The endosymbiotic theory has been widely held as the mechanism of how proto-mitochondrial organisms became the mitochondria we know today.

A professor at my undergrad had us all read this article (same as linked title):
[bmcbiol.biomedcentral.com]

I especially enjoyed it because the endosymbiotic relationship didn't make sense to me, but an ectosymbiotic mechanism seems more probably. They go into depth explaining how & providing support for the theory. I haven't seen it gaining much traction though. But I guess these kinds of things take time to be considered & the ideas spread.

I know it's a long read, but any thoughts/discussion is welcome.