In The vital Question https://en.wikipedia.org/wiki/The_Vital_Question Nick Lane is arguing that the eukaryote symbiotic cell (archea host, bacterial mitochondria symbiot) leads, via sex and the requirement for gene compatibility between the small mitochondrial genome and the host nuclear genome, to a "best fit" pressure on organism with extreme metabolism. Simplified, "Heartbeat hypothesis" (https://en.wikipedia.org/wiki/Rate-of-living_theory) doe snot apply to, say, birds, that have extreme metabolic requirements (= flight). The fact that, among mammals, is bats that fit the same longevity may indicate that the mitochondrial-nucleus gene match idea could be right.
