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.