One random day in December I got thinking about ancestry and numbers. If you work it back, multiplications of two start very quickly to make big numbers. If you give an average of 25 years per generation, and assume no inter-relationships, in 1900 you would have 16 direct ancestors; in 1800 it would be 250; in 1700 it would be 4,000; 1600 would have had 64,000; and in 1500 you'd need a million separate individuals to bring about one person today (work it out) - and if you include the generations of people between 1500 and now it would add another 500,000 people to the mix.
Granted it's never gonna be that much cause pro-creaters along the way will have the same ancestry, but I find it something scary and awe inspiring, because it took so many chance encounters and so many people to create you, and it makes us all such unique creatures.
Of course if you want to go further, the numbers in 1000AD could have been 1,ooo billion, and in the time of Christ you'd need 1000,000,000,000,000,000,000,000 individuals to make one today.