I've often wondered if the frequency of mtDna "H" has something to do with "fitness", and not just in terms of "endurance", either.
Surely to some extent. Haplogroup H isn't just more common among athletes, but men with this mtDNA type also have better sperm motility. MtDNA seems to mobilise energy more efficiently within the cells. The drawback is faster ageing and greater rates of cancer and neurodegenerative diseases. As often in genetics, you can't have it all.