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

by Nick Lane

Selection is all about offspring. If organisms with large and irreplaceable brains leave more viable offspring than organisms with small replaceable brains, then they will thrive. Only when there is a distinction between the germline and soma can selection act in this way; but when it does, the body becomes disposable. Lifespan becomes finite. And cells that can’t meet their metabolic requirements will kill us in the end. That’s why the metabolic rate matters. Cells with a faster metabolic rate are more likely to fail to meet their demands, given the same mitochondrial output. Not only mitochondrial diseases, but also normal ageing and age-related diseases, are most likely to affect the tissues with the highest metabolic demands. And to come full circle – the sex with the highest metabolic demands. Males have a faster metabolic rate than females (in mammals at least). If there is some genetic defect in the mitochondria, that defect will be unmasked largely in the sex with the faster metabolic rate – the male. Some mitochondrial diseases are indeed more common in men than in women; Leber’s hereditary optic neuropathy, for example, is five times more prevalent in men, while Parkinson’s disease, which also has a strong mitochondrial component, is twice as common.