Our human family is growing rapidly. In 1800, about one billion people lived in the world. This number had doubled by 1930, tripled by 1960, and officially reached six billion in October 1999. Because of this population explosion, everyone alive today shares recent common ancestors: the average marriage in Europe is between sixth cousins, who share a great, great, great, great, great grandparent. However, this varies a lot from place to place, and people living in isolated communities will be more closely related.