In mythology, Rome was founded by Romulus, who, with his brother Remus, were brought up by a wolf. He killed his brother when he defied the sacred boundaries of the new city, and became the first king of Rome. In historical times, the civilization of ancient Rome originated in the 8th or 9th century BC, when northern tribes migrated to the Italian peninsula to settle around the River Tiber. For almost a thousand years, Rome was the most important city in the Western world and the largest city in the world (the first to reach and surpass 1,5 millions inhabitants), as the capital of the expansive Roman Empire. With the rise of Christianity, Rome became the center of the Roman Catholic Church and the home of the Popes. The slow decline of the Roman Empire heralded the beginning of the Middle Ages, but the city regained prominence as the political capital of Europe for several hundred years leading up to the Renaissance. Rome remains influential today, as the capital of Italy, of the Catholic Church and as a major world class metropolis.