The ancestry of modern Iberians (comprising the Spanish and Portuguese) is consistent with the geographical situation of the Iberian Peninsula in the south-west corner of Europe. As is the case for most of the rest of Southern Europe, the principal ancestral origin of modern Iberians are Early European Farmers who arrived during the Neolithic. The large predominance of Y-Chromosome Haplogroup R1b, common throughout Western Europe, is testimony to a considerable input from various waves of (predominantly male) Western Steppe Herders from the Pontic-Caspian Steppe during the Bronze Age. The Iberian people's genetic pool largely derives from the pre-Roman inhabitants of the Iberian Peninsula: Pre-Indo-European and Indo-European speaking pre-Celtic groups: (Iberians, Lusitani, Vettones, Turdetani, Aquitani, Conii). Celts (Gallaecians, Celtiberians, Turduli and Celtici), who were Romanized after the conquest of the region by the ancient Romans. There are also some genetic influences from Germanic tribes who arrived after the Roman period, including the Suebi, Hasdingi Vandals, Alans and Visigoths. Due to its position on the Mediterranean Sea, like other Southern European countries, there were also contact with other Mediterranean peoples such as the ancient Phoenicians, Greeks and Carthaginians who briefly settled along the Iberian Mediterranean coast, the Sephardi Jewish community, and Berbers and Arabs arrived during Al-Andalus, all of them leaving some North African and Middle Eastern genetic contributions, particularly in the Southern and Western Iberian Peninsula. Similar to Sardinia, Iberia was shielded from settlement from the Bosporus and Caucasus region by its western geographic location, and thus has lower levels of Western Asian and Middle Eastern admixture than Italy and the southern Balkan Peninsula, most of which probably arrived during historic rather than prehistoric times, especially in the Roman period. Population Genetics: Methods and Limitations One of the first scholars to perform genetic studies, although now questioned in its conclusions, was Luigi Luca Cavalli-Sforza. He used classical genetic markers to analyse DNA by proxy. This method studies differences in the frequencies of particular allelic traits, namely polymorphisms from proteins found within human blood (such as the ABO blood groups, Rhesus blood antigens, HLA loci, immunoglobulins, G-6-P-D isoenzymes, among others). Subsequently, his team calculated genetic distance between populations, based on the principle that two populations that share similar frequencies of a trait are more closely related than populations that have more divergent frequencies of the trait. Since then, population genetics has progressed significantly and studies using direct DNA analysis are now abundant