Hypothetical number of ancestors in 399 BC

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Discussion Overview

The discussion revolves around estimating the number of ancestors a person might have had around the time of Socrates (399 BC) using a hypothetical model based on generational intervals. The scope includes mathematical reasoning and conceptual exploration of ancestry and population dynamics over time.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant outlines a method to calculate the number of ancestors by assuming a consistent generational interval of 30 years, leading to a calculation of 2^79 ancestors by 399 BC.
  • Another participant acknowledges the arithmetic but notes that there would be significant duplication among ancestors, suggesting that many of the calculated ancestors are not distinct individuals.
  • A third participant agrees with the calculation of 2^79 but emphasizes the importance of recognizing common ancestry, indicating that many ancestors may overlap due to familial connections.
  • Some participants point out that while 30 years may be a reasonable estimate for modern times, historical averages for childbirth were often lower, around 17-20 years.
  • One participant proposes a thought experiment where if everyone today performed the same calculation, the total number of ancestors in 399 BC would be around 2^111, raising questions about the implications of such a number.
  • A later reply references the complexity of familial relationships, suggesting that cousin and uncle/nephew connections may add to the confusion in understanding ancestry calculations.

Areas of Agreement / Disagreement

Participants generally agree on the arithmetic involved in the calculations but express differing views on the implications of the results, particularly regarding the uniqueness of ancestors and historical generational intervals. The discussion remains unresolved regarding the accuracy and implications of the model used.

Contextual Notes

Limitations include assumptions about generational intervals, potential overlaps in ancestry, and the historical context of childbirth ages, which may affect the accuracy of the calculations.

blarznik
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I was trying to get some idea of how many ancestors I had around the time of Socrates (399 BC is his death), so I tried coming up with a way to find it.

So I'll start at my birthday which is 1991. I'll pretend that my parents were both 30 when they had me; I'll also pretend every pair in history giving birth was 30 years old when giving birth.

I'll use intervals of 30 years going all way the way back to 399 BC from 1991, which is a 2390 year gap.

There are 79 intervals of thirty years that fit with in 2390 years gap.

So I came up with 2^79= 6.044*10^23
This can't be right?! This is basically Avogadro's number
I used 2^x to find the number of the ancestors at the time of 1781 (5 years after the declaration of independence) which at 7 generation was 128 people. And this seemed right.

I'm not adding the ancestors from previous intervals as I go along, each interval is just the number of people related to me at that particular time.

Am I doing anything wrong?
at birth...
# of present parents (1961)
2^1=2
# of grandparents (1931)
2^2=4
# of great grandparents (1901)
2^3=8...
 
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Your arithmetic is OK. However, once you go back enough generations there is a lot of duplication. For example 10 generations back, there are over a thousand ancestors, but most likely nowhere near a thousand different people.
 
Well, the number 2^79 is (about) correct, with one absolutely huge correction: those people are not all distinct. For instance, your mother's father's father's mother's ... father's mother may well have been the same person as your father's mother's mother's father's ... mother's mother. We are all distant cousins of each other -- we all have common ancestors.
 
Although 30 may be normal today, historically people usually had babies around 17-20.
 
KingNothing said:
Although 30 may be normal today, historically people usually had babies around 17-20.
Historically people started to have babies around 17-20. 30 is not a bad guess for mean generation time.
 
You've calculated how many ancestors you had in 399 BC. Now suppose all 6 billion people on Earth now did the same calculation. The total number of ancestors in 399 BC would be roughly 2^79 * 2^32 = 2^111.

What happened to everybody?
 
Good point. http://www.aisee.com/graph_of_the_month/inbred.gif
I guess it gets confusing when cousin or uncle/nephew relationships happen.
 
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