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

by Nick Lane

So there is a hypothetical death threshold (Figure 34). Above the threshold the cell, and by extension the whole organism, dies by apoptosis. Below the threshold the cell and the organism survive. This threshold is necessarily variable between different species. For bats and birds and other creatures with high aerobic requirements, the threshold must be set low – even a modest rate of free-radical leak from mildly dysfunctional mitochondria (with slight incompatibilities between mitochondrial and nuclear genomes) signals apoptosis and termination of the embryo. For rats and sloths and couch potatoes with low aerobic requirements, the threshold is set higher: a modest rate of free-radical leak is now tolerated, dysfunctional mitochondria are good enough, the embryo develops. There are costs and benefits to both sides. A low threshold gives a high aerobic fitness and a low risk of disease, but at the cost of a high rate of infertility and poor adaptability. A high threshold gives a low aerobic capacity and higher risk of disease but with the benefits of greater fertility and better adaptability. These are words to conjure with. Fertility. Adaptability. Aerobic fitness. Disease. We can’t cut much closer to the grain of natural selection than that. I reiterate: all these trade-offs emerge inexorably from the requirement for two genomes.