Here’s a paper on this topic:
Relative timing of mitochondrial endosymbiosis and the “pre‐mitochondrial symbioses” hypothesis
Toni Gabaldón
IUBMB Life. 2018 Dec; 70(12): 1188–1196.
Published online 2018 Oct 25. doi: 10.1002/iub.1950
PMCID: PMC6282991
PMID: 30358047
The origin of eukaryotes stands as a major open question in biology. Central to this question is the nature and timing of the origin of the mitochondrion, an ubiquitous eukaryotic organelle originated by the endosymbiosis of an alphaproteobacterial ancestor. Different hypotheses disagree, among other aspects, on whether mitochondria were acquired early or late during eukaryogenesis. Similarly, the nature and complexity of the receiving host is debated, with models ranging from a simple prokaryotic host to an already complex proto‐eukaryote.
simple eukaryogenesis models that assume a binary symbiosis between an archaeon host and an alpha‐proteobacterial proto‐mitochondrion cannot explain the complex chimeric nature that is inferred for the eukaryotic ancestor.
The origin of eukaryotes remains one of the most difficult questions in evolutionary biology. Given the scarcity of data, most proposed scenarios for the origin of eukaryotes are necessarily highly speculative.