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"However, based on the absence of crucial virulence genes, unlike the later Y. pestis strains that were responsible for the first to third pandemics, these ancient ancestral Y. pestis strains likely did not have the ability to cause bubonic plague, only pneumonic and septicemic plague. These early plagues may have been responsible for the suggested population declines in the late 4th millennium BC and the early 3rd millennium BC ( Hinz et al.,2012; Shennan et al., 2013)."