Mew is generally considered the 'Ancestor' of most Pokemon (barr stuff like Arceus xD)
Around this time, Mew was thought to have came into being by itself, containing the DNA of every Pokémon to exist and exist in the future.
That quote is from Sinnoh Region on Bulbapedia which can be found here
Since many scientists believe that Mew is the ancestor of all Pokémon, and a number of extraterrestrial Pokémon have been discovered, it is possible that Mew could inhabit other areas of the universe, and whether it originates from Earth at all is debatable.
And that is from Mew's page on Bulbapedia which can be found here