Complex sentence: Fully documented Amerindian settlement dates to between about 350 to 650 AD, by a group known as the Saladoid-Barrancoid, who arrived from mainland South America.
Split into simpler sentences: Fully documented Amerindian settlement dates to between about 350 and 650 AD. The arrivals were a group known as the Saladoid-Barrancoid from mainland South America.