Prehistoric Britain is the period between the arrival of the first humans in Great Britain and the start of recorded British history. The period prior to occupation by the genus Homo is part of the Geology of the British Isles. The history of Britain is conventionally reckoned to begin in AD 43 with the Roman invasion of Britain, though some historical information is available from before this. Prehistory is commonly divided chronologically into distinct periods, based on the development of tools from stone to bronze and iron as well as changes in culture and climate, but their boundaries are uncertain, and the changes between them gradual; the dates of the changes are generally different from those of continental Europe.
Britain has been intermittently inhabited by members of the Homo genus for hundreds of thousands of years and by Homo sapiens for tens of thousands of years. DNA analysis has shown that modern man arrived in Britain before the last ice age but retreated to Southern Europe when much of Britain was ice covered, with the remainder being tundra. At this time the sea level was around 127m (417ft.) lower than today so that Britain was joined to Ireland and to the continent of Europe.
After the end of the last Ice Age (around 9500 years ago) Ireland became separated from Britain and later (around 6500 BC) Britain was cut off from the rest of Europe. By 12,000 BC Britain had been reoccupied, as shown by archaeology. By around 4000 BC, the island was populated by people with a Neolithic culture. However, none of the pre-Roman inhabitants of Britain have any known surviving written language. No literature of pre-Roman Britain has survived, so its history, culture and way of life are known mainly through archaeological finds. Though the main evidence for the period is archaeological, there is a growing amount of genetic evidence which is still changing. There is also a small amount of linguistic evidence, from river and hill names, which is covered in the articles on Pre-Celtic and Celtic.