Every eukaryotic organism retains mitochondria or a reduced version of them.

Eukaryotes are the organisms whose cells contain a nucleus: animals, plants, fungi and many microbes. Mitochondria are best known for supplying usable energy, but they also perform other important chemistry.

Some oxygen-poor environments contain eukaryotes with tiny, highly altered mitochondrial descendants. Finding these remnants in organisms once thought to lack mitochondria helped establish a new consensus: the organelle could shrink dramatically, but every living eukaryotic lineage still retained it in some form.

Then a microbe from a chinchilla’s gut broke the rule.

In 2016, researchers investigated the genome of the organism now named Monocercomonoides exilis. They could not find the machinery normally associated with mitochondria. Instead, a system acquired from bacteria could perform iron–sulfur cluster assembly outside the organelle. These small chemical components help essential proteins work.

This was not a surviving organism from before mitochondria evolved. The evidence pointed to a descendant of mitochondria-bearing ancestors that had lost the organelle. A 2023 comparison supported the same loss in two relatives.

The exception concerns whole organisms and evolutionary lineages. It is different from a specialized cell, such as a mature human red blood cell, losing its mitochondria during development.