Human germline mutation and the erratic evolutionary clock

Our understanding of the chronology of human evolution relies on the \"molecular clock\" provided by the steady accumulation of substitutions on an evolutionary lineage. Recent analyses of human pedigrees have called this understanding into question, by revealing unexpectedly low germline mutation rates, which imply that substitutions accrue more slowly than previously believed. Translating mutation rates estimated from pedigrees into substitution rates is not as straightforward as it may seem, however. We dissect the steps involved, emphasizing that dating evolutionary events requires not \"a mutation rate,\" but a precise characterization of how mutations accumulate in development, in males and females--knowledge that remains elusive.