1. The first Americans came from Asia between 20,000-20,000 years ago during the last Ice Age when lower sea levels exposed a land bridge connecting Asia and North America. They followed animal herds as nomads across this land bridge.
2. Over thousands of years, groups of humans migrated throughout North and South America and developed different cultures based on their environments and available natural resources. These early peoples were the ancestors of Native Americans.
3. Archaeologists study artifacts left by early Native Americans to learn about the cultures since they did not have written records.