These GEITP pages have often discussed evolution, and quite often evolution of Homo sapiens during the Great Human Diaspora (migrations out of Africa, multiple times, and repeated admixture of sublines). Today, even seemingly unrelated humans are distant cousins to each other — because all members of a species are related to each other through a vastly subdivided family tree (their pedigree). We can follow traces of these relationships in genetic data, when individuals inherit shared genetic material (stretches of DNA) from a common ancestor; much interest has been generated recently by advances in genealogy companies (e.g. ancestry.com, 23andMe.com, heritage.com, etc.). Traditionally, population genetics has studied the distant bulk of these genetic relationships, which in humans typically date from hundreds of thousands of years ago.
Although most such genetic relationships among individuals are very old, some individuals can be shown to be related on far shorter time scales. Indeed, given that each individual has 2n ancestors from n generations ago, theoretical considerations suggest that all humans are related genealogically to each other over surprisingly short time scales (e.g. aren’t Meryl Streep, Kim Jong il, Robert De Niro, and Michelle Obama all ‘second cousins, once-removed’?). We are usually unaware of these close genealogical ties, because few of us have knowledge of family histories more than a few generations back; these ancestors often do not contribute any genetic material to us. However, in sufficiently large samples, we can hope to identify genetic evidence of more recent relatedness, and so obtain insight into the population history of the past tens of generations. Authors [see attached article & editorial] investigated such patterns of recent relatedness in a large European dataset (therefore, this study is about Caucasians, not the other four main groups)
Events that likely had significant impact on modern European relatedness include — the Neolithic expansion of farming, the Roman Empire, and the more recent expansions of the Slavs and Vikings; our current understanding of these events is deduced from archaeological, linguistic, cultural, historical, and genetic evidence — with widely varying degrees of certainty. However, the demographic and genealogical impact of these events is still uncertain. Genetic data describing the breadth of genealogical relationships can therefore add another dimension to our understanding of these historical events. Authors used genomic data from 2,257 Europeans [in the Population Reference Sample (POPRES) dataset] to conduct a survey of recent genealogical ancestry — over the past 3,000 years at a continental scale. Authors identified 1.9 million shared long genomic segments, and used the lengths of these to infer the distribution of shared ancestors across
time and geography.
Authors [see attached article] show that a pair of modern Europeans, living in neighboring populations, share between 2 and 12 genetically common ancestors from the last 1,500 years — and upwards of 100 genetic ancestors from the previous 1,000 years. These numbers drop off exponentially. with geographic distance; however, since these genetic ancestors are a tiny fraction of common genealogical ancestors, individuals from opposite ends of Europe are still expected to share millions of common genealogical ancestors over the last 1,000 years.
There is also substantial regional variation in the number of shared genetic ancestors. For example, there are especially high numbers of common ancestors shared between many eastern European populations that date roughly to the Migration Period (which includes the Slavic and Hun expansions into that region). Some of the lowest levels of common ancestry are seen in the Italian and Iberian peninsulas, which may indicate different effects of historical population expansions and wars in these areas and/or more stably structured populations. Population genomic datasets have considerable power to uncover recent demographic history, and will allow a much fuller picture of the close genealogical kinship of individuals across the world. 😊
PLoS Biol Nov 2013; 11: e1001555 & editorial doi:10.1038/nature.2013.12950