Ancient DNA and the rewriting of human history: be sparing with Occam's razor

Abstract

Ancient DNA research is revealing a human history far more complex than that inferred from parsimonious models based on modern DNA. Here, we review some of the key events in the peopling of the world in the light of the findings of work on ancient DNA.

Fig. 1

Populating Europe. aDNA research shows that present-day Europeans are the result of a mixture of different ancient populations (blue diamonds): (1) West hunter-gatherers who had inhabited Europe since Paleolithic times; (2) Early European farmers, who descended from Near Eastern farmers and entered Europe during the Early Neolithic; and (3) Steppe herders, who arrived in Europe during the Bronze Age. The steppe herders themselves were a mixture of eastern Eurasian hunter-gatherers (Eastern hunter-gatherers) and Near Easterners (orange squares). Additionally, Europeans have ~2 % archaic ancestry from mixture with Neanderthals that arose ~50,000–60,000 ya, probably somewhere in the Near East (purple star). There is also evidence that admixture with Neanderthals occurred again in Europe (purple triangle), as evident from the DNA of a 37,000–42,000-year-old human from Romania. However, this population appears not to have contributed detectably to later humans in Europe. Grey arrows represent the model for populating Europe inferred from modern DNA analysis. aDNA research refined this model by adding several additional layers of information, including multiple migrations and mixtures leading to present-day Europeans (black arrows)

Fig. 2

Recent positive selection in Europe and Asia. Change over time in the allele frequency of variants involved in adult lactose tolerance (rs4988235 for LCT), skin pigmentation (rs1426654 for SLC24A5) and immunity (rs4833095 for TLR1 and rs9302752 for NOD2) observed from aDNA (colored solid lines). Allele frequencies in ancient populations are replotted from Allentoft et al. [32]. The black dotted lines represent predictions from theoretical models of selection based only on present-day population information. We assume that the initial frequency of the derived alleles in Eurasians in the past was similar to that in present-day Near Easterners. Increase in the frequency of these alleles was thought to have started in Paleolithic times (SLC24A5) or in Neolithic times (TLR1, NOD2 and LCT)