Simplify the following text for a Grade 9 native English-speaking person.
Original: It is believed that a small human population of at most a few thousand survived the Last Glacial Maximum in Beringia , isolated from its ancestor populations in Asia for at least 5,000 years , before expanding to populate the Americas sometime after 16,500 years ago , during the Late Glacial Maximum as the American glaciers blocking the way southward melted .
Simplified: It was isolated from populations in Asia for at least 5,000 years . Sometime after 16,500 years ago , it started to populate the Americas when the American glaciers that blocked the way southward melted .