The Liir are an air-breathing aquatic species, and bear a strong resemblance to the extinct cetaceans of Old Earth. They are the result of a long-term process of environmental change: an ice age lasting millions of years initially allowed for the development of mammalian species on the isolated tropical islands and huge ice shields of their home world, but eventually an extended warming period resulted in a planet with less than 10% of its surface above water. The vast majority of land-dwelling species returned to the sea—including the early ancestors of the Liir.