Ages computed from nuclear DNA diversity are larger than ages computed from mtDNA diversity.  This is also evidence for an accelerated mutation rate in the past because nuclear DNA mutates much slower and would be more affected.  But what caused the increase?
 There is evidence that small doses of radiation can lead to unexpectedly high mutation rates in humans (Science 8 February 2002 vol. 295 page 946): ... researchers led by geneticist Yuri Dubrova of the University of Leicester, United Kingdom, describe a compelling connection between radioactive fallout and elevated mutation rates in families living downwind of the Semipalatinsk nuclear facility ...