I would appreciate insight into the average, median, RMS or any similar measure of relatedness among the current world population - and perhaps something about how rapidly this may be changing. A similar question is how un-related any two humans can be: i.e., what is the lowest degree of consanguinity between the two most distantly related people. The context is an exploration of how humans have evolved tendencies toward racism and other in/outgroup distinctions, when all humans share such a large fraction of DNA with each other, very nearly as much with non-human primates, and about half even with fruit flies. Might be some helpful lessons in there!
Apologies if the question is ill-formed, or answer readily available someplace - I've browsed the Web for several years on this topic, and found nothing I could understand..
Accessible material about most recent common ancestor and identical ancestor point seems to indicate a wide range of both methodologies and results, based mainly on statistical simulations since "hard" genetic structures apparently do not persist. MRCA datings based on mitochondrial and other genetics seem to line up with human behavioral modernity, ca. 200 kya. But there seem to be extreme estimates as recent as 2.3 kya, which implies a lot of mobility and high fertility by some not-so-distant forebears (like Genghis Khan). In any case, IAP may be a better starting point for this shared-anncestry question.
I'm guessing that no human is further than about a 10th~12th cousin to any other. Can't be more that 32nd, since 2^33 is more than the number of living humans!
All thoughts, including guesses more informed than mine, will be appreciated.
~ ~ ~ ~ ~
Addendum - tried to post this as an answer, but it was deleted:
MANY thanks to Zo-Bro-23 for the time, effort and creativity to create his response. I hope it is well-indexed for future explorers to find!
In case useful or interesting to anyone,or sparks further contributions, here were/are my main motivations for this inquiry:
Though having working knowledge of physical and natural sciences, and having minored in anthropology at Uni, I have never really grasped current estimations of "most recent common ancestor" and "identical ancestors point." This is probably due to my poor understanding of populations statistics, but may also reflect the huge uncertainty in and conflict between various MRCA and IAP methodologies. So I'm always seeking a simpler, if less precise, way to understand these propositions. Time-dependent mean/max degree of consanguinity seems like such a heuristic.
The ancestor cone, as mentioned in the Quartz article linked by Zo-Bro-23: I've been using the same term for decades, so I suppose it's a natural way to describe something that isn't precisely conic or biconic. For me, the description follows naturally from the notion of the light (bi-)cone in Minkowski diagrams, extending the idea of a photon's world-line to that of a genomic cluster.
I've long wondered about both genetic and memetic (e.g. religion, language, cuisine, art, toolmaking) contingency as they result from and induce changes in the topology of the ancestor cone. There are numerous groups that for geographic, religious or other reasons have been largely independent since we all left the Great Rift Valley (or wherever humanity arose). Having studied a bit of cultural diffusion, I've wondered if genetics, linguistics or anything else might give insight into how often and effectively the occasional Marco Polo or Squanto voluntarily or involuntarily acts as emissary between cultures. Many of these exchanges have apparently had significant cultural influence, and in some cases perhaps genetic as well. It might take very few such, over hundreds or thousands of generations, to lower substantially the degree of consanguinity of all humans.
This does not even count the much larger migrations, both voluntary (e.g. Bering Strait crossings that peopled the Americas), semi-voluntary (the Irish famine that so strongly influenced US culture) or entirely unwillingly (the African slave trade). Another question concerns the influence of sex-linked characteristics in Africans on the general American population, where for several centuries, the great majority of interbreeding appears to have been between White males and (usually unwilling, one imagines) Black females. Perhaps too hot a topic for research?
Pedigree collapse is a conflating factor of special interest to me, as most of my known ancestors belonged to a relatively small ethnic group that favored in-marriage, and (due to low social standing) was not attractive for outsiders to join. So other than couplings of necessity (rape, isolated populations), our cone is probably pretty narrow down to the bottom of the historic timeline. Not directly related by mechanism but probably relevant is the Galton–Watson process, the extinction of family names when a named lineage runs out of (usually male now; perhaps less so in "primitive" matriarchal societies) descendants.
Not long ago, it was popularly believed that explosive population growth since the industrial revolution resulted in the number of living humans exceeding the number deceased. Current credible estimates seem to place the latter at slightly more than 100 bn, - a dozen ghosts standing behind each living soul. Theme for an SF movie?
There is also the question of speciation. It is widely accepted that Homo sapiens and Homo neanderthalensis interbred to a considerable degree. This would, of course, violate pre-cladistic notions of what constitutes a species. (The cited Quartz article could have been titled "Everyone on Earth is actually your cousin - including some non-humans." And if all humans are at least 15th cousins, how far are we from other living families, even phyla?) I believe that barring "lost world" scenarios, we have been alone on the planet for all of historic time, hence for the lives of most H. sapiens. But if we date humanity to behavioral modernity, and plot the ancestor cone with a logarithmic or horizontal axis (or even force it into a cylinder of constant width), the influence of some of these factors on present-day consanguinity might be seen as comparable to those of posited bottlenecks like the Toba catastrophe, climate change, pandemics etc.; and on a smaller scale, founder effects like the peopling of the Pacific islands and Australia.
