What is the ‘widest’ ancestral generation?

Submitted by vesuvisian t3_zwg75b in askscience

Each generation back, the number of individuals doubles (two parents, four grandparents, etc.), but eventually, the same individuals start to appear in multiple parts of your family tree, since otherwise you’d be exceeding the population of the world. So the number of unique individuals in each generation grows at first before eventually shrinking. How many unique individuals can we expect in the ‘widest’ generation?

Edit: I’ve found the topic of pedigree collapse, which is relevant to my question.

Edit 2: Here's an old blog post which provides one example of an answer. For a typical English child born in 1947, "the maximum number of “real” ancestors occurs around 1200 AD — 2 million, some 80 percent of the population of England." Here's another post that delves into the concept more. England is more isolated than mainland Europe or elsewhere in the world, so it'd be interesting if these calculations have been done for other places.

463

Comments

You must log in or register to comment.

Uncynical_Diogenes t1_j1v10qx wrote

I think the problem lies with your model/question. You’re taking it for granted that the math is 2^n , because that works for a couple generations at a time that a human can hold in their brain at once. I think that’s leading you astray. The concept of “generations” is also tenuous and mostly only works for a limited number of generations of specific individual ancestors of one specific single organism you’re looking at. As boomer/millennial discourse has proven, generations are not actually, like, a thing, they’re just these constructs we use to explain things. What makes sense to describe a 30yr period in your own life as you relate to your parents and children does not work very well for describing a 300yr period where the timing of births is all over the place.

When you compare two separate peoples’ family trees, they don’t align neatly, you just get a forest. It’s not like the human population just iterates forwards as a group every so many ticks like in Conway’s Game of Life.

You can perform this simple check on your model: If the population has grown, that means at any instant, the “moment of birth rate”, if you will, will on average be positive. There are more babies being born than people dying. How then can the number get larger as you go back? There were always fewer people each year back. We know that because there are always more people each year forward.

−10

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.

242

nicolasknight t1_j1v2sie wrote

I am assuming you mean for an individual not for humanity as a whole.

That's actually a really tough question and is going to be different for each person depending on their ancestry and how often they moved and/or married (Being polite) people from vastly different populations.
based on migration patterns and this I would say you are probably looking at an increase, statistically, every time there was a big colonization push and/or a new travel method became popular enough jack and Jill Average could use it to go somewhere and live for a while.

So based on this I would say 17th Century going forward with a hard stop in 1914.

25

lynmc5 t1_j1v9yrk wrote

Replying to Dorocche (#1,094,822)

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.

74

lookmeat t1_j1vds4s wrote

In theory it's around the area that the generation side spreads to larger than the population, in practice this won't matter much since the size of disconnected populations that su but reach a branch is tiny compared to the greater world, unless you happen to be a member of that specific branch. The other factor is that incest probably covers a good chunk of that tree, which child reduce how quickly your tree widens (that is there could be multiple people in your tree that only have two grandparents) so technically our generation exponentiation is not 2, but some slightly smaller number (the average). It probably is very close to 2 though, so it should be a good enough estimate. Another factor that we're not considering is that not everyone reproduced, and that we really want the population that reproduced, but that's hard to measure and the number is probably pretty close to the total population.

So you find the average gap between generations, lets say 17 years (consider that for the greatest amount of time people got married at 13 and it was common to have had most of your children before 20). So it's mapping the population in year *x* vs *((YOB-x)/17)^2* where *YOB* is the year you where born. The x` is almost certainly with an error of 10 years, though it could easily be a century before you even hit the millennia. I am sure you won't go to BCE, and I'd be surprised if the year was any time before 500CE.

A more interesting question is: assuming modern family conventions, and generally full population mix, how long would it take until someone is probably a descendants of everyone that reproduced?

−6

Dorocche t1_j1vf655 wrote

Replying to [deleted] (#1,096,586)

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.

2

guynamedjames t1_j1w0dsr wrote

Replying to PicardTangoAlpha (#1,098,242)

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

24

SpezPoop t1_j1wbcbs wrote

Oddly, I subscribe to many local photographer pages. The girls from different areas of the US usually have a few distinct features common to many of the other local models of each area of the US the photographer has access to. I speak with one regularly and I can almost always point out which models of his don't have much family history in an area.

