Complex sentence: In 1501 Martyr, at the urgent request of the Cardinal of Aragon, had added to these eight chapters on the voyage of Columbus and the exploits of Martin Alonzo Pinzón, and in 1511 he added a supplement giving an account of events from 1501 to 1511.
Split into simpler sentences: In 1501 Martyr, as requested by the Cardinal of Aragon, added eight chapters on the voyage of Columbus and the exploits of Martin Alonzo Pinzón. In 1511 he added a supplement giving an account of events from 1501 to 1511.