In my last post, I questioned whether a stranger sitting next to you on a train would be more similar to you than an ancestor from 10,000 years ago and suggested that this could be tested genetically.
A few issues arise in testing this. First, as I suggested in the last post, the particular ancestor we choose might affect the result. If an ancestor contributed through only a single ancestral line (of the approximately 10^120 lines), any similarity due to ancestry will be very low to negligible, unless that person is, say, a direct male-male ancestor and has contributed the Y-chromosome, much of which does not engage in recombination (that is, the crossover of genes between the chromosomes inherited from ones parents).
A bigger issue in my mind is convergent evolution. Given the selection pressures that agricultural populations have been under, it is likely that a number of shared phenotypic traits (that is, traits that are expressed in the person) have emerged, with these traits having different genetic origins. Take the ability to digest lactose in adulthood, which has separate origins in Europe and sub-Saharan Africa (and possibly other locations). While the genetic mutations that cause the traits are different, the phenotypic result is similar, which reflects the common selection pressure.
I expect that this is the case for many other traits. Distinct populations developed agriculture in a number of separate locations, which is likely to have resulted in some similar selection pressures in these locations (I won’t describe these events as independent developments of agriculture, as is sometimes done, as they aren’t independent). If the traits favoured by the adoption of agriculture are similar, despite being expressed by different genes or mutations, they would spread and increase similarity between populations in a way which may not be apparent in a genetic test.
Given this, as I did in my last post, I still question whether Seabright’s statement would be generally true. Further, if we could also capture similar traits expressed through different genes or mutations, we may be more similar to the modern stranger than the genetic test would suggest.