Final answer:
The discovery of mitosomes in some eukaryotes suggests that all eukaryotic cells once had mitochondria, supporting the endosymbiotic theory of mitochondrial origin and casting doubt on the idea that mitochondria were acquired late in eukaryotic evolution. Option 3 is correct answer.
Step-by-step explanation:
The question pertains to a discovery that questions the timeline of mitochondria acquisition in eukaryotic cells. The correct option that casts doubt on the idea that eukaryotes acquired mitochondria well after their origin is the presence of mitosomes in some eukaryotes.
This is because findings have shown that mitochondria or their evolutionary precursors such as mitosomes and hydrogenosomes are present in nearly all eukaryotes, even those that appeared to lack mitochondria. These findings support the endosymbiotic theory, which suggests that mitochondria originated from free-living prokaryotes that entered into a symbiotic relationship with primitive eukaryotic cells.
Additionally, mitochondria contain their own DNA, reproduce independently by binary fission, and resemble alpha-proteobacteria in their genomes and ribosomes. Thus, the correct option in this scenario is 3) the presence of mitosomes in some eukaryotes.