In the company of a stranger


Jason Collins


April 22, 2011

I have just left the Social Decision Making: Bridging Economics and Biology conference, with one of the last speakers being Paul Seabright, author of The Company of Strangers. I will post some thoughts on Seabright’s presentation (and some of the other presentations at the conference) after Easter and once I have read the related papers.

In the meantime, the night before Seabright’s talk I flipped through the revised edition of his book (it is a few years since I read the first edition). In the introduction is the following:

If it were somehow possible to assemble together all your direct same-sex ancestors - your father and your father’s father and so on if you’re male, your mother and your mother’s mother and so on if you’re female; one for each generation right back to the dawn of agriculture - you and all of these individuals could fit comfortably in a medium-sized lecture hall. Only half of you would have known the wheel, and only 1 per cent of you the motor car. But you would be far more similar to each other - genetically, physically, and instinctually - than any group of modern men or women who might have assembled there by chance. … [E]xcept in some dimensions such as height and perhaps in skin color, the biological differences between you and your furthest ancestor would be hard to distinguish from random variation within the group. If you are reading this book in a train or an airplane, this means your most distant ancestor from Neolithic times was probably more like you, biologically, than the stranger sitting in the seat next to you now.

As occurred to me the time I first read it, I am not sure that the last sentence is true. I expect that this could be empirically tested on a genetic basis (so this post should be put in the “I might be wrong” category), but there are a couple of issues in my mind.

First, it is likely that a Western European stranger would share many common ancestors with me within the last 10,000 years. If we go back 10,000 years, or around 400 generations, I have approximately 2^400 direct ancestors (which is approximately 10^120, or more than the number of atoms than there are estimated to be in the observable universe). Obviously, this is more ancestors than there were people at that time, which points us to the fact that many of my ancestors from this time are ancestors through multiple ancestral lines. Many ancestors are likely to be shared with anyone of a similar ethnic origin. On this basis, there is no ground to expect that the direct ancestor of 10,000 years ago would be more similar than the stranger.

Applying Seabright’s particular scenario, I wrote this post while sitting on a train in Northern Italy, with an Italian the stranger sitting next to me. As far as I have traced, I have English, Welsh and Irish heritage. That reduces the chance that we share common ancestors within the last 10,000 years compared to a scenario where I sit next to someone from the United Kingdom or Ireland. Even so, there is still a significant probability of common ancestors, as would be suggested by my Y-chromosome haplogroup (R1b1a2a1a1a under the 2011 nomenclature), with earlier versions of this haplogroup present in both United Kingdom and Italian populations (and that, of course, is just the male-male lineage).

A second issue is who this ancestor is. Depending on the genes of each ancestor and the traits expressed by those genes, selection will have resulted in certain ancestors contributing more to my genome than others. If we picked one of my ancestors who contributed through a single line 10,000 years ago, they are likely to be less similar to me than an ancestor who has contributed through ten orders of magnitude more lines (remembering that I have 10^120 ancestral lines from 400 generations ago). Whether the ancestor we use for the comparison is more similar to me than the modern stranger might depend on whether we picked an ancestor randomly, the ancestor who made the median or mean contribution to my genome or the ancestor who made the greatest.