Correlated and geographically predictable Neanderthal and Denisovan legacies are difficult to reconcile with a simple model based on inter-breeding

Although the presence of archaic hominin legacies in humans is taken for granted, little attention has been given as to how the data fit with how humans colonised the world.  Here I show that Neanderthal and Denisovan legacies are strongly correlated and that, like heterozygosity, distance from Africa predicts legacy size.  Simulations confirm that, once created, legacy size is extremely stable: it may reduce through admixture with lower legacy populations but cannot increase detectably through neutral drift.  Consequently, populations carrying the highest legacies must also be those whose ancestors inter-bred most with archaics.  However, the populations with the highest legacies are globally scattered and are unified, not by having origins within the known Neanderthal range, but instead by living in locations that lie furthest from Africa.  Furthermore, the Simons Genome Diversity Project data reveal two very distinct correlations between Neanderthal and Denisovan legacies, one that s...