# billions of ancestors **URL:** \ **Category:** Factual Questions\ **Created:** [June 16, 2016, 3:40am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495 "2016-06-16T03:40:07Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Vicsage](https://avatars.discourse-cdn.com/v4/letter/v/e68b1a/32.png) [@Vicsage](https://boards.straightdope.com/u/Vicsage)\ **Post date:** [June 16, 2016, 3:40am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/1 "2016-06-16T03:40:07Z")
I have 2 parents. I have 4 grandparents. 8 great grandparents. If I go back 30 generations, I have millions of ancestors. 40 generations billions if not trillions. 50 generations, its astronomical. Why is this not possible? ---
**Author:** ![spamforbrains](https://avatars.discourse-cdn.com/v4/letter/s/e274bd/32.png) [@spamforbrains](https://boards.straightdope.com/u/spamforbrains)\ **Post date:** [June 16, 2016, 3:43am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/2 "2016-06-16T03:43:57Z")
Because if you go back far enough (not very far) they are all the same people? Plus humanity hasn’t been around all that long and there are very limited number of generations one can go back to. ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [June 16, 2016, 3:45am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/3 "2016-06-16T03:45:47Z")
They mix. To begin with, lots of people have siblings, so you and your brothers and sisters all have the same great-grandparents. But, seriously, they really start to mix it up going back a few generations. Your great-great-great-grandparents might also easily be my ggggps. There’s no reason for each of us to have his own unique set. Toss in divorces and remarriages, extra-marital affairs, premarital affairs, rape…and then toss in adoption and step-parentage…and the tree looks more like a bramble thicket. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [June 16, 2016, 3:50am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/4 "2016-06-16T03:50:35Z")
This is known as [pedigree collapse.](https://en.wikipedia.org/wiki/Pedigree_collapse) Going back 1000 years, pretty much everyone with European ancestry will have the same set of ancestors. ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [June 16, 2016, 4:07am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/5 "2016-06-16T04:07:32Z")
Yeah, sorta like / / with you at the bottom. ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [June 16, 2016, 4:10am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/6 "2016-06-16T04:10:19Z")
[The Master Speaks!](http://www.straightdope.com/columns/read/412/2-4-8-16-how-can-you-always-have-more-ancestors-as-you-go-back-in-time) ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [June 16, 2016, 4:11am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/7 "2016-06-16T04:11:03Z")
> [@Chefguy](#): > > Yeah, sorta like > > / > / > > with you at the bottom. According to my link (and to Cecil), most people have their maximum number of ancestors at around 1200 AD. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [June 16, 2016, 4:26am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/8 "2016-06-16T04:26:30Z")
pdf of research paper here: [Modelling the recent common ancestry of all living humans (Rohde, Olson & Chang)](http://www.stat.yale.edu/~jtc5/papers/CommonAncestors/NatureCommonAncestors-Article.pdf) and a summary of that paper here: [Pedigrees for all humanity](http://people.sc.fsu.edu/~pbeerli/bsc5932/Rohde-et-al.pdf) The most recent person who was one ancestor of all living humans (MRCA) probably lived around 76 generations ago. At around 169 generations ago we all have exactly the same _set_ of ancestors: about 80% of people then alive were the ancestors of us all, and the other 20% were the ancestors of noone (their lines died out). ---
**Author:** ![Just\_Asking\_Questions](https://avatars.discourse-cdn.com/v4/letter/j/ba9def/32.png) [@Just\_Asking\_Questions](https://boards.straightdope.com/u/Just_Asking_Questions)\ **Post date:** [June 16, 2016, 6:24am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/9 "2016-06-16T06:24:35Z")
Since the question has been answered… You are all wrong! the fact is, the population of the earth in 2000BC was 75 billion, and in 5000BC was 1 trillion. There has been a huge population implosion. At this rate, there will be only one person alive by the year 2200. ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [June 16, 2016, 10:02am UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/10 "2016-06-16T10:02:14Z")
I blame the Flood. ---
**Author:** ![Chihuahua](https://avatars.discourse-cdn.com/v4/letter/c/dec6dc/32.png) [@Chihuahua](https://boards.straightdope.com/u/Chihuahua)\ **Post date:** [June 16, 2016, 12:32pm UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/11 "2016-06-16T12:32:03Z")
Cecil’s last point is something that always had me scratching my head. I hear someone say, “X is a descendant of Y famous person…” and I’m like, “Well, mathematically speaking, there’s a darn good chance all of us are.” ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [June 16, 2016, 1:15pm UTC](https://boards.straightdope.com/t/billions-of-ancestors/757495/12 "2016-06-16T13:15:20Z")
> [@Colibri](#): > > According to my link (and to Cecil), most people have their maximum number of ancestors at around 1200 AD. Right. It’s just a simple graphic; the top portion would be much, much larger.