at the location of geographic barriers such as the Sahara, the Oceans, and the Himalayas. Nonetheless, stated that their findings "should not be taken as evidence of our support of any particular concept of biological race... Genetic differences among human populations derive mainly from gradations in allele frequencies rather than from distinctive 'diagnostic' genotypes." Using a sample of 40 populations distributed roughly evenly across the Earth's land surface, found that "genetic diversity is distributed in a more clinal pattern when more geographically intermediate populations are sampled." Guido Barbujani has written that human genetic variation is generally distributed continuously in gradients across much of Earth, and that there is no evidence that genetic boundaries between human populations exist as would be necessary for human races to exist. Over time, human genetic variation has formed a nested structure that is inconsistent with the concept of races that have evolved independently of one another. Social constructions As anthropologists and other evolutionary scientists have shifted away from the language of race to the term population to talk about genetic differences, historians, cultural anthropologists and other social scientists re-conceptualized the term "race" as a cultural category or identity, i.e., a way among many possible ways in which a society chooses to divide its members into categories. Many social scientists have replaced the word race with the word "ethnicity" to refer to self-identifying groups based on beliefs concerning shared culture, ancestry and history. Alongside empirical and conceptual problems with "race", following the Second World War, evolutionary and social scientists were acutely aware of how beliefs about race had been used to justify discrimination, apartheid, slavery, and genocide. This questioning gained momentum in the 1960s during the civil rights movement in the United States and the emergence of numerous anti-colonial movements worldwide. They thus came to believe that race itself is a social construct, a concept that was believed to correspond to an objective reality but which was believed in because of its social functions. Craig Venter and Francis Collins of the National Institute of Health jointly made the announcement of the mapping of the human genome in 2000. Upon examining the data from the genome mapping, Venter realized that although the genetic variation within the human species is on the order of 1–3% (instead of the previously assumed 1%), the types of variations do not support notion of genetically defined races. Venter said, "Race is a