I googled the amount of people that lived on earth throughout its entire history, it’s roughly 108 billions. If I take 1 person and multiply by 2 for each generation of ancestors, at the 37th generation it already outnumbers that 108 billions. (it’s 137 billions). If we take 20 years for 1 generation, it’s only 740 years by the 37th generation.
How??
(I suck at math, I recounted it like 20 times, got that 137 billions at 37th, 38th and 39th generation, so forgive me if it’s not actually at 37th, but it’s still no more than 800 years back in history)
In: 1391
Google pedigree collapse, that’s what it’s called. Basically it’s why we are all technically related and we can go back far enough and have found “mitochondrial Eve” who is a relative to every single person alive today and known as the mother of all modern humans. There used to be a lot more inbreeding also.
A lot of people never have any children, either by choice, or because they die. So not every person doubles in 20 years.
I also don’t know how they estimated that number.
The secret ingredient is incest. Distant incest so we don’t worry about it, but ultimately your parents share some ancestors.
You’ve touched on a really interesting concept in ancestry called “pedigree collapse.” Here’s how this works:
If you double the number of ancestors in each generation, you quickly arrive at a number larger than the historical world population. But there’s a logical explanation for this.
Imagine you have two parents, four grandparents, eight great-grandparents, and so on. By the time you get to the 37th generation, as you pointed out, the math suggests you would have 137 billions ancestors, which is larger than the estimated 108 billion people that have ever lived.
The flaw in this logic is the assumption that every person in your family tree is a distinct individual. In reality, the farther you go back in time, the more likely it is that you will have the same individual occupying multiple slots in your family tree. This means that your ancestors start marrying their own distant cousins, and the “tree” is not as branching as we imagine but rather collapses in on itself.
Think of it this way: Let’s assume every human has two parents (a basic assumption). If you go back 1,000 years, the simple doubling math suggests you’d have over a trillion ancestors. Since that’s many times the total number of people who’ve ever lived, it’s clearly impossible.
So, what’s happening is that the same people are getting counted multiple times in your tree because lines of descent converge. This is pedigree collapse. As you trace back, many of your ancestral pathways lead to the same individual. This becomes especially true in more isolated communities or in places/times where people didn’t travel far from where they were born.
The key takeaway is that while the math of doubling ancestors every generation is accurate in the abstract, it doesn’t account for the way human populations and family trees really work. Pedigree collapse ensures that we all have far fewer distinct ancestors than the simple math would suggest.
Once you go back far enough in time, everyone who’s alive now is descended from everyone who was alive back then (and had children who had children who had children etc.) in multiple different ways. We’re all inbred but in a way that’s socially acceptable and doesn’t cause health problems.
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