Some time after the Great Flood it says in Genesis 10:25 that in the days of Peleg there was a great division, which would indicate that the continents were divided. We would imagine that before this division the wild life would probably scatter across the earth and reach the area that is now the western hemisphere. However when it came to domestic animals, this would be less likely. Notice that it is just domestic flocks and domestic fowls and freshwater fish that the Jaredites were told by the Lord to take with them to replenish the promised land. The wild life had apparently reached the promised land already.