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Researchers sequencing the genomes of modern-day relatives of the first eukaryotes have found many unexpected genes that don’t seem to come from either the host or the endosymbiont.

The ancestral host and endosymbiont belonged to different branches of the tree of life — archaea and bacteria, respectively — that use different molecules to build their membranes.

All those foreign genes could have arrived in a single package with the endosymbiont that evolved into the mitochondrion.

Instead of being digested, the bacterium took up permanent residence within the other organism as what biologists call an endosymbiont.

But there are many more recent, looser endosymbioses where the origin of foreign genes is easier to identify, says John McCutcheon, an evolutionary cell biologist at Arizona State University in Tempe who wrote about endosymbiont evolution in the 2021 Annual Review of Cell and Developmental Biology.