Every human being has ancestors, the number of which doubles with each preceding generation.
Since there are approximately 30 years between generations, the number of ancestors grows rapidly as we go back in time. This number is far in excess of the total world population of that time. Obviously, many ancestral lines stem ultimately from a relatively small group of people, and the lines cross and re-cross due to marriages between cousins. Our most famous ancestor in the early years of the current millennium was Duke William II of Normandy, who invaded England on Thursday 28 September 1066 and defeated King Harold at the Battle of Hastings two weeks later on Saturday 14 October. William the Conqueror was then crowned King William I of England.