Complex sentence: The Meadowcroft Rockshelter contains the earliest known signs of human activity in Pennsylvania, and perhaps all of North America, as it contains the remains of a civilization that existed over 10,000 years ago and possibly pre-dated the Clovis culture.
Split into simpler sentences: The Meadowcroft Rockshelter contains the earliest known signs of human activity in Pennsylvania, and perhaps all of North America. The site contains the remains of a civilization that existed over 10,000 years ago and possibly pre-dated the Clovis culture.