QUOTE : Much of our internal heat is generated by dissipating the proton gradient across the mitochondrial membranes (see page 183). Since the proton gradient can either power ATP production or heat production, we are faced with alternatives: any protons dissipated to produce heat cannot be used to make ATP. (As we saw in Part 2, the proton gradient has other critical functions too, but if we assume that these remain constant, they don’t affect our argument.) If 30 per cent of the proton gradient is used to produce heat, then no more than 70 per cent can be used to produce ATP. Wallace and colleagues realized that this balance could plausibly shift according to the climate. People living in tropical Africa would gain from a tight coupling of protons to ATP production, so generating less internal heat in a hot climate, whereas the Inuit, say, would gain by generating more internal heat in their frigid environment, and so would necessarily generate relatively little ATP. To compensate for their lower ATP production, they would need to eat more. Wallace set out to find any mitochondrial genes that might influence the balance between heat production and ATP generation, and found several variants that plausibly affected heat production (by uncoupling electron flow from proton pumping). The variants that produced the most heat were favoured in the Arctic, as expected, while those that produced the least were found in Africa.