Complex sentence: Originally Seine-et-Marne was very rural, but the population has tripled in 50 years, a key reason being the development of the Paris conurbation and the building of new towns.
Split into simpler sentences: Originally Seine-et-Marne was very rural and lightly populated. Over the past 50 years, however, its population has tripled, due to the development of the Paris conurbation and the building of new towns.