Years ago, it was perceived that facultative bacteria living inside other organisms (facultative endobacteria) were an intermediate, transitional stage leading to more obligate mutualisms. Facultative relationships are interactions between two species in which their functions benefit the others fitness but are not required for life cycle completion. On the other hand, obligate relationships are required for the species survival. Before this 2012 paper by Stephen Mondo and company, it was thought that over time, facultative endobacteria shift to playing more and more obligatory roles. Though, this research has upended this apparent evolutionary trajectory.

The most notorious endobacteria we should all be familiar with are the mitochondria within eukaryotic cells. These organelles supply energy to our cells and without them we could not function. The leading hypothesis describing mitochondrial origin is that tiny prokaryotes with complex oxidative mechanisms made their way into early eukaryote cells. They gained a secure place to live, more sheltered from the elements, while their symbiote received energy through these oxidative processes. After millions of years, these once facultative endobacteria became obligate mutualists. They don’t function as individual bacteria anymore either, now working seamlessly as cellular organelles, even though they have a separate genome from us.