What does the endosymbiotic theory propose?

• A. Eukaryotes arose from prokaryotes
• B. Prokaryotes are the ancestors of all life
• C. Early life forms were unicellular
• D. All cells are fundamentally the same

Answer: (A)

The endosymbiotic theory proposes that eukaryotic cells, which are more complex than prokaryotic cells, originated through a process where certain prokaryotic cells, such as primitive bacteria, were engulfed by a host cell. Rather than being digested, these engulfed cells formed a mutualistic relationship with the host, eventually evolving into organelles like mitochondria and chloroplasts. This theory explains how eukaryotic cells inherited features from their prokaryotic ancestors, suggesting that the organelles originated from free-living prokaryotes that became integrated into a larger cell. This connection highlights the evolutionary progression from simpler life forms toward more complex organisms, demonstrating a pivotal moment in the history of life on Earth.

The other options do touch on aspects of biological evolution and cell structure but do not encapsulate the specific mechanism of eukaryotic evolution as described by the endosymbiotic theory.