Thus, C dates must be converted to calendar years using calibration curves based on other sources, including annual tree rings (dendrochronology) or uranium-series dating of coral (2).
Such calibrations, however, may not fully capture the variation in atmospheric carbon.
The idea is that an ancient genome has evolved less than the genomes of present-day individuals and thus has experienced fewer recombination events since the common ancestor.
To implement this idea, we take advantage of the insight that all non-Africans have a common heritage of Neanderthal gene flow into their ancestors.
Thus, we can estimate the date since Neanderthal admixture for present-day and ancient samples simultaneously and use the difference as a direct estimate of the ancient specimen’s age.
In addition, instead of using a distant outgroup, such as chimpanzees, we rely on a more recent shared event that has affected both extant and ancient modern humans and is therefore a more reliable fixed point on which to base the dating.In addition, contamination of a sample by modern carbon, introduced during burial or by handling afterwards, can make a sample seem younger than it actually is (2).The problem is particularly acute for samples that antedate 30,000 y ago because they retain very little original C.Because recombination occurs at an approximately constant rate per generation, the accumulated number of recombination events provides a molecular clock for the time elapsed or, in the case of an ancient sample, the number of missing generations since it ceased to evolve.This idea is referred to as “branch shortening” and estimates of missing evolution can be translated into absolute time in years by using an estimate of the mean age of reproduction (generation interval) or an independent calibration point such as human–ape divergence time.
The idea of our method is to estimate the date of Neanderthal gene flow separately for the extant and ancient genomes.