What does the endosymbiont theory propose?

The endosymbiont theory proposes that chloroplasts and mitochondria originated from free-floating prokaryotic cells that entered into a symbiotic relationship with a host eukaryotic cell. This theory is supported by various pieces of evidence, such as the presence of their own circular DNA, double membranes, and similarities in ribosomal structures to prokaryotes.

According to this theory, these prokaryotic ancestors of chloroplasts and mitochondria were engulfed by ancestral eukaryotic cells. Instead of being digested, these engulfed prokaryotes provided the host cell with advantages such as enhanced energy production or the ability to perform photosynthesis, thus leading to a mutually beneficial relationship. Over time, these prokaryotes became integral parts of the eukaryotic cells, evolving into the organelles we observe today.

Other choices do not accurately capture the essence of the endosymbiont theory. For example, while all organisms may trace back to a common ancestor, this concept is broader than what the endosymbiont theory specifically addresses. The proposal that eukaryotes evolved before prokaryotes contradicts the widely accepted view of evolutionary timelines, where prok