Based on the recent evidence that modern human & Neanderthal hybrids suffered from reduced fertility and male offspring might have been sterile, two questions that I think should be asked are that what relationship the fact that late Denisovans were hybrids (having mixed with both their Neanderthal cousins and that unknown species) had with their higher rate of mutations and observed decrease in population? So, could the decrease in population point towards the timing of these hybridization events?