How did mitochondria and chloroplasts most likely arise in eukaryotic cells?

Mitochondria and chloroplasts most likely arose in eukaryotic cells through a process called endosymbiOSis, where a host cell engulfed a prokaryotic cell. Over time, the prokaryotic cell evolved to become an organelle within the host cell, forming a symbiotic relationship. This theory is supported by the similarities between mitochondria and chloroplasts and modern-day bacteria.