What is ancestry?

Ancestry connects genetics and society in fundamental ways.For many people it has cultural, religious or even political significance, and can play a key role in shaping personal and public identities.People's desire to discover their own ancestry drives the multibillion-dollar genealogy industry, which has grown rapidly in the era of consumer genomics.Companies such as 23andMe and Ancestry now claim tens of millions of customers worldwide.In parallel, our scientific understanding of the human past is being transformed by studies of ancient and modern genetic data, which allow us to track changes in ancestry over space and time.Sophisticated methods have been developed to infer and visualise these relationships.Thus, it seems that both scientists and the wider public are learning more and more about ancestry, and there is an optimistic sense that genetic data provide an exhaustive repository of ancestral information.However, although frequently discussed, ancestry itself is rarely defined.We argue that this reflects widespread underlying confusion about what it means in different contexts and what genetic data can really tell us.This leads to miscommunication between researchers in different fields, and leaves customers open to spurious claims about consumer genomics products and overinterpretation of individual results.In wider usage, the terms ancestry and ancestors often indicate a general connection to people or things in the past.But in a genetic context they have a more specific meaning: your ancestors are the individuals from whom you are biologically descended and ancestry is information about them and their genetic relationship to you.Even here however, confusion arises from the way that ancestry is presented and discussed.Rather than emphasising its complex structure, results are often simplified in terms of discrete categories.While convenient and sometimes useful, ultimately this is misleading about the nature of ancestry.These labels can also impose contemporary political or cultural divisions which may be misrepresentative of ancestral relationships.Another source of confusion is that three distinct concepts-genealogical ancestry, genetic ancestry, and genetic similarity-are frequently conflated.We discuss them in turn, but note that only the first two are explicitly forms of ancestry, and that genetic data are surprisingly uninformative about either of them.Consequently, most statements about ancestry are really statements about genetic similarity, which has a complex relationship with ancestry, and can only be related to it by making assumptions about human demography whose validity is uncertain and difficult to test.Genealogical ancestry probably reflects the most common and intuitive understanding of the term ancestry.Consider your parents, grandparents, or even great-grandparents.You likely have a sense of these people as individuals, even if you have never met them.If one of them belonged to a particular group X, you might say that you have some "X" ancestry.You might even be able to claim ancestry in this way from more distant ancestors, based on