Final answer:
Endosymbiosis is the event that significantly contributed to the evolution of eukaryotes, leading to the development of mitochondria and chloroplasts, which are essential organelles in eukaryotic cells.
Step-by-step explanation:
The event thought to have contributed to the evolution of eukaryotes is endosymbiosis. This process involves one cell engulfing another, with the engulfed cell surviving and both benefiting from the relationship. Over time, this can lead to two organisms that are interdependent. This endosymbiotic event likely contributed to the evolution of the last common ancestor of modern eukaryotes, leading to the development of mitochondria and chloroplasts. In the context of eukaryotic cells, the mitochondrial DNA, which bears resemblance to bacterial genomes, suggests an endosymbiotic origin. Additionally, horizontal gene transfer (HGT), while more common among prokaryotes, may play a role in eukaryotic evolution but does not describe the origin of mitochondria and chloroplasts.