A nondescript plant has overturned one of the bedrock assumptions of biology. Unlike every other animal and plant ever studied, its cells do not have the equipment to make energy. It is not clear how it survives.

All animals, plants and fungi are “eukaryotes”: they are made up of cells that are much more complex than those of bacteria. All eukaryotic cells contain tiny sausage-shaped objects called mitochondria, which are their energy source.

Mitochondria are almost entirely essential for eukaryotes. Some single-celled …