Rock of Gibraltar
In The Humans Who Went Extinct the author makes much of the fact that Neandertals obviously lacked skill at crossing the water, insofar as their range was constricted by barriers to their south in Iberia. This sort of issue is kind of confusing to me, insofar as it seems probable that very ancient humans did make water crossings of a more arduous nature in Southeast Asia, if the Hobbits finds are valid.
I don’t really know what to think about the general issue of water crossings, but it does seem that the short distance between North Africa and Iberia has had a big impact. Bodies of water tend to serve as a major check on conventional gene flow between adjacent populations, because they limit “casual encounters.” An analogy can be made with the inbreeding coefficients in the mountainous regions of southern Italy. They were rather high until modern transportation made travel between isolated regions much easier, because the typical peasant simply wasn’t likely to venture far, or have a social network which would span valleys (rather, often the minimal transit avenues tended to lead back to …