Trying to explain to people how everyone can be related, especially if they have no interest in research genealogy can be a little challenging.
It is not until individuals take on the challenge to research the family tree that they find out that as little as four generations ago, tracing their roots to to small towns and villages, that the likely hood that some of the great-great ancestors married and had families with their own cousins.
Endogamy: the custom of marrying only within the limits of a local community, clan, or tribe.
The further back in history we go, the younger women were getting married (some places had young women as young as 16 married off, with the intent of having children as soon as possible. Also the further back in history we go, the more likely a couple would continue to have children for the duration of a woman’s fertility. If she was married off at 17, and can have children until 40, and no available birth control, it is possible that one couple could produce up to 20 children.
Not all 20 children would be expected to reach adulthood, with many likely dying as infants.
However, if even 6 children reached adulthood long enough to marry and repeat the cycle, eventually, all of their kids and grand kids, still limited to their territories would likely end up marrying a distant cousin.
Finally, let’s add to all this, the idea of world populations. The further back in time we go, the less people there were on planet earth. However, the further back we go in a family tree, the higher the number of people that contribute in your dna (the number doubles with each generation going back. To make this point clear, consider that you are 1 individual person. You have 2 parents (1 doubled is 2). Then you have 4 grandparents (2 doubled is 4), and then you have 8 great grandparents (4 doubled is 8) and then 16 great great grandparents (8 doubled is 16)…and so on.
Eventually, some of your great-great-etc…grandparents happen to be the same ones counted more than once. If your parents are distant cousins to each other, it means they shared at least one set of great-great-etc..grandparents, and thus the same couple is counted twice.
So here is a chart to help explain all this:
This breakdown starts with someone who is born in 2020, and using 30 years as an estimate for a generation. (mothers as young as 16 and as old as 40, (16+40) and then divided by 2, gives about 28, and rounded to 30 for easier math.
|Family Tree Genealogy||World Population|
|Person born in 2020 (one person)||7.8 Billion|
|People born in 1990 (2 parents)||approx 5 billion|
|People born in 1960 (4 grandparents)||approx 3 billion|
|People born in 1930 (8 great grandparents)||approx 2 billion|
|People born in 1900 (16 great grandparents)||approx 1.6 billion|
|1870 ( 32 great great grandparents)|
|1840 (64 great great great grandparents)||approx 1.2 billion|
|1810 (128 great great great great grandparents)||approx 1 billion|
|1780 (256 G G G G G grandparents)|
|1750 (512 GGGGGG Grandparents)||approx 700 million|
|1720 (1024 GGGGGGG Grandparents)|
|1690 (2048 GGGGGGGG Grandparents)|
|1660 (4096 GGGGGGGGG Grandparents)||approx 500 million|
|1630 (8192 GGGGGGGGGG Grandparents)|
|1600 (16 384 GGGGGGGGGGG Grandparents)|
|1570 (32 768 GGGGGGGGGGGG Grandparents)|
|1540 (65 536 GGGGGGGGGGGGG Grandparents)|
|1510 (131 072 GGGGGGGGGGGGG Grandparents)||approx 450 million|
|1480 (262 144 your 15th G Grandparents)|
|1450 (524 288 your 16th G Grandparents)|
|1420 (1 048 576 your 17th G Grandparents)||approx 380 million|
|1390 (2 097 152 your 18th G Grandparents)|
|1360 (4 194 304 your 19th G Grandparents)||approx 420 million|
|1330 (8 388 608 your 20th G Grandparents)|
|1300 (16 777 216 this is your 21st G Grandparents)||approx 400 million|
|1270 (33 554 432 this is your 22nd G Grandparents)|
|1240 (67 108 864 this is your 23rd G Grandparents)||approx 420 million|
|1210 (134 217 728 this is your 24th G Grandparents)||approx 395 million|
|1180 (268 435 456 this is your 25th G Grandparents)||approx 420 million|
|1150 (536 870 912 this is your 26th G Grandparents)||approx 450 million|
|1120 (1 073 741 824 -over 1 billion) 27th G Grandparents)||approx 350 million|
|1090 (2 147 483 648 this is your 28th G Grandparents)||approx 320 million|
|1060 (4 294 967 296 this is your 29th G Grandparents)||approx 300 million|
|1030 (8 589 934 592 this is your 30th G Grandparents)||approx 283 million|
|1000 (17 179 869 184 your 31st G Grandparents)||approx 275 million|
Based on the chart above, using population of world stats I found on the Internet (take it with a grain of salt), as well as an estimated calculation of an average of 30 years per generation (given that historically, it would be less than that), the chart shows that the number of Great-Grandparents is higher than the existing population at the time at about 26 generations ago around the year 1150 AD.
It stands to reason due to endogamy and limited travel options that ALL of our ancestors likely married their own cousins (and a few families inter-breed over the course of 20+ generations) at some point in everyone’s family history.
And since the number of great grandparents exceeds the existing population at the time, it surely indicates that a number of our ancestors were so related as to reduce the number of actual individuals involved.
To be more clear, yes you have as of 1000 AD, you technically have 17 179 869 184 billion 31st G Grandparents, but there is a maximum of 275 000 000 INDIVIDUALS only that could have had that role, so you are likely your own 31st cousin, 62x over!