How many ancestors did I have 1000 years ago?

Powers2Someone recently posed the question “How many ancestors did I have 1000 years ago?” in conjunction with the assumption that the various genetic origins programs include about 1000 years worth of ancestry … my answer, of the billion possible maybe a million, maybe far fewer …

Very simply, if you postulate that 1000 years was 30 generations ago then your theoretical number of ancestors is two to the 30th, or just over a billion: 1,073,741,824.This is impossible as nowhere near that many people were alive back then. Plus not everyone who lived a thousand years ago has descendants today. So your ancestors must be duplicated numerous times on your family tree; this is known as pedigree collapse. Brian Pears points out in his article The Ancestor Paradox, that “even if every marriage in every generation was between second cousins, a quite unbelievable situation, we would still run out of people to be our ancestors within 29 generations.”

Kenneth W. Wachter came up with an interesting mathematical model for this, described in Stephen Lewis’ blog post How many ancestors do I have. To somewhat paraphrase, “Going back 30 generations… Wachter’s model calculates that [an Englishman] would have 952,279 distinct ancestors in 1077 – only around 0.09% of the maximum but representing fully 86% of the total estimated English population [at that time] of 1.1 million.”

More recently (1999), Yale statistician Joseph Chang wrote a paper analyzing pedigree collapse that postulated that we Europeans all have a common ancestor who lived in about 1400 AD. Warning, that paper has lots of math in it.

I am not so convinced of these generation estimates however, since in my own tree my ancestors living in about 1000 AD are only my 25th  grandparents; so 1000 years is 27 generations for me rather than 30. But then I am descended from many a youngest child. Click here for my relationship path to a Norwegian 25th grandfather who lived in the year 1000, the grandson of King Harald the Finehair, and another 25th grandparent in France from that same time period.

Here are a few very readable articles about the number of ancestors we might have had 1000 years ago:

In tracing my Norwegian side, I have found much evidence of pedigree collapse among my Etne, Hordaland ancestors, many of them starting to repeat at 7-9 generations back. The DNA shared with other folk from Etne almost always makes the relationships look closer than they are, when in fact there are multiple common ancestors, often on the same lines. Farms Ve, Fatland, Frette, and the Koll (Skiftun) clan keep reappearing in the tree. Plus the Rieber and Galtung family which traces back to royalty, of course.

15 thoughts on “How many ancestors did I have 1000 years ago?

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  1. This of course depends on the nature of your pedigree.

    To take a trivial example, if your father’s parents are Swedish and Nigerian, while your mother’s parents are Chinese and Arab, you will have many more ancestors than if all your grandparents come from one stock.

  2. Darel on the DNA-NEWBIEs list had this to say: “To put some perspective on the number of ancestors 1000 years ago being nearly 1 billion or 500 million pairs (husband & wife), estimates for the total human population in Europe is around 40 million people in 1000 AD and total world population is estimated to have been between 300 or 400 million. Unless those estimates are WAY off, the “numbers” of ancestors may be around a billion, but more likely far less than that if you filter duplicate names….The family tree is actually shaped more like a diamond or one of those old lava lamps?”

  3. And as Israel Pickholtz says, the curve goes up faster or slower depending on the population(s) your ancestors belong to. Eventually we all get to the identical ancestors’ point, but some more isolated lines will merge with the rest of our common ancestral population when the global population was lower.

  4. “I am not so convinced of these generation estimates however, since in my own tree my ancestors living in about 1000 AD are only my 25th grandparents; so 1000 years is 27 generations for me rather than 30.”

    This is only one of millions of paths of your direct ancestors. 1 path is 27 generations, there may be thousands more paths that are 35 generations. To say your ancestors 1000 years ago is only 27 generations for you is nonsense and defeats the object of this article.

  5. Whilst the figures appear logical and mathematical I dont believe them, back track maybe 5 generations but no more. As an aside your back at the tenth generation… with 1000 plus ancestors whom in turn will have children etc etc… so just how many descendants would be roaming the Earth today.

    • Yes Kevin, that is the point. Our ancestors start duplicating after a while. Read the Joseph Chang paper or the more readable Brian Pears series.
      Also we are all cousins … so share most of those ancestors 1000 years ago.

  6. I’m from Cherokee NC and I’m half Cherokee,my mother was fullblood.the term local convention use to describe the pedigree. The other side or half is Scottish,irish,German and English. Western north Carolina in the Appalachian mountains was settled by this side or half. According to are legend we have always been here from the beginning of time. The scientists have proof that we have been here 12,000 to 14,000 years. My ancestors thought of seven generation ahead when making decisions. This article has made me look at my own generations in a new light

  7. It is important to remember that the number of ancestors lived through a thousand year period. The final number doesn’t mean that there were more than a billion people on the planet 1000 years ago but a billion people that lived over the course of 30 generations. Lets break it down to basic math. My parents each had parents meaning going back two generations I had 2+4=6. To figure out 30 generations you would multiply 4 (mothers parents+fathers parents)×30(number of generations )+2 (mom and dad)=122. The chart doubles the previous amount. That is simply contrary to common sense. Each child can only have 4 grandparents and 2 parents. The chart suggests that children have multiple parents and grandparents. It’s not compound math but simple math. Basic addition… Even if you go back 100 generations there would be 402 ancestors.

    • Michael –
      No the chart does not suggest multiple parents and grandparents. It is just counting the number of ancestors in each generation.
      Your 4 grandparents each had 2 parents which makes 8 people in that generation. Those 8 people also each had 2 parents so 16 in the next generation. Thus each generation of your ancestors going back is double the size of the previous generation.
      Read some of the linked articles.

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