Sea in culture of barbarian invaders and knowledge of geography took a backward step. In the seventh century, Isidore of Seville produced a "wheel map" in which Asia, Africa and Europe were arranged like segments in an orange, separated by the "Mare Mediterranean", "Nilus" and "Tanais" and surrounded by "Oceanus". It was not until the fifteenth century that Ptolemy's maps were used again and Henry the Navigator of Portugal initiated ocean exploration and maritime research. Encouraged by him, Portuguese navigators explored, mapped and charted the west coast of Africa and the Eastern Atlantic and this knowledge prepared the way for the great voyages
African admixture in Europe Europe may have a common origin in Asia (e.g. Y haplogroups R1, and some paternal haplogroup T and U subclades). One subclade of haplogroup U, namely U6a1, is known to have expanded from northern and eastern Africa back into Europe even though haplogroup U6 is considered to have originated in the Middle East. Other lineages are known to have moved from Europe directly into Africa, for example mitochondrial haplogroups H1 and H3. Such bidirectional migrations between Africa and Eurasia complicate the task of defining admixture. One proposed example of Holocene gene flow from Africa to Europe, via the Middle East,