4

Rannie333 t1_j1wd81p wrote

Yes, there are too many variables. One, after WWII, lot of people got married and had babies, food, safety net. Two, during power outs, lots of couples made love back a few decades ago when technology was not at it's best as today, and ended up having babies. Three, pandemics, like Covid-19 took away some lives, as did war in some countries. Four, medical research prolonged some lives, as in the past it didn't because the research was not there. Too many variables. I, like you, would like to put it into some kind of statistic, or whatever you call it. I lack the proper terminology.

−7

Octavus t1_j1wgus9 wrote

Replying to WilliamMorris420 (#1,101,446)

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.

24

Frozen_Watcher t1_j1x4fy6 wrote

Replying to Octavus (#1,101,285)

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.

15

Artanthos t1_j1xbtka wrote

Replying to lynmc5 (#1,095,931)

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.

7

lynmc5 t1_j1xdjit wrote

Replying to lynmc5 (#1,095,931)

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.

3

brad_l_taylor t1_j1xj8zi wrote

This may not be relevant, but I once calculated that at the 7th generation there is only a 50% chance of inheriting DNA from an ancestors. This is because DNA breaks are chunky and at a certain point you can just lose all the DNA from an ancestor . So when you go back you are actually only related to a subset of your ancestors

Interestingly 7 generations is also the max number of generations in a human lifetime for most people

  • Great Grandpa
  • Grandpa
  • Father
  • Me
  • Daughter
  • Granddaughter
  • Great granddaughter

I've always wondered if this is fine tuned by our DNA crossings

26

swami78 t1_j1xsoic wrote

Replying to WilliamMorris420 (#1,096,400)

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.

14

WilliamMorris420 t1_j1xuwpj wrote

Replying to swami78 (#1,108,263)

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.

3

Spanks79 t1_j1ydoj3 wrote

In the end we all descend from just a few women (mitochondrial eve)

https://en.m.wikipedia.org/wiki/Mitochondrial_Eve

And a handful of men (y-chromosomal Adam)

https://en.m.wikipedia.org/wiki/Y-chromosomal_Adam

There was a period humans were almost extinct and of those only a few people are responsible for all of our genetic material.

Even so: mitochondrial dna might come from ‘eve’ , some of the other humans or even humanoids (like Neanderthals) might have given us part of their dna by breeding. It’s just mixed into the y- or mitochondrial lines.

Still there are many factors of which I know only a few. Must important would be distance / travel. Micronesian people might share a Europe’s and African forefather, but after they separated ages ago they didn’t breed back in. Same for native Americans or Australian natives.

Still it’s very interesting to see there’s still a lot of things being found out. For instance that denisova man interbred with early humans and Neanderthals and many Asian people have some denisova dna and Europeans/Caucasian have up to 5% Neanderthal dna.

2

Jedi_Emperor t1_j1yih8y wrote

Replying to Dorocche (#1,094,822)

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.

4

Rolldal t1_j1yrojg wrote

Replying to lynmc5 (#1,095,931)

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.

2

thephoton t1_j20ak83 wrote

Replying to Octavus (#1,101,285)

> 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?

2

Octavus t1_j20c6b6 wrote

Replying to thephoton (#1,123,324)

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.

0

thephoton t1_j24ijax wrote

Replying to Octavus (#1,123,618)

> 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?

1

Octavus t1_j24z5w5 wrote

Replying to thephoton (#1,149,536)

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.

1

Tidorith t1_j25ti7w wrote

Replying to Dorocche (#1,094,822)

>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.

2

Spanks79 t1_j25uy96 wrote

Replying to Additional-Fee1780 (#1,159,112)

Yes, as I state in my post other DNA might still be mixed in into the lines. As the egg organelles come from eve, there might be much different dna in the chromosomes.

Same for the male thing.

Actually there’s many humans with Neanderthal and Denisova dna. Interbreeding took place and introduced different dna.

1

Octavus t1_j25ymfi wrote

Replying to Additional-Fee1780 (#1,159,001)

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

1