The Vital Question: Why is life the way it is?

by Nick Lane

The more tissues there are in an adult, the greater the likelihood that a vital tissue will accumulate all the worst mitochondria. Conversely, with only one tissue type, this is not a problem, as there is no interdependence – no organs whose failure can undermine the function of the whole individual. In the case of a simple organism with a single tissue, then, increased variance is unequivocally good: it’s beneficial for the germline and not particularly detrimental to the body. We therefore predicted that the first animals, with (presumably) low mitochondrial mutation rates and very few tissues, should have had biparental inheritance and lacked a sequestered germline. But when early animals became slightly more complex, with more than a couple of different tissues, increased variance within the body itself becomes disastrous for adult fitness, as it inevitably produces both good and bad tissues – the heart attack scenario. To improve adult fitness, mitochondrial variance must be decreased so that nascent tissues all receive similar, mostly good, mitochondria.