In the last post I discussed the idea that that we are all related in the recent past (building off the work of Chang, Derrida, and colleagues). This idea can be confusing; for many of us our ancestors all seem to come from one or a few geographic locations. How does this geographic restriction affect the relatedness between modern day humans?
I’m originally from the UK, but I’ve been in the States for a third of my life. However, in general my ancestors weren’t big travelers. My family is from Yorkshire and Staffordshire in England. My mum traced our family tree back a few years ago; my photocopy of it is stuffed in a drawer somewhere. A bit further back, apparently many generations of my granddad’s side of the family are buried in a churchyard in a village (I think) somewhere outside of Melton Mowbray. No seafaring life with a kid in every port for my ancestors. Unsurprisingly then my ancestry report from 23&me makes for dull reading, and says my recent ancestry is all from the UK. How then do I have ancestors all over the world just a few thousand years back? Is it really possible that I am related to nearly everyone who lived in the entire world?
The key to this is that I, and you, have vast number of ancestors just a short time into the past. Fourteen generations back –roughly four hundred years ago– you have over sixteen thousand ancestors. Twenty generations back you have (potentially) over million different people as ancestors. Even if only a few people in the past emigrated from a specific country to the country you’re from, you are likely descended from those immigrants.
To illustrate this, consider the following simulation. We track your ancestors back over the generations as we did before. But now instead of coming from a well-mixed population, I’ve divided up the population of a million individuals into ten regions. These regions are arrayed along a line for simplicity, and the boundaries are shown as vertical lines. Each generation back, there’s a 1/50 chance that an individual’s parent comes from a neighbouring region. We see our first local migration event 4 generations back; one of your 16 great-great-grandparents is from the neighbouring region. See how their pedigree in that region rapidly expands; you soon have many ancestors in this second region.
On top of the local migration, in these simulations there’s a 1/5000 chance that an individual’s parent comes from some more distant region (chosen at random). We only see these long distance migrants deep in your pedigree. These migration events are occurring in the population all the time. However, It’s unlikely that any of your recent ancestors is one of these immigrants, as there’s only a low rate of immigration. But you have vast numbers of ancestors further back, and so further back you start to be descended from them too. See how eleven generations back you have over two thousand ancestors, and a couple of them are from distant regions. Looking slightly further back, each of your immigrant ancestors has many ancestors from his or her distant homeland. You’ll soon be descended from nearly everyone in these distant regions.
This rapid spatial expansion of your ancestors means also that you share recent genealogical ancestors with present-day individuals in distant locations, as both your and their ancestors are found all over the place. To illustrate this, I’ve run our simulation for another individual who lives at the other end of the set of regions from you. Below I plot your two family trees together.
Maybe you think 1/5000 individuals being an immigrant from some distant location is too high, and it likely is for distant locations or other continents. However, even if it were as low as 1 in a million, we only have to go back roughly 600 years to find you descended from one of these rare long distant immigrants. A thousand years back I’m descended from nearly every traveler of the high seas who set foot in Europe. Well at least those that left descendants there; if they had an unfortunate accident with a short-sword before conceiving a child, then they’re out of luck. As a result of the ones who had kids, I have millions of ancestors on every habitable continent just a few thousands of years ago.
I’m not an anthropologist of distant oceanic islands, so I can’t tell you for sure that there’s nowhere in the world so remote (and so long isolated) that we can rule out that you recent shared ancestry with people from these remote regions. However, I can confidently tell you that you’re related to nearly everyone in the world via ancestors just a few thousand years back. Even for the remotest locations in the world, I suspect that they too are soon part of our family tree. as nowhere has been completely isolated for many thousands of years.
Some links to related topics:
Simulations by Brian Pears of the spread of ancestors across the UK.
Jerome Kelleher et al explore technical aspects of the spatial spread of your ancestors, and calculate the rate of spread of the rapidly expanding geographic region your ancestors are drawn from. We’ve used related ideas to calculate dispersal distances from genetic data (see Harald Ringbauer et al.).
Thanks to Vince Buffalo, Doc Edge, Emily Josephs, and Jeff Ross-Ibarra for feedback on an earlier draft of this post.