Establishing a timescale for plant evolution is a prime goal of palaeontology, but ever since the advent of gene sequencing the prospect of molecular clocks has provided an enticing alternative. Now clocks are increasingly taking the place of rocks. Advances in technology mean that vast amounts of DNA sequence can be obtained rapidly and cheaply from large numbers of living species, informing both our understanding of the pattern of branching of the tree of life (phylogeny) and, when calibrated, the timing of those branching events (timetree). As the nature of molecular data becomes better understood, methods have developed to address recognized weaknesses, modelling the reality of molecular evolution more accurately (Hedges & Kumar, 2009). Fossils still play a crucial role, forming the link between molecules and an actual chronology. At least one fossil calibration point is needed to set the basic rate at which the clock ticks, and increasingly more are deployed to act as constraints on rate changes across different limbs of the tree (Benton et al., 2009). Getting the fossil calibrations right is critical, because they greatly influence outcomes, but this apparently simple step is not nearly as straightforward as it may seem. In this issue of New Phytologist, Clarke et al. (pp. 266–301) take a novel approach to setting and handling fossil calibrations for the plant tree of life, and one that is likely to be widely adopted and developed. Results are controversial, indicating substantially earlier origins of land plants and flowering plants than is supported by direct fossil evidence.
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