Marcin Woźniak and colleagues (2010) searched for specifically Slavic sub-group of R1a1a [M17]. Working with haplotypes, they found a pattern among Western Slavs which turned out to correspond to a newly discovered marker, M458, which defines subclade R1a1a7. This marker correlates remarkably well with the distribution of Slavic-speakers today. The team led by Peter Underhill, which discovered M458, did not consider the possibility that this was a Slavic marker, since they used the "evolutionary effective" mutation rate, which gave a date far too old to be Slavic. Woźniak and colleagues pointed out that the pedigree mutation rate, giving a later date, is more consistent with the archaeological record.
What did Peter Underhill suspect M458 was?