See Omni 1:12-19. When Mosiah I led the more righteous portion of the Nephites northward from the land of Lehi-Nephi (about 200 B.C.) he found on the west bank of the river Sidon a city inhabited by a people whose language he could not understand. They were ruled by a king named Zarahemla.
When the two races developed the ability to communicate, it was discovered that the people of Zarahemla were the descendants of a colony which had been led by the Lord out of Jerusalem in the year when that city was destroyed by the king of Babylon (587 B.C.).
After wandering in the wilderness they were brought across the ocean and landed in the Americas. In the years that followed, they migrated southward to the place where they were found by Mosiah. Because they had brought no scriptural or written records with them their language, society, and spiritual nature had all devolved or become corrupted.