Australia is unusual because the animal population evolved largely out of contact with the other main continents. A very high percentage of Australian animals are endemic (found nowhere else) including about 70% of its birds and 95% of its mammals. Over time, the marsupials filled most of the ecological niches that are normally occupied by placental mammals. Australia is also home to the only three monotreme species in the world - two echidnas and the platypus. The only placental mammals naturally found on the Australian mainland are bats and some primitive rodents; plus the dingo, which came to Australia somewhere around 7000 years ago.