There are three large clusters of related organisms, called domains from which living things have evolved. These domains are archaea, bacteria and eukaryota. Archaea and bacteria are prokaryotes. They are small simple cells surrounded by a membrane and a cell wall. They have a circular strand of DNA that contains their genes.

Almost all of the everyday things that we see and notice everyday, belong to the third domain, which includes plants and animals. This domain is eukaryota. The cells in eukaryota are much more complex then prokaryotes. The DNA is linear and found within a nucleus. Eukaryotic cells also have mitochondria, which are their own personal power plants. Mitochondria not only produce energy but also hold the secret of the understanding of the eukaryotic cells evolution.

The eukaryotic cells ushered in a whole new era for life on earth by evolving into multicellular organisms. How did this evolution take place? Endosymbiosis seems to be the answer.

There is evidence, that millions of years ago, two separate prokaryotic cells were joined together in a symbiotic union. The mitochondrion seems to have once been free-living bacteria that was “eaten” or taken over by another cell engulfing it. It ended up staying as a permanent houseguest. The mitochondrion produced energy that the host cell needed and the host cell offered a protected nutrient rich environment for the mitochondrion. In this way, both profited from this relationship. This is an example of Endosymbiosis.

Endosymbiosis occurs when one cell takes up the residence inside of another cell and eventually evolving into a single cell.

There are plenty of evidences to support endosymbiosis. Although it was not until the 1980s when biologist Lynn Margulis’ book, “Symbiosis in Cell Evolution”, came out with the theory that was given any real consideration. After all, no one was around over a billion years ago to observe the start of endosymbiosis. Why should we believe that a mitochondrion used to live free in its own right?

The most important piece of evidence is the similarities between mitochondria and prokaryotes. They both have their own cell membranes. Although mitochondrion has a smaller DNA genome, it is circular like that in bacteria. Also, mitochondria pass its DNA to its offspring separately from the host cell’s genome in the nucleus. The way mitochondria multiply is by breaking itself in half. Every new mitochondrion has to be produced from parent mitochondria. If the mitochondrion is removed from the host cell then the host cannot build new mitochondria. Bacteria also reproduce by breaking itself in half.

Mitochondria look a lot like tiny little bacteria living inside of eukaryotic cells. Endosymbiosis seems to be the best explanation of eukaryotic cells. The scientific community has decided to support Lynn Margulis’s theory.

Endosymbiosis could also apply to other cellular organelles, such as chloroplasts. Chloroplasts are tiny little organelles living within plant cells that convert the energy from sunlight into sugars. Evidence suggests that these chloroplast organelles were also free-living bacteria once.

All eukaryotes have mitochondria suggesting that the event that triggered the endosymbiotic relationship happened many billions of years ago.