Complex sentence: DNA analysis has shown that modern humans arrived in Britain at least 25,000 years ago, before the last Ice Age, but then retreated to Southern Europe when much of Britain was ice covered, with the remainder being tundra.
Split into simpler sentences: DNA analysis has shown that modern humans arrived in Britain at least 25,000 years ago, before the last Ice Age. This evidence also shows that the first humans living in Britain then retreated to Southern Europe when much of that land mass was covered with ice, with the remainder being tundra.