The region was settled by Asian people around 50,000 years ago when New Guinea was still part of the main Australian landmass. 10,000 years ago, at the end of the last ice age, the waters of the Torres Strait cut off New Guinea from Australia, giving rise to separate development of the indigenous people. Plant-based agriculture developed about 9,000 years ago in the New Guinea highlands. 2,500 years ago, a large migration of Austronesian-speaking people settled in the coastal areas. These communities developed animal husbandry, pottery and fishing and there is evidence of trade with the southeast Asian mainland. The rugged topography meant that contact between communities was limited and they retained their separate languages and customs. This heterogeneity continued down the centuries, so that today Papua New Guinea has an estimated 1,000 different cultural groups. Spanish and Portuguese explorers reached the island in the early 16th century and the Portuguese are thought to have introduced the kaukau (sweet potato), which became a staple crop. Spain laid claim to the western half of the island in 1545, naming it ‘New Guinea’ because of the supposed resemblance of its inhabitants to the people of Africa’s Guinea coast.