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Dorocche t1_j1v2q61 wrote

There's too many variables to answer universally. It would vary wildly depending on the individual you started with.

Edit: See /u/Tidorith's comment below, the rest of what I'm saying here isn't necessarily relevant.

For what it's worth, the point where unique ancestors would outnumber the population is precisely 30 generations. Whereas if we limited it to just the UK, it would be a number in the low twenties. So the possible variance here isn't dozens of generations, but more like fives.

So probably around 15-20 generations back? But again, it's impossible to give a universal answer.

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lynmc5 t1_j1v9yrk wrote

Given the propensity of people to stay near where they were born and also the propensity of people to marry within social circles, the "expectation" of the number of generations back for every ancestor being unique is probably quite small.

2**15 = 32,768, 15*20 years/generation = 300 years. So 300 years ago, if your community of eligible ancestors was 32,768 or more, each one could be unique. I guess that's not unreasonable depending where they lived, but it doesn't seem likely.

2**20 = 1,048,576, 20*20 years/generation = 400 years. It seems unlikely to me that your community of eligible ancestors 400 years ago would be over 1 million.

Anyway, that's my uneducated guess.

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WilliamMorris420 t1_j1vdlat wrote

There was one teacher, in Somerset, England. Whose relatives have moved about 0.5 miles, in 9,000 years.

https://www.thearchaeologist.org/blog/mesolithic-skeleton-known-as-cheddar-man-shares-the-same-dna-with-english-teacher-of-history

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Octavus t1_j1wd7i8 wrote

Cheddar Man lived before the human isopoint, if he has one living descendant then every single person on the planet is also his descendant. At some point 7,300 to 5,300 years ago if someone had a living descendant, then all of humanity is their descendant.

https://www.scientificamerican.com/article/humans-are-all-more-closely-related-than-we-commonly-think/

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WilliamMorris420 t1_j1wef86 wrote

So why is he the only one, noted as a descendant and not everybody else?

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Octavus t1_j1wgus9 wrote

He has the same mitochondrial DNA haplotype as Cheddar Man, which isn't passed down by males so isn't actually evidence at all that he is a descendant of Cheddar Man. Only that the share the same female ancestor.

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Minniechild t1_j1x4a8f wrote

I would suggest because he lives within walking distance of where Cheddar Man’s remains were found, and also due to the similarities in their faces which make for a nice Personal Interest Piece

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WilliamMorris420 t1_j1x7xdb wrote

They already knew that he was a descendant or at least related. The visual reconstruction, came after the DNA tests.

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Frozen_Watcher t1_j1x4fy6 wrote

This is an optimistic estimate that casually ignores physical/cultural barrier and lack of movements. This estimate only applies if people move around often and for a long distance to leave descendants around a big area all over the world like in modern world which isnt really applicable to ancient past. I seriously doubt some native american living at that time is a common ancestor of most people living in eurasia right now.

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thephoton t1_j20ak83 wrote

> At some point 7,300 to 5,300 years ago if someone had a living descendant, then all of humanity is their descendant.

Weren't, for example, Native Americans, isolated from Europe for more than 7,300 years?

So if you consider someone living in the Andes with pure Native American ancestry, how are they descended from Cheddar Man?

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Octavus t1_j20c6b6 wrote

There isn't believed to be anyone left in the Americas or Tasmania who does not have any European ancestry from the last 500 years.

Going the other direction Paleo Eskimo bridged the gap for a while between the Americas and Asia. Their culture spanned from Russia through Alaska into Greenland.

There was a continuous but some gene flow between Australia and South East Asia. Any other isolated groups of humans have only been isolated for a few hundred years.

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thephoton t1_j24ijax wrote

> There isn't believed to be anyone left in the Americas or Tasmania who does not have any European ancestry from the last 500 years.

OK, but take the Andean's great-to-the-nth grandmother from 7300 years ago (one of the ones who lived in the same region all those centuries ago). Is that grandmother also an ancestor of the teacher in Somerset? And of some villager in a remote village in Tibet?

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Octavus t1_j24z5w5 wrote

Their ancestry would spread to Alaska present day Alaska on only a few hundred years. Paleo Eskimo, who lives from Russia through Alaska into Greenland. They acted as the bridge between the old and new worlds 4,500 and 1,500 years ago.

The world has been much more interconnected than what most would believe. It takes only one person after complete mixing to spread an entire continent of ancestry. Do not underestimate just how much mixing occurs in 1,000 years, that is enough time to completely mix all of Europe.

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Additional-Fee1780 t1_j25u0tt wrote

That’s not true. Australian aborigines have been isolated for something like 50 ky.

EDIT: this is now known untrue. Thanks /u/Octavus!

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Octavus t1_j25ymfi wrote

They have not been completely isolated for 50,000 years, there has been several periods of limited contact.

The most significant is ~10,000 years ago was when Australia was finally culturally split from New Guinea, there is also linguistic evidence as 90% of Australian languages are within the same family and split only a few thousand years ago. However this is before the isopoint so not related.

What is important is genetic and trade evidence between India, South East Asia, and the northwest cost of Australia. This trade and gene flow occurred ~4,300 years and gave enough time for Australia and Tasmania to become completely mixed in the 1,000-3,000 years before the contact.

This is technically only evidence of India -> Australia but the evidence points towards continue contact and not a one off event. Continued contact points to the people returning from Australia to the homelands which allows for gene flow the other direction. It simply takes one person to make the trip and have descendants.

