“Modelling the Recent Common Ancestry of All Living Humans”, 2004-09-30 ():
If a common ancestor of all living humans is defined as an individual who is a genealogical ancestor of all present-day people, the most recent common ancestor (MRCA) for a randomly mating panmictic population would have lived in the very recent past1,2,3. However, the random mating model ignores essential aspects of population substructure, such as the tendency of individuals to choose mates from the same social group, and the relative isolation of geographically separated groups.
Here we show that recent common ancestors also emerge from two models incorporating substantial population substructure:
One model, designed for simplicity and theoretical insight, yields explicit mathematical results through a probabilistic analysis.
A more elaborate second model, designed to capture historical population dynamics in a more realistic way, is analysed computationally through Monte Carlo simulations.
These analyses suggest that the genealogies of all living humans overlap in remarkable ways in the recent past. In particular, the MRCA of all present-day humans lived just a few thousand years ago in these models. Moreover, among all individuals living more than just a few thousand years earlier than the MRCA, each present-day human has exactly the same set of genealogical ancestors.
…For example, in a panmictic population of one million people, the genealogical MRCA would have lived about 20 generations ago, or around the year AD 1400, assuming a generation time of 30 years. The MRCA along exclusively maternal lines would have lived something like 50,000× earlier—in the order of one million generations ago.
…The major problem in applying these results to human populations is that mating is not random in the real world. Mating patterns are structured by geography, proximity, culture, language and social class. Nevertheless, even in populations with considerable internal structure, the time to the MRCA can be remarkably brief. To demonstrate this in a tractable mathematical model, consider a population of size n divided into randomly mating subpopulations that are linked by occasional migrants. The population is represented by a graph, G, with a node for each subpopulation. Edges indicate pairs of nodes that exchange a small number (for example, one pair) of migrants per generation. Let R denote the radius of G, and let Δ be a quantity ranging 0–1 that depends on the structure of G (see Box 1). A probabilistic analysis (see Supplementary Information) shows that as n → ∞, Tn ≈ (R + Δ)log2n. Furthermore, if we let D denote the diameter of the graph, then the number of generations, Un, since the IA point satisfies Un ≈ (D + 1.77)log2n
…These estimates would suggest, with the exchange of just one pair of migrants per generation between large panmictic populations of realistic size, that the MRCA appears in about the year 300 BC, and all modern individuals have identical ancestors by about 3,000 BC. Such estimates are extremely tentative, and the model contains several obvious sources of error, as it was motivated more by considerations of theoretical insight and tractability than by realism. Its main message is that substantial forms of population subdivision can still be compatible with very recent common ancestors.
…In the case of Tasmania, which may have been completely isolated from mainland Australia between the flooding of the Bass Strait, 9,000–12,000 years ago, and the European colonization of the island, starting in 180313, the IA date for all living humans must fall before the start of isolation. However, the MRCA date would be unaffected, because today there are no remaining native Tasmanians without some European or mainland Australian ancestry.
No large group is known to have maintained complete reproductive isolation for extended periods. The populations on either side of the Bering Strait appear to have exchanged mates throughout the period documented in the archaeological record14. Religious isolates such as the Samaritans occasionally have absorbed migrants from outside the group15. Even populations on isolated Pacific islands have experienced occasional infusions of newcomers16. Even if rates of migration between some adjoining populations are very low, the time to the MRCA tends not to change substantially. For example, with a migration rate across the Bering Strait of just one person in each direction every 10 generations, rather than the 10 per generation in the more conservative simulation described earlier, Tn only increases 3,415 years → 3,668 years.