Complex sentence: Though most famous for Christian burials, either in separate catacombs or mixed together, they began in the 2nd century, Toynbee: 39-40 much as a response to overcrowding and shortage of land.
Split into simpler sentences: Though most famous for Christian burials, either in separate catacombs or mixed together. They began in the 2nd century, Toynbee: 39-40 much as a response to overcrowding and shortage of land.