Biology
The Endosymbiotic Theory of Mitochondrial Evolution
Quick fact
Mitochondria have their own DNA, which is circular like bacteria's, and they reproduce independently of the cell—strong evidence that they were once free-living organisms.
Why this is interesting
You carry around tiny power plants inside your cells that were once independent bacteria. How did they get there?
Read the full explanation
Understanding The Endosymbiotic Theory of Mitochondrial Evolution
Imagine a large cell that engulfs a smaller bacterium, intending to eat it. Instead of being digested, the bacterium survives inside and forms a mutually beneficial relationship: the bacterium provides extra energy to the host, and the host provides a protected environment. Over millions of years, the bacterium became a permanent resident—the mitochondrion. This is the core of the endosymbiotic theory. You can see clues: mitochondria have their own circular DNA, similar to bacterial chromosomes; they have ribosomes that resemble bacterial ones; and they are surrounded by two membranes, the inner one likely from the original bacterium and the outer one from the host's membrane. This explains why mitochondria are passed down maternally and why they replicate independently.
A deeper explanation
The mechanism begins with an ancestral eukaryotic cell phagocytosing an aerobic bacterium, likely an alpha-proteobacterium. Instead of being lysed, the bacterium was retained, providing the host with ATP through aerobic respiration—a huge advantage in an increasingly oxygen-rich environment. The symbiosis became obligate as genes were transferred from the endosymbiont to the host nucleus, making the mitochondrion unable to live independently. Evidence includes the mitochondrial genome's genetic code similarities to bacteria, its sensitivity to antibiotics that target bacteria, and the presence of cardiolipin in the inner membrane, also found in bacterial membranes. This theory, championed by Lynn Margulis, explains why mitochondria are essential for complex multicellular life, as they provide the energy to support larger genomes and cells.