Simplify the following text for a Grade 8 native English-speaking person.
Original: The Great American Interchange was an important paleozoogeographic event in which land and freshwater fauna migrated from North America via Central America to South America and vice versa , as the volcanic Isthmus of Panama rose up from the sea floor and bridged the formerly separated continents .
Simplified: The volcanic Isthmus of Panama rose up from the sea floor and bridged the two continents .