Complex sentence: Due to fluctuating sea levels, mainland Eurasia was not separated completely from the western part of the Indonesian archipelago, allowing animals from the Eurasian mainland passage to the archipelago, and new species evolved.
Split into simpler sentences: Due to fluctuating sea levels, mainland Eurasia was not separated completely from the western part of the Indonesian archipelago. As a result, animals from the Eurasian mainland could travel to the archipelago, and new species evolved.