Ancestor Paradox

Rmulvany

Apologies if this has been discussed previously, this is my recent fascination and would like to discuss/share.

I have gotten back 5/6 generations on my family tree and while I was thinking over these ancestors I phrased it to myself along the lines of "if I had a time machine and went back to c.1850 I would have 32 people to track down who were my 3x Great-grandparents"

Me - 0
(- 1 Gen / 25 years) Parents - 2
(- 2 Gens / 50 years) Grandparents - 4
(- 3 Gens / 75 years) 1 x G-Grandparents - 8
(- 4 Gens / 100 years) 2 x G-Grandparents - 16
(- 5 Gens / 125 years) 3 x G-Grandparents - 32

This is all based on some presumptions such as:
- A new generation occurs every 25 years
- There are no shared relations/implex

So then I was started thinking that if I went back further again, the ancestors would have doubled per generation and so the further you go back, the more theoretical ancestors you would have.

I looked into it and it's called the genealogical / ancestor paradox.

This is the part that really confused me.
In theory, if you went back 1,000 years that would be roughly my 38x great grandparents, and taking into account how the amount of ancestors doubles every generation you go back, you would have 1,099,511,628,000 (1 Trillion) 38x great-grandparents...

It makes sense on paper but in no way can it be true as that would be more than the earths population...

Anyway... excuse the rant

touts

In theory yes. But 200 years ago never mind 1000 years ago most people lived in small communities and rarely travelled more than 20miles from home. To many a second cousin was alluring and exotic.

Rmulvany

Yeah I know that explains it in reality, the small community and the attractive/unbeknownst cousins etc.

pinkypinky

Pedigree collapse.

Multiple people are your ancestors several times over. We'll likely never be able to prove it.

I have seen it with landed gentry families. In one case a pair of siblings were both a person's ancestor, not because of incest, but because many generations later 2 people married each other and each was separately a descendant of one of the pair.

Ipso

Rmulvany wrote: »

Yeah I know that explains it in reality, the small community and the attractive/unbeknownst cousins etc.

Second cousin, well la de da

Cragaun

If you have roots in rural Ireland and could locate all 32 great great great grandparents I would be willing to bet better than evens that it would comprise less than 32 people. From my own experience of a wide range of trees in the areas of Waterford, Tipperary and Limerick.

The number usually falls in twos, there was not so much remarriage and NPE is hard to find, it surely existed but very hard to find it.

Population bottlenecks such as after the wars and famines of the 1640s and famine of early 1700s would narrowed your available ancestors significantly

My experience is that an average generation would be 33 years or more for rural. People did not marry that young over the range of the two centuries and families were large.
I can't find a number of my GGG grandparents because of late marriage age.

srmf5

Cragaun wrote: »

If you have roots in rural Ireland and could locate all 32 great great great grandparents I would be willing to bet better than evens that it would comprise less than 32 people. From my own experience of a wide range of trees in the areas of Waterford, Tipperary and Limerick.

The number usually falls in twos, there was not so much remarriage and NPE is hard to find, it surely existed but very hard to find it.

Population bottlenecks such as after the wars and famines of the 1640s and famine of early 1700s would narrowed your available ancestors significantly

My experience is that an average generation would be 33 years or more for rural. People did not marry that young over the range of the two centuries and families were large.
I can't find a number of my GGG grandparents because of late marriage age.

My ancestors are from rural and I have 32 different 3x great grandparents. However, in my case, I do have two 4x great grandparents (couple) that both appear twice as 4x great grandparents. I got back one further generation but I don't think that I'll ever be able to prove any other repeats.

ps200306

The human population is extremely inbred. It has to be when you think about it. If all your ancestors were from different lineages you would have an implausible trillion of them going back just forty generations, barely a thousand years.

Endogamy was (and is) commonplace among certain groups. We've all heard the stories about the European monarchy. Alfonso XII of Spain had only four great-grandparents instead of the usual eight. Charles II of Spain was the cousin of his own mother, and the great nephew of his father. Inbreeding encouraged "royal diseases" -- porphyria in the House of Hanover and haemophilia among the descendants of Queen Victoria.

But inbreeding has its benefits too. Tribal societies have long promoted marriage among cousins. If the evolutionary explanation is to be believed, you are more likely to protect those who share more of your genes, resulting in strong tribal bonds.

Genetic studies map how closely related different populations are. By understanding how these genes mutate over time we can estimate how long ago the most recent common ancestor of the entire human race lived. Y-chromosomal DNA maps male lineages, while mitochondrial DNA maps female ones. So we have a male "Y-chromosomal Adam" and a female "mitochondrial Eve".

Some points about this Adam and Eve can be counterintuitive. They weren't married to each other, nor even necessarily lived at the same time. They are also only the most recent common ancestors of all Y-chromosomes and all mitochondrial DNA, respectively. The best guess is that they were around some 200 to 300 thousand years ago.

An easy mistake to make is to suppose that you got all your genes from "Y-chromosomal Adam" and "mitochondrial Eve". That's not true. Apart from the Y chromosome which you get only from your father and only if you are male, your somatic genes are a random assortment from both parents. Each gene independently hitches a ride on the offspring produced. In Richard Dawkins' The Selfish Gene he makes the point that natural selection should not be thought of as selecting individual humans, but individual genes. While your particular set of genes has a common destiny by being hitched to you at this moment in time, each of them has an independent lineage via your offspring and will go on to become more or less prevalent in the population.

If we don't restrict ourselves to individual ancestors, there are some very small so-called founder populations more recently than 200kya. About 1,500 individuals from 40,000 years ago are estimated to be the ancestors of the entire Eurasian population. That is, all your genes are inherited from this group if you are a purebred Eurasian (but you're probably not -- probably nobody is). There is an even more recent common ancestor of all humans which scientists estimate could be as recently as 5,000 years ago. They are not the ancestor of all your genes, like those Eurasian founders, nor the ancestor of all existing Y-chromosomes like the aforementioned Adam. But they are your ancestor through some line of descent.

It seems incredible that someone from just 5,000 years ago (younger than Newgrange) could be the ancestor of all living humans. But as we said at the outset, the population must have been pruned aggressively in every generation as exponential growth in the human population is a recent phenomenon.

pinkypinky

Adam Rutherford's A brief history of everyone who ever lived is a good pop science read on this topic.