Considering everyone’s related uf we go back far enough ti our african origins, at which point can ancestry tests and the like go “ah you are 20% scandinavian”? Were is the cutoff time period or how does this work?

Question

I may not be asking the rifht questions but i just dont get this at all

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In: 5

Answer 1

I’ll try to explain.

First, dna tests on ancestry and 23andme don’t have a lot of dna data from 100, 200 years ago. There are some dna from people living a long time ago that is on record but not very many. So the majority of what ancestry is telling you is where the descendants of your ancestors live not necessarily where your ancestors actually lived.

You can use the dna to cross check family trees from distant cousins and also to fatten your tree.

Anyway to answer the question in a different way is to look at the question like how many ancestors do you have? Well as you may know every generation your ancestors will double. You have 2 parents, 4 grandparents and 8 great grandparents. That’s all fine but when you go back to about the 30th generation then you have over a billion ancestors. Who probably lived around 1050 CE.

So you may think well was there over a billion people around at that time and the answer is no. Also remember that the ancestors double so generation 31 would have over 2 billion.

Obviously, you cannot have had more ancestors than the population living on the planet. So the reasonable conclusion is that your pedigree collapsed. A pedigree collapse happens when two people knowingly or unknowingly share an ancestor. An example is queen Elizabeth II and Philip were third cousins and shared a great great grandmother.

Now if you researched all of your ancestors and were able to find all of them without any issues. Eventually you will find an ancestor that your parents share in common. This would happen with any two people you met. Your boss, your neighbor, your BFF. Eventually you will find a common ancestor with them. There is also a person who everyone alive today is related to. This person is referred to as the most recent common ancestor. If you have European ancestors than the person probably 1,000 years ago.

Mathematically it is estimated that the most recent common ancestor of all people alive today may have lived as early as 300 BCE. This doesn’t take into genetics, but it did take into account that most people do not mate randomly and in the past people almost always mated with people that lived nearby usually in the same town or village, but there are rare people that would have mated with people far away so no population is truly isolated.

From there, there is an identical ancestry point. This is the point where all people alive are related to everyone alive today or no one alive today. This point is surprisingly recent as early as 5,000 to 15,000 years ago based both on math and genetics.

So at this point everyone has the exact same ancestors. So this is where your question is: how do people that have the same ancestors have different precent of relatedness? Well, its the pedigree collapse!

Say you have a Japanese person and a Norwegian person they have the same ancestors 7,000 years ago. But for the Japanese person his ancestors that lived in Japan would be in his tree 84% of the time, but the ancestors that lived in Norway 7,000 years ago would only be in his tree 0.00049% of the time. The Norwegian on the other hand his ancestors in Norway would be on his tree 92% of the time and his ancestors in Japan would only appear 0.00044% of the time. So even though they share the same ancestors the number of times the ancestor appears on the person’s tree is different.

Also you may be curious about how many unique ancestors that you likely have and it’s estimated to be around 6 million.

Finally, I want to say that the most recent common ancestor is not the same as Y-chromsome Adam or mitochondria-Eve. Adam is the most recent common ancestor for Y chromosomes. Eve is the common ancestor of the mitochondria. These two didn’t live at the same time. Also they lived with other people who had y chromosomes and mitochondria but those y chromosomes and mitochondria did not survive to today. Also the person isn’t fixed it changes as family lines die out. Finally the most common recent ancestors and the identical ancestors point will always be after y chromosome Adam and mitochondria-Eve existed.