Symbiotic relationships, where two or more organisms benefit from each other, are everywhere in nature. Once in a while, this relationship is taken a step further, and one organism actually engulfs the other. If all goes well, they live happily ever after, each benefiting from the other.
For example, the protist (single-celled animal) Hatena arenicola, has the ability to engulf a green algae cell, and then use it to gain photosynthetic nutrition. In the beginning of a relationship like this, both participants remain separate organisms, still capable of surviving on their own. Sometimes, though, they begin to take over vital functions for each other, and become stuck with one another for life.
Something just like this must have happened over a billion years ago, at the time when the very first Eukaryotic (animal, plant and fungi, i.e. ‘higher organism’) cells were evolving. Within every one of our cells, we personally have the evidence of this long-ago endosymbiotic event.
The mitochondria, present in almost all higher organisms, are responsible for producing most of the chemical energy that cells need to function, through aerobic respiration. They are essential, and without them, we couldn’t survive… or use oxygen. The ability to conduct aerobic respiration was a significant evolutionary advantage to that first eukaryotic cell. Anaerobic bacteria still use fermentation for their energy, which is far less efficient, but still handy if you want to make wine.
But it wasn’t always this way for the mitochondria…
Mitochondria were once independent bacteria, living side by side with what would become the first plant and animal (eukaryotic) cells, until they began forming an eventually irreversible partnership after one engulfed the other. Once the bacterium was within the eukaryotic cell, changes took place over millions of years as two separate organisms learned to live together.
The mitochondrion still bears the mark of its origin as a bacterial cell. It has its own cell membrane and its own DNA, completely separate from the cell it resides in, and when the cell divides, the mitochondria divides too, separately. It makes a copy of its DNA and then pinches in half, exactly like a bacterial cell does.
The DNA of our mitochondria is very similar to the Rickettsial bacterium, making this genus its closest living relative today. The Rickettsia which survives today still needs to enter a eukaryotic cell in order to live and reproduce, even if it doesn’t stay there permanently. It causes diseases such as typhus and Rocky Mountain Spotted Fever.
In plants, a second endosymbiosis occurred, this time of a bacterium capable of using sunlight to produce energy. Thus, the chloroplast was born.
Because the mitochondrium has its own genome, separate from that of its host cell, it’s inherited differently in both plants and animals. When an egg and sperm meet, each has one-half of the full genome, and at fertilization, these genes recombine to form a completely new and unique genetic combination. But the mitochondria inherited by the offspring come only from the egg cell, and are transferred intact. Thus, you have exactly the same mitochondrial DNA as your mother, and her mother before her, etc., with only occasional small and random changes. This is why mitochondrial DNA is a favorite tool for scientists trying to trace the movement of populations over long periods of time.
So thank you, long ago Rickettsia species, for allowing us to breathe air, because quite frankly, the anaerobes haven’t gotten very far at all.