

Primitive bacteria extract energy by the relatively simple process of glycolysis. To this process some bacteria have added respiration using sulfate, oxygen, or nitrate. Other bacteria have developed photosynthesis with a single photocenter, employing HS, H, or organic molecules as reductants. From these possibilities blue-green algae have selected O respiration and have developed a twocenter photosynthesis, using HO as a source of reducing electrons.

It is a reasonable working hypothesis that eucaryotic cells evolved from an initial symbiosis between a large, nucleated, but nonphotosynthetic and possibly nonrespiring host, and small respiring bacteria that became the ancestors of mitochondria. Photosynthesis in eucaryotes probably developed from a symbiotic relationship between early nucleated, mitochondria-containing eucaryotic cells and blue-green algae. The traces of carbohydrate metabolism are preserved in the dark reactions of chloroplasts, and a rudimentary genetic machinery remains in both chloroplasts and mitochondria.

With the "invention" of the eucaryotic cell 1.2 - 1.4 billion years ago, the way was clear for the evolution of large, multicelled organisms.