What does the endosymbiotic theory propose?

The endosymbiotic theory proposes that eukaryotic cells originated from prokaryotic organisms through a symbiotic relationship. This theory suggests that certain organelles within eukaryotic cells, such as mitochondria and chloroplasts, were once free-living prokaryotes that entered into a mutually beneficial relationship with an ancestral eukaryotic cell. Over time, these prokaryotic cells became essential components of the eukaryotic cell, leading to the complex cellular structures that characterize eukaryotic organisms today.

This theory is supported by various pieces of evidence, including the presence of their own DNA in mitochondria and chloroplasts, which is similar to bacterial DNA, as well as double membranes surrounding these organelles, indicative of an engulfing process. The concept of symbiosis is crucial here, as it highlights how organisms can evolve new features and capabilities through advantageous interactions rather than solely through lineage or descent.