What role does the endosymbiont theory play in explaining the origin of eukaryotic cells?

The endosymbiont theory explains that eukaryotic cells evolved from a symbiotic relationship between different types of prokaryotic cells. This theory suggests that mitochondria and chloroplasts, which are organelles in eukaryotic cells, were once independent prokaryotic cells that were engulfed by a larger host cell. Over time, these prokaryotic cells became integrated into the host cell and eventually evolved into the organelles we see in eukaryotic cells today. This theory helps to explain the presence of these organelles in eukaryotic cells and provides insight into the evolutionary history of these complex cells.