Complex sentence: This river was known as the Astura to the Ancient Romans, from which the Asturian people took their name, the ancestors of the modern inhabitants of Asturia and León, living on both sides of the Cantabrian Mountains.
Split into simpler sentences: This river was known as the Astura to the ancient Romans, from which the Asturian people took their name. They were ancestors of the modern inhabitants of Asturia and León, living on both sides of the Cantabrian Mountains.