Eons ago, two single-celled organisms, a bacterium and an archaeon, combined to form the first complex cell. This symbiosis gave birth to all multi-cellular organisms, but new evidence suggests this wasn't about cooperation. That bacterium was actually a parasite.
Two billion years ago, an archaeon swallowed a bacterium, and for whatever reason it forgot to digest its meal. This was the birth of the eukaryotes, and now every last one of our cells act as a silent monument to this first complex cell that formed two billion years ago. In this symbiotic partnership, the archaeon developed into the cells of complex organisms, while the bacterium that it ingested became the mitochondria, the energy producing powerhouses found inside all our cells.
But it seems the entire eurkaryotic domain might be built on a lie. Far from a moment of impromptu partnership, the first complex cell might actually have been the accidental byproduct of an attempted invasion by a parasitic bacterium. That's the contention of researchers Nathan Lo at Australia's University of Sydney and Claudio Bandi at Spain's University of Valencia, who have done extensive genetic studies on the bacteria most closely related to our mitochondria, the Rickettsiales.
Some of the bacteria within this group, including a species with the evocative name Midichloria mitochondrii, carry the genes necessary for a flagellum, a whip-like tail used by some bacteria to move about. The problem is that the "some bacteria" here are almost exclusively parasites. If the Rickettsiales are any indication, then the bacteria that became our mitochondria also carried flagella - and if that's true, then odds are they were parasites.
Of course, we're talking at a remove of two billion years from that first complex cell, which is plenty of time for these Rickettsiales to pick up a flagellum elsewhere, particularly considering how readily bacteria share genetic information with each other. But the researchers reconstructed the evolutionary tree for the Rickettsiales, and it appears the truth is that once all these bacteria carried flagella, and most simply lost them in the last two billion years.
That bolsters the argument for ancient parasitism, but there are still some unanswered questions. One possible objection is that a parasitic relationship is, by its nature, destructive to the host cell, and so it's unlikely all complex cells could be descended from such a coupling. But Nathan Lo counters, "Many symbiotic relationships start as parasitism - they give up fighting each other and then work together." If so, then we're looking at two billion years of unbroken peace between parasite and host. That has to be some sort of record.