The table below illustrates how the number of your ancestors rises dramatically as you go back in time.
Around 1450 the number of your theoretical ancestors exceeds the population of the British Isles.
Keep going back to the year 0 and you have more theoretical ancestors, than the number of people who have ever been born!
How?
Families whose history has been well documented, such as Royalty, show individuals appearing at two or more places in the family tree.
For example, HRH Prince William's father's mother's father's father's father's parents were Queen Victoria and Prince Albert; but they were also his father's father's father's mother's monther's parents. He has another set of duplicate great great great great grandparents, and so only has 60 great4 grandparents not 64.
Wikipedia — Ahnantafel
Your father's father's father's father's father's father could easily be the same person as your mother's mother's mother's mother's mother's father. Two of his children may have gone their separate ways, and the families would have lost touch over numerous generations. When two descendents met a hundred or more years later they might have had no idea they were distant cousins.
As you go further back, the number of such individuals will increase, as may the quantity of each persons duplications. As soon as you find one duplicate, all that person's ancestors are also duplicates, significantly reducing your actual ancestors.
For example, were you to find that two of your 128 potential gt5 grandparents were the same person, you would have only 126 gt5 grandparents, but this would mean that your gt6 grandparents would reduce from 256 to 252, your gt7 grandparents from 512 to 504, your gt8 grandparents from 1024 to 1008, etc. The reduction would double each generation, just as the potential doubles each generation.
All of which means that if your family originated in the British Isles, at least one of your 4,194,304 great- great- great- great- great- great- great- great- great- great- great- great- great- great- great- great- great- great- great- great- grandparents was the same person as one of mine! Hello 21st cousin!
All figures in columns 3 and 4 are approxiamate! Years based on an average 25 year gap between generations.
| Generation | Potential people | Approx Year | Population of British Isles |
| You | 1 | 2000 | |
| Parents | 2 | 1975 | |
| Grandparents | 4 | 1950 | 50,000,000 |
| Gt Grandparents | 8 | 1925 | |
| g2gparents | 16 | 1900 | 40,000,000 |
| g3gparents | 32 | 1875 | |
| g4gparents | 64 | 1850 | |
| g5gparents | 128 | 1825 | |
| g6gparents | 256 | 1800 | 16,500,000 |
| g7gparents | 512 | 1775 | |
| g8gparents | 1,024 | 1750 | 12,100,000 |
| g9gparents | 2,048 | 1725 | |
| g10gparents | 4,096 | 1700 | 8,500,000 |
| g11gparents | 8,192 | 1675 | 8,200,000 |
| g12gparents | 16,384 | 1650 | |
| g13gparents | 32,768 | 1625 | |
| g14gparents | 65,536 | 1600 | 7,000,000 |
| g15gparents | 131,072 | 1575 | |
| g16gparents | 262,144 | 1550 | |
| g17gparents | 524,288 | 1525 | |
| g18gparents | 1,048,576 | 1500 | 6,000,000 |
| g19gparents | 2,097,152 | 1475 | 5,500,000 |
| g20gparents | 4,194,304 | 1450 | 5,000,000 |
| g21gparents | 8,388,608 | 1425 | 4,500,000 |
| g22gparents | 16,777,216 | 1400 | 4,000,000 |