But Petersen isn’t convinced the loss is beneficial at all. It could be that something like a catastrophic mutation befell the mistletoes early in their group’s evolution, something that accidentally wiped out their ability to make complex I. “And then mistletoe just managed to cope with what happened,” she said, perhaps because mistletoe’s parasitic lifestyle offered ways to compensate. She is curious about whether additional genetic sequencing might let researchers suss out when the loss happened and whether the plants were already parasitic.

Mistletoes have all but shut down the powerhouses of their cells. Scientists are still trying to understand the plants’ unorthodox survival strategy.