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

[Maciamo]

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.

Generations	Ancestors	Time lapse (min)	Time lapse (max)	Time lapse (avg)	Year (CE)	World Population
1	2	16	40	28	1952	2.6 billion
2	4	32	80	56	1924	1.9 billion
3	8	48	120	84	1896	1.6 billion
4	16	64	160	112	1868	1.3 billion
5	32	80	200	140	1840	1.1 billion
6	64	96	240	168	1812	900 to 1000 million
7	128	112	280	196	1784
8	256	128	320	224	1756	700 to 800 million
9	512	144	360	252	1728
10	1,024	160	400	280	1700	600 to 1000 million
11	2,048	176	440	308	1672
12	4,096	192	480	336	1644	500 to 580 million
13	8,192	208	520	364	1616	500 to 600 million
14	16,384	224	560	392	1588
15	32,768	240	600	420	1560
16	65,536	256	640	448	1532
17	131,072	272	680	476	1504	425 to 500 million
18	262,144	288	720	504	1476
19	524,288	304	760	532	1448
20	1,048,576	320	800	560	1420
21	2,097,152	336	840	588	1392	350 to 500 million
22	4,194,304	352	880	616	1364
23	8,388,608	368	920	644	1336
24	16,777,216	384	960	672	1308	360 to 500 million
25	33,554,432	400	1000	700	1280
26	67,108,864	416	1040	728	1252	± 400 million
27	134,217,728	432	1080	756	1224
28	268,435,456	448	1120	784	1196	360 to 500 million
29	536,870,912	464	1160	812	1168
30	1,073,741,824	480	1200	840	1140
31	2.14 billion	496	1240	868	1112	300 to 450 million
32	4.29 billion	512	1280	896	1084
33	8.58 billion	528	1320	924	1056
34	17.1 billion	544	1360	952	1028
35	34.3 billion	560	1400	980	1000	250 to 400 million
36	68.7 billion	576	1440	1008	972
37	137 billion	592	1480	1036	944
38	274 billion	608	1520	1064	916	225 to 270 million
39	549 billion	624	1560	1092	888
40	1.1 trillion	640	1600	1120	860
41	2.2 trillion	656	1640	1148	832
42	4.4 trillion	672	1680	1176	804	220 to 260 million
43	8.8 trillion	688	1720	1204	776
44	17.6 trillion	704	1760	1232	748
45	35.2 trillion	720	1800	1260	720
46	70.4 trillion	736	1840	1288	692	200 to 225 million
47	141 trillion	752	1880	1316	664
48	281 trillion	768	1920	1344	636
49	563 trillion	784	1960	1372	608	200 to 235 million
50	1130 trillion	800	2000	1400	580
51	2250 trillion	816	2040	1428	552
52	4500 trillion	832	2080	1456	524
53	9010 trillion	848	2120	1484	496	190 to 280 million
54	1.80E+16	864	2160	1512	468
55	3.60E+16	880	2200	1540	440
56	7.21E+16	896	2240	1568	412	190 to 210 million
57	1.44E+17	912	2280	1596	384
58	2.88E+17	928	2320	1624	356
59	5.76E+17	944	2360	1652	328
60	1.15E+18	960	2400	1680	300
61	2.31E+18	976	2440	1708	272
62	4.61E+18	992	2480	1736	244
63	9.22E+18	1008	2520	1764	216	190 to 250 million
64	1.84E+19	1024	2560	1792	188
65	3.69E+19	1040	2600	1820	160
66	7.38E+19	1056	2640	1848	132
67	1.48E+20	1072	2680	1876	104
68	2.95E+20	1088	2720	1904	76
69	5.90E+20	1104	2760	1932	48
70	1.18E+21	1120	2800	1960	20	150 to 330 million

 

[Maciamo]

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.