Today I got the results back from testing my blood for Rickettsial antigens and while looking up the species of Rickettsia my blood was tested against I discovered a very interesting fact which totally blew my lab-mate and my mind.
R. prowazekii is the closest free-living relative of mitochondria.
Let me translate... The statement above is saying that an ancestral strain of a current species of parasite called Rickettsia prowazekii is the closest common ancestor of mitochondria. This ancestral species was a Rickettsia-like intracellular symbiont, meaning it relied on its relationship with another organism (in this case another cell) to live. The term "free-living" is a little misleading, as R. prowazekii cannot live outside of a host or vector, but, R. prowazekii is "free-living" in the fact that it is no longer intracellular, as its closest common ancestor to mitochondria was.
As scientists studying DNA we learn a lot about mitochondria (circular DNA within eukaryotic cells responsible, in most part, for energy production). We are taught that the most likely origin of mitochondria was when one single-celled organism engulfed another creating an endosymbiont (cell living within a cell). There are multiple theories about the engulfing versus the engulfed cells but the typical explanation is an anaerobic (can live without oxygen) bacteria was engulfed by a nucleus-bearing cell. This cell-in-a-cell combo evolved to be able to create its own energy and later developed the capability to become multicellular organisms.
So, this ancestral species evolved into both mitochondria and R. prowazekii: mitochondria when it became fully incorporated into the other cell (mutual symbiosis) and R. prowazekii when it became a parasite (non-mutual symbiosis). In all of the times we have been taught about the origins of mitochondria, it was never mentioned that the common ancestor isn't just any bacteria but the common ancestor of a parasite. It puts a whole new dynamic on the origin story and the evolution of symbiotic relationships as well.
The original statement was found on Wikipedia but was verified by this scientific publication: