About this doubling
Your Ancestors starts from something nobody argues with. You have two parents, four grandparents and eight great-grandparents. Each generation back, the number of places in your family tree doubles.
Thirty doublings is about a billion. With roughly 28 years per generation, thirty generations takes you back about 840 years, to a world of perhaps 400 million people. So somewhere around the 1100s or 1200s, your family tree has more places in it than there were people alive. A few generations later it has more places than the roughly 117 billion humans estimated to have ever been born.
None of that means anything has gone wrong. It means a family tree is the wrong shape.
Pedigree collapse
The answer is that the same people fill many slots. Third cousins marry, fifth cousins marry, people in the same valley marry for centuries, and each time two branches of your tree join. Genealogists call this pedigree collapse. Go back far enough and a single person can sit in thousands or millions of places in your tree at once.
The page shows how extreme this has to be: once the slots pass the population, it works out how many times each person alive at that time would need to appear in your tree, even if every single one of them were your ancestor. In reality many people alive then left no descendants at all, so the real repetition is larger still.
Everyone's shared ancestors
If your tree is folding in on itself, so is everybody else's, and the trees run into each other. In 2004, Douglas Rohde, Steve Olson and Joseph Chang published a model of human ancestry in the journal Nature. It suggested that the most recent person who is an ancestor of everyone alive today probably lived only a few thousand years ago. A few thousand years before that is the identical ancestors point: everyone living then either has no descendants alive today, or is an ancestor of every single person alive today.
That is a model result with real uncertainty, and it is about genealogy, not genes. DNA is passed on in large shuffled chunks, so once you have more ancestors in a generation than you have chunks of DNA, many of your genuine ancestors contributed none of it to you. The page's DNA line gives the average share per ancestor, which quickly becomes a vanishingly small number.
The numbers used
| Figure | Value and source |
|---|---|
| Generation length | 28 years by default, adjustable from 20 to 35. Studies of human generation intervals mostly land between about 25 and 30 years. |
| World population | Mid-range historical estimates, interpolated between dates, with a wide uncertainty band before 1800 |
| Humans ever born | About 117 billion, the Population Reference Bureau's estimate |
| Common ancestors | Rohde, Olson and Chang (2004), Nature 431, 562–566 |
Questions
What is Your Ancestors?
A free interactive page about the arithmetic of family trees. You have two parents, four grandparents and eight great-grandparents, and the number of ancestor slots doubles with every generation. Step back one generation at a time from your own birth year and the page compares the slots with how many people were alive at the time, draws your tree as a fan, and explains why the tree has to fold back on itself.
When does my number of ancestors pass the world population?
With the default of 28 years per generation, the number of slots overtakes the estimated world population about 29 or 30 generations back, somewhere in the 1100s or 1200s for someone born around 2000. It passes the roughly 117 billion people estimated to have ever lived a few generations later. Change the generation length and your birth year to see your own crossover.
So how can that be possible?
It is possible because most slots are filled by the same people over and over. Distant cousins marry, usually without knowing it, and the same ancestor ends up on many branches of your tree. Genealogists call this pedigree collapse. A family tree is really a tangled net, and the further back you go the more tangled it gets.
How long is a generation?
It depends on when and where your family lived and on how old each parent was, which varies a lot. Studies of human generation intervals generally land between about 25 and 30 years, a little longer through fathers than mothers. The page uses 28 years by default and lets you choose anywhere from 20 to 35.
Is everyone alive today related?
Genealogically, very likely yes, and more recently than most people expect. In a 2004 paper in Nature, Douglas Rohde, Steve Olson and Joseph Chang modelled human ancestry and found that the most recent common ancestor of everyone alive today probably lived just a few thousand years ago, and that a few thousand years before that, everyone alive now has exactly the same set of ancestors. These are model results, not a family tree anyone has traced.
Are the population numbers exact?
No. They are mid-range figures from widely used historical estimates, and nobody counted. Before about 1800 the uncertainty is large, and for the ancient world the real number could be half or double the figure shown. The page shows that range, and the doubling overtakes even the high estimate within a generation or two, so the conclusion does not depend on the exact figures.
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