A world in a grain of sand: human history from genetic data

To see a world in a grain of sand ...William Blake, Auguries of InnocenceThe genome of each individual is a temporary assemblage of DNA segments brought together for a single generation by a combination of chance, ancestry, recombination and natural selection. These segments have different histories because of recombination and can thus provide independent information about ancestry, the focus of this review. However, the ancestry of different segments is not entirely independent. Humans are not a single randomly mating population: we are subdivided, and these subdivisions into bands, tribes, clans, ethnic groups, nations and so on are of great interest to both scientists and non-scientists. Thus the thousands of different genomic segments in any individual do not trace back to ancestors randomly spread around the globe; segment ancestry is constrained by population history. Two non-recombining segments of the genome, mitochondrial DNA (mtDNA) and the Y chromosome, have been used for decades to study genetic histories [1,2]. Sometimes mtDNA and the Y chromosome share the same history, but often they do not, and such differences alert us to some of the complexities of the human past [3]. But mtDNA and the Y chromosome provide only two perspectives. Recent advances in technology provide access to most of the genome, and increasingly to the genomes of companion species. Here, we consider how this wider perspective is beginning to inform our view of human history. We will see that it is possible to probe much further back into the past, into a period in which the uniparental markers are uninformative yet key evolutionary events took place, and even to speculate about when humans might have begun to wear clothes or to start reconstructing the genomics of former populations before their contact with modern expansions.Genome-wide data can be obtained by either genotyping samples or re-sequencing them. Genotyping provides information about the allelic state of positions in