Swimming Cow (link)
Leaving for the moment, the swimming cow theory, the Henn et al 2012 paper supports the idea of a human (not cow) Back-to-Africa migration from Europe more than twelve thousand years ago. For various technical reasons, the authors are not able to pin point the nexus of this migration as Gibraltar or the Strait of Sicily.
This paper also unfortunatlely claims that they "estimate that a migration of western African origin into Morocco began about 40 generations ago (approximately 1,200 ya); a migration of individuals with Nilotic ancestry into Egypt occurred about 25 generations ago (approximately 750 ya)" and that these "sub-Saharan ancestries appear to be a recent introduction into North African populations, dating to about 1,200 years ago in southern Morocco and about 750 years ago into Egypt, possibly reflecting the patterns of the trans-Saharan slave trade that occurred during this period." The problem is with the word began. These statements would leave one with the impression that there was no gene flow between the people of Africa and the people of European origin until at least 1,200 years ago. Needless to say, this notion is very implausible. In spite of that flawed conclusion, again, I do think that the authors are onto something regarding there being an ancient European contribution to the ancestry of North Africans.
Many have been following the discussion of the more recent paper on Neanderthal admixture in North Africans (Sánchez-Quinto). Again, there are some technical weaknesses in this paper which have been discussed by John Hawks (The North African Neandertal descendants) and also by Joshua Gatera (here) in the comments. What is most interesting to me in this discussion is that, as John Hawks points out, "When they sorted out parts of the genome in Tunisians that ADMIXTURE determines to be most likely from pre-Neolithic North Africans, they found these parts of the genome had more Neandertal ancestry than typical of the CEU sample of northern European ancestry. Is it possible that ancient North Africans had more Neandertal similarity than today's Europeans? ". In a word, yes. One possibility is that, as the higher level of Neanderthal admixture in Tuscans suggests, a nexus of Neanderthal admixture was in Italy. Again, John Hawks discussed this on his blog back in February (here). So did Ice age Italians carry this higher level of Neandertal to Tunisia when they crossed at the Strait of Sicily? No one has yet taken DNA from the different populations of archaic Neanderthals and compared them to the traces of Neanderthal DNA in different populations of modern humans. As yet, there is not a definitive answer as to the source of Neanderthal ancestry in North Africans.
Migrations across the Mediterranean would not have been one-way-heading-south tickets. People from North Africa likely also crossed back into Europe whenever the sea level was low enough both at Gibraltar and at the Strait of Sicily. That was apparent to me the moment I started looking at the E Y-chromosome haplogroup distribution which I wrote about in Gazelle Hunters and in the Mediterranean Coastline During the LGM. The Atlas mountains, the Saharan megalake corridor, Gibraltar and the Straight of Sicily must have provided intermittent byways to Europe. The Levant was not the only ticket north.
So what was the driving force behind these Mediterranean crossings? The distribution speaks to hunting of ungulates. The distribution of Ounanian Point hunters, as discussed by Drake et al (2010), bears a surprising congruence to the distribution of the E Y-chromosome haplogroup. What is more, the preponderance of ancient rock art drawing of gazelles, antelopes and bovines in the Atlas Mountains and North Africa also follows this distribution (Le Quellec, 1993). The abundance of rupricapra, bos primigenius, capra ibex, cervus elaphus and other now extinct antelopes and gazelles on both sides of the Strait of Sicily would have drawn hunters across the Straits in both directions.
As the sea level rose after the glacial maximum, populations on both sides were gradually cut off from their nexus of expansion. Over time, the "arrivals" on either side begin to fuse with the locals. Regarding the Ounanian culture that developed in North Africa after the Ice Age, Drake expounds: "We hypothesize that the other economic revolution that occurred in the Sahara at approximately the same time was the southward spread of the bow and arrow. North African hunters would have observed the new abundance of large and unfamiliar land mammals to the south, notably the elephant and the giraffe. In a dispersal inverse to that of the Nilo-Saharans [expanding from the southeast during the Holocene Climatic Optimum], they would have been attracted southward to hunt these animals with the bow and arrow. The "Ounanian" of Northern Mali, Southern Algeria, Niger, and central Egypt at ca. 10 ka is partly defined by a distinctive type of arrow point. These arrowheads are found in much of the southern Sahara and are generally considered to have spread from Northwest Africa. This view is supported by the affinity of this industry with the Epipalaelithic that also appears to have colonized the Sahara from the north. No Ounanian points occur in West Africa before 10ka, suggesting the movement of a technology across the desert from the north to south around this time."
One of many remaining questions regarding movements between Western Europe and North Africa is the means by which both animals, humans and plants came to cross Gibraltar and the Straight of Sicily. Gibraltar is a narrow crossing, but the possibility of the Strait of Sicily as a crossing point requires some consideration. The channel has depths below 200 meters, and as the sea level at the last glacial maximum never fell below 120 meters, no land isthmus could ever have existed. However, recent studies of the Marbled White butterfly (Habel et al, 2011), linaria (Fernández-Mazuecos, Vargas, 2011), and maniola jurtina (Dapport et al, 2011) indicate that some plants and butterflies sporadically were able to cross the Strait of Sicily during the last glacial maximum. To date, no studies on mammals definitively indicate a crossing of the Strait of Sicily prior to the Neolithic and during the LGM.
Further studies of ancient and modern plant, mammal and insect DNA will be needed to illucidate the picture of these ancient mariners. Swimming ice age cows across the Strait of Sicily? It certainly would explain the odd phylogenic distribution of those T1 cattle. Perhaps the surprising finding of high levels of "Neanderthal" in both Tuscans and Tunisians also speaks to this ancient crossing.