The rate at which mutations accumulate in a genome, referred as a "molecular clock," is an instrumental tool in molecular evolution and phylogenetics. Different types of mutations occur via distinctive molecular pathways. In particular, while most mutations occur from errors in DNA replication, spontaneous deamination of methylated CpG dinucleotides is another important source of mutation in mammalian genomes. Molecular clock studies typically combined all types of mutations together. In this paper, the authors analyze molecular clocks of replication-origin and methylation-origin mutations separately. By utilizing high-quality sequence data from several primate species and fossil calibration, the authors demonstrate that the two types of mutations follow statistically different molecular clocks. Methylation-origin mutations accumulate relatively constantly over time, while replication-origin mutations scale with generation-times. Therefore, the genomic molecular clock, as a whole, is shaped by the molecular origins of mutations that have accumulated over time. The authors' results have direct implications on phylogenetic analyses, estimation of species divergence dates, and studies of the mechanisms and processes of evolution, where molecular clocks are imperative.
I really don't have much to add, their own summary covers all the bases. Molecular clocks are critical in reconstructing phylogenies, and in human evolution their utility is so great that they have arguably revolutionized paleoanthropology. Obviously the great "rate debate" within phylogenetics matters here, as a million years here and there is essential in framing and generating hypotheses about the environment in which our hominid ancestors evolved and speciated.