Technically, every living thing is endemic to Earth; biologists mean something narrower
A species is endemic when it lives naturally in one defined place and nowhere else, like the Cape sugarbird of southwestern South Africa. Charles Darwin first used the word that way in 1872. Before then, endemic described diseases that simmer steadily in a country rather than exploding as epidemics.
The word descends through French from Greek endemos, native, built from en, within, and demos, the people. Its medical sense made one entomologist uneasy: in 1900 David Sharp, describing Hawaiian insects, coined precinctive as an alternative, and a Hawaii botanist adopted it in 1917, though it never caught on widely. Words like indigenous are sometimes used loosely in its place, but they do not imply that a species lives only in one location.
In principle the scale is arbitrary. Cougars qualify for two whole continents, and by the same logic every organism ever found belongs to this planet alone. In practice biologists reserve the term for species with small or restricted ranges, in contrast to cosmopolitan species found worldwide. Two common confusions are worth untangling: endemic species can be abundant where they live, and rare species may be spread thinly across huge areas. Animals kept in zoos or plants grown in botanical gardens elsewhere still count as endemic to their native range.
Isolation is the great engine. When populations are cut off from one another, they drift apart into separate species, a process called allopatric speciation. That is why remote islands such as Hawaii, the Galápagos and Socotra are crowded with endemics, Darwin's finches being the classic case. Isolated highlands like Ethiopia's and ancient, distant lakes like Baikal show the same pattern.
Climate history adds another route. Regions with unusually stable conditions served as refuges during ice ages, collecting species that retreated there repeatedly. Some endemics are survivors of once-wide ranges, called paleoendemics, like the ginkgo tree; others, neoendemics, are young species that have not yet spread. For conservation biologists, counting endemics helps measure a place's biodiversity and gauge extinction risk, and for evolutionary biologists, they reveal how environmental change drives range shifts, extinction and diversification.
Source: Endemism
