The researchers raised hundreds of generations of C. elegans, carefully making sure they knew which generation was which by selecting a single organism to parent each new generation. (C. elegans is hermaphroditic and capable of self fertilization.) They then sequenced portions of the genome in each generation.
This direct measurement revealed a mutation rate an order of magnitude higher than had been previously estimated.
If this turns out to be correct, it has implications for everything from evolution to cancer mechanisms to aging.