The earliest known evidence of human presence in the area now known as England was that of Homo antecessor, dating to approximately 780,000 years ago. The oldest proto-human bones discovered in England date from 500,000 years ago. Modern humans are known to have inhabited the area during the Upper Paleolithic period, though permanent settlements were only established within the last 6,000 years. After the last ice age only large mammals such as mammoths, bison and woolly rhinoceros remained. Roughly 11,000 years ago, when the ice sheets began to recede, humans repopulated the area; genetic research suggests they came from the northern part of the Iberian Peninsula. The sea level was lower than now and Britain was connected by land bridge to Ireland and Eurasia. As the seas rose, it was separated from Ireland 10,000 years ago and from Eurasia two millennia later. The Beaker culture arrived around 2,500 BC, introducing drinking and food vessels constructed from clay, as well as vessels used as reduction pots to smelt copper ores. It was during this time that major Neolithic monuments such as Stonehenge and Avebury were constructed. By heating together tin and copper, which were in abundance in the area, the Beaker culture people made bronze, and later iron from iron ores. The development of iron smelting allowed the construction of better ploughs, advancing agriculture (for instance, with Celtic fields), as well as the production of more effective weapons.