The dingo has only been in Australia for 4,000-10,000 years. If Australians have been isolated for 50,000 years where did this non-native animal come from?

Genome-wide data substantiate Holocene gene flow from India to Australia

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swami78 t1_j1xsoic wrote

Many years ago I was contacted by a German researcher who told me I am a descendant of 40 odd skeletons found in the Lichenstein Hole (a family tomb in a sealed cave) dating from 2500 years ago. They found another descendant less than 5kms from that cave. They were thought to have been Visigoths or Frisians.

I have read that some 60% of English people can find Edward the Confessor in their lineage - and even slightly more French can find Charlemagne in theirs. We're all related anyway - Mitochondrial Eve.

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WilliamMorris420 t1_j1xuwpj wrote

What you find is that as you go back, that you have so many great, great, great.... grandparents. So Edward was born 1020 years ago, which is roughly 40 generations. In that time you "should" have had 1 trillion ancestors. Which is about 10 times more people who have ever lived. Because the same people keep turning up on different branches of your family tree. Especially when you get to about the tenth generation.

Essentially we have so many ancestors that as long you have children and they have children. Your line will continue indefinetly and you'll have millions of descendants.

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sagramore t1_j1yjfdh wrote

Isn't what you've described here exactly what OP's original question states and asks about?

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swami78 t1_j21f2bs wrote

I was hoping someone would do the math! Thanks.

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PicardTangoAlpha t1_j1vqya6 wrote

Whats the widest it can be in a place like North America with large influxes from overseas?

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guynamedjames t1_j1w0dsr wrote

You'll get your widest from interracial households, especially when you get multiple generations of interracial mixing. So your African grandma, dutch grandpa, Chinese grandma and Indian grandpa type families are realistically going to be very far apart generically. And within that, historically people from cities will have more diverse genes than those from the country

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MetricJester t1_j1xr8dq wrote

Also: three of those grandparents could also be children of Genghis Khan.

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Artanthos t1_j1xbtka wrote

If you are talking a small island (Guam) it would be much lower numbers.

If you are talking about Hollars in WV, it might only be 3-4 generations. The Hollar I am familiar with had 2nd cousins marrying and only about a dozen families in total.

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lynmc5 t1_j1xdjit wrote

On the other hand, if you don't require every ancestor to be "unique", that is, not relatives by ancestry, but just want to know the number of unique persons who are ancestors, my simple formula isn't very helpful. If cousins marry and produce a grandchild, that grandchild has 2**3 - 2 great-grandparents instead of 2**3.

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Rolldal t1_j1yrojg wrote

It largely depends on when. During the middle ages under the feudal system it was difficult for all but itinerant workers (tinkers, travelling merchants, etc.) to move but as the feudal system collapsed this changed. During the English civil war many ordinary people moved about, travelled to the new world etc. Also often occupation dictated movement. My Potter ancestors generally stayed put but my miner ancestors moved as mines ran out of coal, lead, tin or whatever they happend to be mining.

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A_Notion_to_Motion t1_j1yeorb wrote

This stuff is hard for me to wrap my head around tbh. It doesn't help that I now have that family tree song stuck in my head for some reason...

"I'm my own grandpaaaaa!!!"

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spunkmobile t1_j1x3jzh wrote

How would a couple of generations of inbreeding effect the precise 30 gens statistic, does it go up precisely by some amount?

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Jedi_Emperor t1_j1yih8y wrote

Plus even the most die-hard English isolationist traditionalist "we're more English than longbows" are bound to have at least a couple of ancestors from mainland Europe once you're ten generations back.

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[deleted] t1_j1veun0 wrote

[removed]

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Dorocche t1_j1vf655 wrote

My math put 30 generations back at c. 1400 AD. Yet another reason why the general case of this question isn't answerable.

Edit: The math can very quite a lot, but 5000 BC would require ~230-year generations.

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EvanDaniel t1_j1vfz0j wrote

Wow, I definitely misread an extra zero somehow. Thanks.

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Tidorith t1_j25ti7w wrote

>For what it's worth, the point where unique ancestors would outnumber the population is precisely 30 generations.

I don't think it's necessarily safe to use this as a lower bound for the posed question in terms of how far back we'll be going. Obviously the max number of unique ancestors alive at the same time must be lower than 2^30 (*Edit, maybe not technically, but probably in practice on Earth today - see my edit below), but it does not follow from that that the widest generation must be less that 30 generations ago. What happens instead is that very quickly as you go back generations, the number of additional ancestors in the next generation back starts to grow very slowly, and then in fits and starts as you have individual ancestors who migrated a significant distance.

The more determinative fact will be the distribution of the size the population over time - both the global ones, and the sizes of the populations to which you are likely to be genetically linked. Populations that have grown significantly in the somewhat recent past (but not too recently) will have the max number of ancestors being very recent, because you can hit a number of ancestors alive at the same time that outnumber the people that actually lived in early generations in that population.

But for sparsely interconnected populations that have had stable sizes for centuries/millennia, you could expect to very slowly attain small numbers of new ancestors each generation for well over 30 generations, if the growth rate is low enough.

Edit: Come to think of it, perhaps more important is that the number of unique ancestors in generation X back from yourself can actually outnumber the people who were alive at the same time as any given one of those ancestors. Go back far enough and you'll have ancestors from different parts of your family tree in the same ancestral generation who lived centuries apart. The further back you go, the more pronounced this effect becomes.

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