The endosymbiosis of the mitochondrion likely occurred only once in evolutionary history.

“as far as we know endosymbiosis of the mitochondrion only took place once oh so there are lots of eukaryotes almost not all but almost all eukaryotes have a mitochondrion and if you look at the DNA of those mitochondria that still have a genome the phylogenetic analysis tells us that this was a monophyletic event this took place only once if that's not true for something like the chloroplast that also arose through serial end of symbiosis and that likely took place at least two independent times in evolution but for the mitochondrion you probably only took place once as far as we know”