Quick Answer
Studies estimate that you only have to go back around 35 generations or 900 years to be related to every person alive today. This is because your number of ancestors grows exponentially with each generation further back in time. After around 35 generations, the number of ancestors you have surpasses the total world population at the time, meaning ancestors begin repeating and everyone becomes related.
How Many Direct Ancestors Do You Have Each Generation Back?
You have two parents, four grandparents, eight great-grandparents, and so on. The number of ancestors you have in each generation going back doubles as you go back another generation. Here is how it grows:
| Generations Back | Number of Direct Ancestors |
| 1 generation | 2 parents |
| 2 generations | 4 grandparents |
| 3 generations | 8 great-grandparents |
| 4 generations | 16 great-great-grandparents |
| 5 generations | 32 great-great-great-grandparents |
| 10 generations | 1,024 ancestors |
| 20 generations | Over 1 million ancestors |
| 30 generations | Over 1 billion ancestors |
As you can see, the number of direct ancestors you have doubles each generation, growing exponentially.
When Do Your Ancestors Begin Repeating?
Although the number of your ancestors grows exponentially with each generation further back, the actual population of the earth was much smaller in the past. This means that as you go back far enough, your number of ancestors surpasses the population of the earth at the time, and ancestors begin repeating themselves in your family tree.
For example, by 30 generations back (around 900 years ago), you would have over 1 billion direct ancestors just from that generation alone. However, the world population was only around 350 million people at that time. Therefore, many of those billion ancestors must be the same people repeating multiple times in your family tree, because there weren’t enough unique people alive then for each ancestor to be distinct.
This repetition of ancestors is responsible for the fact that everyone is related if you go back far enough. Once ancestors begin repeating multiple times in your tree, those ancestors then connect you with thousands of other people who also have repeats of those same ancestors in their trees.
At What Point Are You Related to Every Person Alive Today?
Several studies have estimated that you only have to go back somewhere between 30-35 generations to be related to every person alive today. This is around 900-1000 years ago.
One study by Joseph Chang at Yale University estimated that you only need to go back about 32 generations or around 800 years for everyone alive today to share at least one common ancestor. Another study estimated that 40 generations or 1000 years was enough time for everyone today to not just share one ancestor, but literally be related to every person alive at the time.
So in summary, scientific estimates state that if you go back just 35 generations or around 900-1000 years ago, you will be related in some way to every single person alive on Earth today. The exponential growth in your ancestors combines with the repetition of ancestors due to small historical populations to create these surprisingly short time frames for universal relatedness.
Visualizing how Many Generations Back You Need to be Related to Everyone
Here is a helpful visualization of how many generations back you need to go for your ancestors to surpass the world population and make everyone related:
In this visualization, you can see the exponential growth in direct ancestors (blue line) versus the relatively flat world population (green line) over the generations. Within 30-35 generations, the blue ancestry line surpasses the green population line, meaning ancestors begin repeating and connecting everyone as relatives.
Is It Possible to Actually Trace Your Family Tree Back This Far?
While the math shows that you only need to go back around 35 generations to be related to everyone, it would be incredibly difficult to actually document a family tree going back that far with detailed records proving the connections.
Some of the challenges include:
- Written family records tend to only go back a few hundred years at most.
- It’s impossible to know for sure that your records connect accurately over hundreds of generations back to specific ancestors who lived over a thousand years ago.
- Genetic relationships get diluted after many generations as DNA is mixed and matched in unpredictable ways.
- Marriage and children were not always well documented historically, making lines difficult to trace.
- Wars, migrations, and other events mean records do not exist tracing most family lines back more than a few hundred years.
While you can find some noble families (like royalty) that claim to trace their genealogies back hundreds of years, these lines often have gaps or inaccurate connections when examined closely. And for most everyday people, documented family trees do not extend more than a few centuries back, if that.
So while mathematically we are all related if you go back 35 generations, actually proving a specific family connection that far back to every person would likely be impossible with the limited historical records available. Genealogists consider a well-documented family tree going back 15-20 generations to be quite an impressive feat.
Genetic Genealogy Offers Some Insights Into Distant Ancestry
Although documented paper records may not be able to trace your ancestors back over a thousand years, new DNA analysis techniques allow glimpses further back in time.
Services like 23andMe can analyze your DNA to look for genetic matches with other people to estimate relationships as distant as fifth cousins or farther. This allows connections to ancestors from hundreds of years ago who you likely will not find in any written genealogy record.
Y-DNA and mtDNA testing look at genetic markers passed down unchanged for many generations along paternal and maternal lines. They can identify ancient genetic lineages and connections to geographic ancestral groups from thousands of years ago.
While this DNA evidence does not prove documented lines of descent like a paper family tree, it provides some insights into our distant genetic kinship and ancestry not found in written records. Ancient DNA evidence supports mathematical models showing that we are all distant cousins related if you go back far enough in time.
Conclusion
Mathematical models show that we are all related to one another if you go back roughly 35 generations or around 900-1000 years. This is because of the exponential growth in direct ancestors versus the relatively small historical world populations. While paper genealogies may not document specific family connections back that far, DNA evidence confirms our shared genetic kinship and descent from common ancestors if you go back far enough. So while it may be difficult to trace your family tree to every specific ancestor lived a thousand years ago, the math says we are definitely all cousins!
