How did mitochondria end up inside our cells?

Mitochondria were once free-living prokaryotes. According to the endosymbiotic theory, one cell took another up and the two evolved together until they could no longer be told apart as separate cells. The idea was comprehensively examined in the 1960s by Lynn Margulis and is now widely accepted. Mitochondria cannot survive outside the cell today, and the cell can no longer live without them. [1] [2] [3]

Several features point to a bacterial origin. Most mitochondria are shaped like a specific group of bacteria and are surrounded by two membranes, and the inner membrane has folds called cristae. Mitochondria divide by a process resembling binary fission in prokaryotes. They carry their own circular DNA chromosome with genes similar to those expressed by bacteria, and they have ribosomes and transfer RNAs that resemble prokaryotic ones. [1]

Gene transfer is further evidence. Many proteins the mitochondrion needs are imported from the rest of the cell, because the genes for them moved from the mitochondrion to the host's genome during the long relationship. [4]

The evidence suggests that as photosynthesis oxygenated the atmosphere and aerobic prokaryotes evolved successfully, an ancestral cell engulfed and kept alive an aerobic prokaryote. That gave the host the ability to use oxygen to release energy stored in nutrients. One popular analogy compares it to a landlord and tenant: the host offers a safe home, and the organelle pays rent by making energy the host can use. [1] [2]

The fact of endosymbiosis is not in doubt, but the mechanism of entry is debated. A review in the journal literature on eukaryote origins describes three candidate processes: bacteriovory (one bacterium infiltrating another), syntrophy (two organisms interacting biochemically without one living inside the other), and phagocytosis (entry through the cell's engulfing machinery). Popular summaries mention that the ancestor may have been eaten or may have been a parasite of the host. [5] [2]

The review's authors argue for phagocytosis. Their reasoning is that it is the only process known to place cells of one domain inside cells of another, since eukaryotes host endosymbionts from all three domains. Bacteria are known to host only other bacteria, and no archaea are known to carry endosymbionts. They also note that the basic phagocytic and endocytic machinery traces back to the last eukaryotic common ancestor. They therefore favor a phagocytosing archaeon as host, while stating that the same logic holds whether eukaryotes derive from archaea or are their sister group. They also argue that the links between specific models of eukaryote origins and genomic data are weaker than many suppose. This is one research group's argument, and the search results do not show how widely it is held. [5]

The timing, the nature of the host, and the mechanism have all been debated. It is now uncontroversial that the acquisition happened before the last eukaryotic common ancestor, and that it was not the final event before modern eukaryotes diversified. Many nuclear genes and the machinery for handling them look closely related to Archaea, while the energy-harvesting genes and organelles come from bacteria. Several endosymbiotic events likely contributed to the eukaryotic cell, and the exact steps are still being worked out. [5] [1]