13-20,000 years 13-20,000 years National Geographic National Geographic article Oldest Burial Yields DNA Evidence of First Americans 13-20,000 years Pleistocene child graves Pleistocene child graves Oldest Burial Yields DNA Evidence of First Americans Pleistocene child graves Oldest Burial Yields DNA Evidence of First Americans Oldest Burial Yields DNA Evidence of First Americans Anzick Clovis site 13-20,000 years National Geographic article