Mitochondria and Chloroplasts

Margulis believes that mitochondria were originally parasites (or predators - the distinction is not important at this level) which attacked the larger bacteria that were destined to provide the shell of the eucaryotic cell. There are still some bacterial parasites that do a similar trick, burrowing through the prey's cell wall, then, when safely inside, sealing up the wall and eating the cell from within. The mitochondrial ancestors, according to the theory, evolved from parasites that kill to less virulent parasites that keep their host alive to exploit it longer. Later still, the host cells began to benefit from the metabolic activities of the protomitochondria. The relationship shifted from predatory or parasitic (good for one side, bad for the other) to mutualistic (good for both). As the mutualism deepened, each came to depend more thoroughly on the other, and each came to lose those bits of itself whose purpose was best served by the other.

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The other grinning relic which is now pretty uncontroversial is the chloroplast. Chloroplasts are small bodies in plant cells that do the business of photosynthesis - storing solar energy by using it to synthesize organic molecules. These organic molecules can then be broken down later and the energy released in a controlled way when required. Chloroplasts are responsible for the green colour of plants. It is now widely agreed that they are descended from photosynthetic bacteria, cousins of the 'blue-green' bacteria that still float free today and are responsible for 'blooms' in polluted water. The process of photosynthesis is the same in these bacteria and in (the chloroplasts of) eucaryotes. Chloroplasts, according to Margulis, were captured in a different way from mitochondria. Where the mitochondrial ancestors aggressively invaded larger hosts, the ancestors of chloroplasts were prey, originally engulfed for food, only later evolving a mutualistic rapport with their captors, doubtless again because their DNA became transmitted longitudinally down host generations.