First, the whole argument is a little silly. You could walk from, say, Calais on the English Channel to Pusan in South Korea without dying of thirst. At either end of your vast journey, however, the people look quite different. In between you might run into, say, Boris Yeltsin, a blond man with features slightly reminiscent of East Asia, and other people of varying degrees of European and East Asian admixture. But, in the big picture, so what? Frenchmen and Koreans are still different and nobody would mistake one for the other.

Second, the geneticists’ statement applies only “along colonization routes,” and most possible directions were not major colonization routes. If you walk in the majority of directions, you will eventually fall into the ocean and drown. This causes the “obvious genetic discontinuity” that we see with our lying eyes. …

In contrast, up through 1492, there was a relatively massive genetic discontinuity between West Africa and South America, which are only 1,600 miles apart at their closest points. Why? Because the out-of-Africa colonization routes went the other way around the world. The Atlantic Ocean got in the way of walking directly from Africa to South America.

With water covering 7/10ths of the earth’s surface, the out-of-Africa dispersal pathways were, in reality, few and far between.

Even on dry land, there are vast regions where paths were few and arduous. For instance, between the peoples of West Africa and of the Maghreb (Northwest Africa) there was only a small amount of mating until historic times, because the Sahara got in the way. If you tried to walk from Senegal to the Pillars of Hercules, you would likely die of thirst. The eastern end of the Sahara, though, is more porous because of the Nile and some wetter highlands.

Likewise, the Himalayas form a sharp border even today between Caucasians and East Asians.

I’ll try to be semi-methodical about this. Let’s split the world up, quick and dirty, into seven effective continents: Sub-Saharan Africa, West Asia, Europe, East Asia, Australia, North America, and South America. (You can suggest other breakdowns of continents, but the results will all be about the same).

Here is a table showing seven major continents and my guess as to how easy the potential direct colonization routes between each of them were during early human prehistory: A “2” means easy, “1” means difficult but used, and 0 means there was virtually no direct contact between the two continents before historic times. As you can see, it’s a sparse grid:

W Asia Europe E. Asia Australia N. Am Sub-Saharan Africa 1 0 0 0 0 0 West Asia 2 1 0 0 0 Europe 1 0 0 0 East Asia 1 1 0 Australia 0 0 North America 2

[Note: last column is South America.]

Of the 21 possible connections between continents, there were 14 where there was virtually no contact until the last millennium. Above is an image you’ve never seen before: a map of the many intercontinental roads not taken by prehistoric man:

For example, there are relatively big genetic distances between Australians and Sub-Saharan Africans because the Indian Ocean got in the way of their mating. Similarly, Australians aren’t closely related to Europeans because Asia came between them, which was full of tribes that didn’t particularly want outsiders marching through their lands.

This is not to say that there was no contact at all along these routes. For example, prehistoric Southeast Asians rode rafts 4,000 miles across the Indian Ocean to colonize Madagascar, and they likely went on to leave a tiny genetic imprint on the Southeast African mainland.