Two thousand years ago the population of Belgium, as mentioned by Julius Caesar in his book on the Gallic wars, was of Celtic stock. This population was displaced or lost its identity, however, during the great invasions that brought down the Roman Empire. The Salian Franks, who settled there during the 4th century AD , are considered the ancestors of Belgium's present population. The origin of the language frontier in Belgium has never been satisfactorily explained. In the indigenous population, the ratio of Flemings (Dutch speakers) to Walloons (French speakers) is about 5 to 3. In 2002, the Flemings constituted about 58% of the total population; Walloons accounted for 31%. The remaining 11% was comprised of those with mixed ancestry or other groups.