Ever wonder what it would be like to be related to royalty of ancient times past? What if I told you that if you’re European, then you’re related to King Charlemagne? Seems crazy, right? Well, lets examine an arbitrary person’s family tree. It would start with the person themselves, their 2 parents, 4 grand parents, etc… and resemble a complete binary tree. But if we think about this for a bit, we see that there is a bit of a contradiction. Going 30 generations back, so approximately 600 years if we assume a generation is approximately 20 years, a person would have 2^30 = 1,073,741,824 ancestors in that generation. But hold on! That is more than the number of people that would be alive in the whole world at that time, so what’s going on?
It turns out that our family trees aren’t just simple complete binary trees. The explanation lies in the fact that our family tree doesn’t have all unique ancestors, some ancestors take up multiple spots on the tree. This happens when parents of an ancestor are (knowingly or unknowingly) related to each other, which collapses the two parents’ family trees into a single tree.
This is known as pedigree collapse. After considering this, our family tree turns out to be a directed acyclic graph, since an individual cannot be their own ancestor, and a single ancestor can have multiple paths to the individual. How does this relate to King Charlemagne? Considering he lived from the years 742-814, there would have to have been a lot of pedigree collapse since no one individual could have distinct ancestors from those years until now. It was shown in 2013 by geneticists Peter Ralph and Graham Coop that all Europeans have the same common ancestors, going back around 1000 years. Thus all Europeans are related to King Charlemagne! Not only that, go back far enough and it is very likely that everyone is in some way related. It begs the question of how many degrees of separation there are between two individuals through their ancestry.
References:
https://waitbutwhy.com/2014/01/your-family-past-present-and-future.html
https://www.theguardian.com/science/commentisfree/2015/may/24/business-genetic-ancestry-charlemagne-adam-rutherford
https://en.wikipedia.org/wiki/Pedigree_collapse
Interesting. I’ve also heard that we have twice as many female ancestors than male ancestors.
While I’m not sure on the exact numbers, during my research I did also come across the concepts of a “Y-chromosome Adam” and “mitochondrial Eve”, which was essentially, if you trace someone’s lineage back far enough by only their mother, their mother’s mother, etc… (for Eve) / only their father, their father’s father, etc… (for Adam), then everyone’s lineage eventually converges to a single man/woman, being every living human’s most living common ancestor. The mitochondrial eve lived before the Y-chromosomal Adam by some thousands of years, so by pedigree collapse it would make sense that we have more female ancestors than male ancestors. Add on that in the past, it wasn’t uncommon that some men had multiple mates, like kings or nobility for example, and we see that there are more distinct women in the family tree than males. It’s quite an interesting conclusion!