At times during the last interglacial period (130,000–70,000 BC) Europe had a climate warmer than today's, and early humans may have made their way to what is now Scotland, though archaeologists have found no traces of this. Glaciers then scoured their way across most of Britain, and only after the ice retreated did Scotland again become habitable, around 9600 BC. Mesolithic hunter-gatherer encampments formed the first known settlements, and archaeologists have dated a site near Biggar to around 8500 BC. Numerous other sites found around Scotland build up a picture of highly mobile boat-using people making tools from bone, stone and antlers, probably with a very low density of population. Neolithic farming brought permanent settlements, such as the stone house at Knap of Howar on Papa Westray dating from 3500 BC, and greater concentrations of population. Extensive analyses of Black Loch in Fife indicate that arable land spread at the expense of forest from about 2000 BC until the period of the first century AD Roman advance into lowland Scotland, suggesting an expanding settled population. Thereafter, there was re-growth of birch, oak and hazel for a period of five centuries, suggesting that the Roman invasions had a negative impact on the native population.