The evidence within
Every cell in your body, excluding red blood cells, contains mitochondria—organelles that function as cellular power plants. Plant cells and algae have an additional organelle called a chloroplast, the site of photosynthesis. The Endosymbiotic Theory, championed by biologist Lynn Margulis in the 1960s, posits that these organelles were once free-living prokaryotic microbes. An ancestral host cell engulfed these bacteria around 1.5 billion years ago, establishing a symbiotic relationship that became permanent. The evidence for this deep history resides within the organelles themselves.
One of the strongest lines of evidence is their independent genetic material. Both mitochondria and chloroplasts contain their own circular DNA, much like the chromosomes of bacteria. Human mitochondrial DNA (mtDNA) is a loop of 16,569 base pairs containing 37 genes that encode for proteins essential for cellular respiration. This is distinct from the billions of base pairs of linear DNA housed in the cell's nucleus. Similarly, chloroplasts have their own circular DNA (cpDNA), which is typically 120,000–170,000 base pairs long and contains about 100-200 genes related to photosynthesis.
Relics of an independent life
The physical structure of these organelles tells a story of ancient engulfment. Mitochondria and chloroplasts each have two membranes. The inner membrane has a composition, including proteins like porins and lipids like cardiolipin, that is similar to a bacterial cell membrane. The outer membrane is more akin to the host cell's own membrane, consistent with it being formed when the host enveloped the bacterium in a vesicle.
Evidence comes from how these organelles reproduce. They multiply independently of the host cell's division cycle through a process called binary fission, the same method of asexual reproduction used by bacteria. A cell cannot create new mitochondria or chloroplasts if they are removed; they must be inherited from a parent cell during division.
Their protein-making machinery is a prokaryotic relic. Ribosomes inside mitochondria and chloroplasts are the 70S type, the same size found in bacteria. The ribosomes floating in the eukaryotic cell's cytoplasm are larger, designated 80S. The presence of bacteria-sized ribosomes shows of their distinct, prokaryotic origins.