Archaeological evidence suggests that Trinidad was the first Caribbean island to have been settled as early as 9000/8000 BCE. However, the first settlers most likely arrived in Trinidad when it was still attached to South America by land bridges. It was not until about 7000/6000 BCE, during the early Holocene that Trinidad became an island due to a significant jump in sea level by about 60 m. Climate change may have been a cause for this sea level rise. Hence Trinidad was the only Caribbean Island that could have been colonised by indigenous people from the South American mainland by not traversing hundreds or thousands kilometres of open sea. The earliest major habitation sites discovered in Trinidad are the shell midden deposits of Banwari Trace and St. John, which have been dated between 6000 and 5100 BCE. Both shell middens represent extended deposits of discarded shells that originally yielded a food source and stone and bone tools. They are considered to belong to the Ortoiroid archaeological tradition, named after the similar but much more recent Ortoire site in Mayaro, Trinidad.