Earth’s early atmosphere did not contain oxygen. The earliest organisms were anaerobic. Photosynthesis produced oxygen.6CO2 + 6 H2O + Energy ? C6H12O6 + 6O2
Aerobic organisms evolved because oxygen enables organisms to obtain much more energy from their food then they could without oxygen.
The endosymbiotic theory states that some eukaryotic organelles evolved from prokaryotes. For example, the mitochondrion may have arisen from aerobic prokaryotic cells that came to reside within an anaerobic host cell. This enabled the host cell to use aerobic cellular respiration. In return, the host cell provided the necessary nutrients, a stable chemical environment, and protection. Evolutionary change eventually resulted in this relationship being obligate.
For example, the mitochondrion, chloroplast, and other plastids originated as prokaryotic cells that came to reside within a host cell. They enabled the host cell to use sunlight as an energy source (the chloroplast) and use aerobic cellular respiration (the mitochondrion). In return, the host cell provided the necessary nutrients, a stable chemical environment, and protection. Evolutionary change eventually resulted in this relationship being obligate.
Below: Mitochondria are eukaryotic organelles that function in aerobic cellular respiration. They may have arisen by anaerobic prokaryotes engulfing aerobic prokaryotes.The anaerobic cell benefited by the extra energy available from aerobic respiration. The engulfed cell benefited by nutrients provided by its host.
The mitochondrion may have arisen early in the evolution of eukaryotes because all known eukaryotes have them.
Base sequences of DNA taken from the plastids of red and green algae resemble those of cyanobacteria. This suggests that the chloroplast and other plastids of these two groups of algae may have arisen from a symbiotic relationship between a heterotrophic cell and a cyanobacterium. The relationship enabled the host cell to use sunlight as an energy source. Eventually the relationship became obligate.
Below: Heterotrophic prokaryotes that engulfed (or became infected by) photosynthetic prokaryotes were able to benefit by using the sugars produced using solar energy. The prokaryotes benefited by receiving necessary nutrients from their host cell. The chloroplast and other plastids evolved from this photosynthetic endosymbiont.
The origin of the double membrane surrounding plastids can be seen in the diagram above. The outer membrane originated as part of the plasma membrane of the heterotrophic cell when the photosynthetic cell was engulfed. The inner membrane originated as the plasma membrane of the photosynthetic cell.
Secondary endosymbiosis may have occurred when red and green algae became symbionts within other eukaryotic cells.