According to the Telegraph (UK), Neandertals became extinct because their mitochondria leaked excess heat:
Professor Patrick Chinnery, a neurogeneticist at Newcastle University, believes the differences in this mitochondrial DNA could have caused Neanderthals to be inefficient at producing energy, meaning their cells leaked heat.
He said: "The question is why did Neanderthals disappear? There are lots of explanations to do with changes in climate and the food supply.
"Differences in these mitochondrial DNA sequences might explain why modern humans were able to survive while Neanderthals were not.
So, is it true? Did Neandertals go panting into that long good night?
Well, Siberians may have mtDNA alleles that leak extra heat, and they’re not extinct. It seems like a good idea if you don’t live in the tropics and have enough food. Because it’s not like Europeans lack opportunities to take off a layer to deal with the heat.
Plus, as the Neandertal morphology waned in Europe, the climate was getting colder, not warmer.
The real mistake here is assuming that the mtDNA necessarily shared the same fate as the rest of the genome. Sure, there aren’t any living members of the Neandertal mtDNA clade, at least that we know of. But that suggests selection favoring human mtDNA, not necessarily Neandertal extinction. The idea of selection is supported by the finding of functional variations between human and Neandertal COX2, which I discussed in August.
The current research seems like it probably adds detail to this comparison, but that’s not an argument for Neandertals going extinct in the heat.