In the 1990s, genomic studies of the world's peoples found that the Y chromosome of San men share patterns of polymorphisms that are different from those of all other populations. Because the Y chromosome changes little, this type of DNA test is used to determine when different human subgroups separated from one another. This shows their last common ancestry. The authors of these studies suggest that the San may have been one of the first populations to differentiate from the most recent common ancestor of all living humans, estimated to have lived 60,000 to 90,000 years ago.