Final answer:
The endosymbiotic theory explains the origin of eukaryotic organelles like mitochondria and chloroplasts, proposing they evolved from symbiotic relationships between ancient prokaryotes and primitive eukaryotic cells, supported by genetic and structural similarities to bacteria.
Step-by-step explanation:
The Endosymbiotic Theory
The endosymbiotic theory is a scientific hypothesis that explains how eukaryotic cells, which are cells with a nucleus, may have evolved. According to this theory, eukaryotic organelles such as mitochondria and chloroplasts are thought to have originated from ancient, free-living prokaryotes that entered into a symbiotic relationship with primitive eukaryotic cells. These prokaryotes would eventually become an integral part of the cell, evolving into the organelles we observe today.
The theory suggests that mitochondria evolved from aerobic bacteria capable of performing oxygen-based respiration, providing the host cell with a more efficient way to produce ATP – the molecule that stores and transfers energy in cells. Similarly, chloroplasts are believed to have evolved from photosynthetic bacteria, giving the host cell the ability to perform photosynthesis and produce its own food from sunlight.
Evidence supporting the theory includes the fact that mitochondria and chloroplasts have their own DNA, which is distinct from the DNA in the cell's nucleus and closely related to bacterial DNA. Additionally, these organelles reproduce through binary fission like bacteria, and their ribosomes are more similar to those of bacteria than to eukaryotic ribosomes. Although initially controversial, the endosymbiotic theory by American biologist Lynn Margulis is now widely accepted within the scientific community.