Starting about two million years ago and the last phase ending only some ten thousand years before the present, the Glacial and Interglacial periods are contemporaneous with homo himself. As he spread over the face of the earth he was prodded into moving hither and yon by the harsh alternations of climate and weather. Had remote populations remained isolated man might have split into several species. However the intermixture of colliding groups kept the gene pool a world-wide reservoie. Nevertheless different environments have left their mark on his physique.
Tropical peoples tend to be ectomorphic-slenderly constructed, the better to radiate body heat.
There are differences in skin color, a function of exposure to solar radiation. Northern peoples have thin nostrils to warm and moisten cold air before it reaches fragile lung tissues. Tropical man has broad, open nostrils for the opposite reason. Peoples of northeast Asis, cut off by Himalayan mountains during the last glaciation acquired eyes deeply buried in their sockets and protected by fatty folds, noses flattened into their faces and a diminution of facial hair, as defenses against frost bite. Baltic fog and reduced exposure to sunlight produced the Nordics.