How many ancestors do we have in each period of history?


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Have you ever wondered how many ancestors you had 200 years ago, or 500 years ago, 1000 years ago, 2000 years ago? Venture to take a guess? It's easy enough to count how many ancestors we have after 3 generations (8 great-grand-parents), 4 generations (16 g-g-grandparents), etc. But what about the number of ancestors at the 20th or 50th generation? And just how many years do 20 or 50 generations represent? I have made a table to summarise all this.

I estimated the number of years that elapsed for each generation, giving a minimum and maximum range using 16 years as the youngest age of the parents and 40 as the oldest. This is especially for women, who are more limited than men in their reproductive years. There are always exceptions, but that seems like a reasonable range. That gives us an average of 28 years old to become a parent, which is about right for European society, at least in the last 500 years, based on what genealogy websites have calculated. People used to have children earlier than today, but also had more children and the last ones were often born when the mother was in her late thirties to mid forties.

I wanted to deduct the average time lapsed to show around what year each generation would have happened. But in order to do that I had to choose a starting year. It didn't make any sense to choose 2024 as none of us were born this year, and even less our parents. As each of us is generation 0, generation 1 below represents our parents. I chose 1980 as year 0 as I think it's a reasonable average age for members of this forum. Feel free to adjust according to your birth year accordingly. Anyway; once going back centuries a few years don't make any difference as these are estimates and averages.

The rightmost column shows the estimated world population at that time. I used the range of the 11 studies listed on Wikipedia.

You will see that at the 20th generation, we have just over 1 million ancestors. Many of them are probably the same, as people used to marry distant cousins all the time (and still do, often without realising it, especially outside big cities). If you have any doubt about this, think about how many 4th or 5th cousins of yours you might have (surely hundreds, if not thousands), and how many you actually know as such thanks to genealogy.

Genealogy is a great example of the power of exponentials. Every 10th generation we have 1000 times more ancestors. We have 1024 ancestors at the 10th generation, 1 million at the 20th, and going back 30 generations we reach 1 billion ancestors, far more than the number of people alive in all the world at that time. We are now around the 12th century, at the age of Richard the Lionheart of England, Philip II Augustus of France and Emperor Frederick Barbarossa.

We reach one trillion ancestors at the 40th generation, around year 860, soon after the partition of the Carolingian Empire into what would become France and Germany. That's several times the total number of humans who have ever lived. That number has been estimated to be around 117 billion if we go back the the appearance of Homo sapiens some 200,000 years ago.

GenerationsAncestorsTime lapse (min)Time lapse (max)Time lapse (avg)Year (CE)World Population
1216402819522.6 billion
2432805619241.9 billion
38481208418961.6 billion
4166416011218681.3 billion
5328020014018401.1 billion
664962401681812900 to 1000 million
82561283202241756700 to 800 million
101,0241604002801700600 to 1000 million
124,0961924803361644500 to 580 million
138,1922085203641616500 to 600 million
17131,0722726804761504425 to 500 million
212,097,1523368405881392350 to 500 million
2416,777,2163849606721308360 to 500 million
2667,108,86441610407281252± 400 million
28268,435,45644811207841196360 to 500 million
312.14 billion49612408681112300 to 450 million
324.29 billion51212808961084
338.58 billion52813209241056
3417.1 billion54413609521028
3534.3 billion56014009801000250 to 400 million
3668.7 billion57614401008972
37137 billion59214801036944
38274 billion60815201064916225 to 270 million
39549 billion62415601092888
401.1 trillion64016001120860
412.2 trillion65616401148832
424.4 trillion67216801176804220 to 260 million
438.8 trillion68817201204776
4417.6 trillion70417601232748
4535.2 trillion72018001260720
4670.4 trillion73618401288692200 to 225 million
47141 trillion75218801316664
48281 trillion76819201344636
49563 trillion78419601372608200 to 235 million
501130 trillion80020001400580
512250 trillion81620401428552
524500 trillion83220801456524
539010 trillion84821201484496190 to 280 million
567.21E+1689622401568412190 to 210 million
639.22E+18100825201764216190 to 250 million
701.18E+2111202800196020150 to 330 million
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Not only are we likely to descend many times from the same medieval ancestor, it is also likely that there will be big gaps in the number of generations separating us from that ancestor depending on the genealogical pathway that link us to them. For example, according to my very detailed genealogy, I descend over 360,000 times from Charlemagne. He is is my ancestor at the 34th, 35th, 36th, etc. until the 55th generation! It's easy to see how. Charlemagne had five subsequent wives as well as at least four more concubines. I descend from four of his children with two different wives, 8 of his grandchildren and 16 of his great-grandchildren. I don't have the birth year for all of them, but there is already 55 years of difference (the equivalent of about two generations) between the youngest (Rothilde, born in 871) and eldest (Pepin of Vermandois, born c. 817) of the great-grandchildren. And that's only after 3 generations.

Charlemagne and I were born about 1250 years apart. So 34 generations would give an average age of 36.7 years for the parents. At the 55th generation that would be only 22.7 years. Both extremes are indeed possible. The line at the 55th generation would be the eldest, those closest in average to the first-born at each generation. The line that only has 34 generations between us would be have counted mostly younger children - those born the last at each generation, and therefore with the highest average parental age at conception.
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