England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe by the North Sea to the east and the English Channel to the south. The country covers five-eighths of the island of Great Britain, which lies in the North Atlantic, and includes over 100 smaller islands, such as the Isles of Scilly and the Isle of Wight. The area now called England was first inhabited by modern humans during the Upper Paleolithic period, but takes its name from the Angles, a Germanic tribe deriving its name from the Anglia peninsula, who settled during the 5th and 6th centuries. England became a unified state in the 10th century and has had a significant cultural and legal impact on the wider world since the Age of Discovery, which began during the 15th century. The English language, the Anglican Church, and English law—the basis for the common law legal systems of many other countries around the world—developed in England, and the country's parliamentary system of government has been widely adopted by other nations. The Industrial Revolution began in 18th-century England, transforming its society into the world's first industrialised nation. England's terrain is chiefly low hills and plains, especially in central and southern England. However, there is upland and mountainous terrain in the north (for example, the Lake District and Pennines) and in the west (for example, Dartmoor and the Shropshire Hills). The capital is London, which has the largest metropolitan area in the United Kingdom. England's population of 56.3 million comprises 84% of the population of the United Kingdom, largely concentrated around London, the South East, and conurbations in the Midlands, the North West, the North East, and Yorkshire, which each developed as major industrial regions during the 19th century. The Kingdom of England – which after 1535 included Wales – ceased being a separate sovereign state on 1 May 1707, when the Acts of Union put into effect the terms agreed in the Treaty of Union the previous year, resulting in a political union with the Kingdom of Scotland to create the Kingdom of Great Britain. In 1801, Great Britain was united with the Kingdom of Ireland (through another Act of Union) to become the United Kingdom of Great Britain and Ireland. In 1922 the Irish Free State seceded from the United Kingdom, leading to the latter being renamed the United Kingdom of Great Britain and Northern Ireland.