It was thought all complex cells, including animals, plants and fungi, contained the tiny power-producing organs called mitochondria.
But a team in the Czech Republic has discovered a microscopic creature that contains no trace of them at all, making them a biological oddity.
The creature, a type of single-celled organism called monocercomonoides, is at odds with the idea that mitochondria are essential components of eukaryotic cells.
This group includes animals, plants, fungi and a host of others.
Mitochondria generate the power for cells by using oxygen and sugar to produce packets of chemical energy called ATP.
But instead of using these organs to produce energy, scientists believe the single-celled creatures use a chemical process involving sulphur, which they may have pilfered from bacteria.