In many of the areas presently occupied by European cities, settlements were formed already in Neolithic times, when the continent was colonized by agri culturalists (9500 B.C. onwards). The re-colonization of European plants and animals after the last Ice Age, which covered large areas of Europe, was not completed before human infl uence began to cause local disturbances, meaning that the native biodiversity has evolved under human infl uence. The long history of urban development in Europe, and the location of cities in fertile river valleys, are at least two reasons of why many European cities are often characterized by higher species richness of plants and animals than some of the surrounding rural areas. The long history of co-evolution may be a particular factor explaining why European plants and animals worldwide tend to successfully establish in areas with dense human population.