# Hypothetical number of ancestors in 399 BC

• blarznik
In summary, the conversation discusses the method used to estimate the number of ancestors one may have had in 399 BC. The speaker uses a calculation based on their age and the age of their parents, assuming a mean generation time of 30 years. However, it is mentioned that this number may not be accurate due to duplicated ancestors and the fact that people historically had children at a younger age. The conversation also touches on the concept of distant cousins and the potential for inbreeding.
blarznik
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...

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.

Last edited by a moderator:

## 1. How many ancestors did humans have in 399 BC?

It is impossible to determine the exact number of ancestors humans had in 399 BC. It is estimated that every person has approximately 128 ancestors in the 11th generation, which would mean around 2,097,152 ancestors in 399 BC. However, this number can vary greatly due to factors such as intermarriage and population movements.

## 2. What is the significance of determining the number of ancestors in 399 BC?

Determining the number of ancestors in a specific time period can provide insight into human population dynamics, migration patterns, and genetic diversity. It can also help in understanding the evolution of human societies and cultures.

## 3. How do scientists estimate the number of ancestors in a specific time period?

Scientists use mathematical models and genetic algorithms to estimate the number of ancestors in a given time period. These models take into account factors such as population size, intermarriage, and genetic relationships between individuals.

## 4. Why is it difficult to accurately determine the number of ancestors in ancient times?

In ancient times, records of human populations were not as well-maintained as they are today. Many cultures did not keep written records, making it challenging to trace lineage and accurately determine the number of ancestors. Additionally, intermarriage and population movements make it difficult to accurately track a person's entire ancestry.

## 5. Can the number of ancestors in 399 BC be determined for every individual?

No, it is not possible to determine the number of ancestors for every individual in 399 BC. This is because not all individuals have a complete and documented family tree, and the accuracy of the estimation also depends on the reliability of the available records.

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