Why did it take so long for early people to reach south america

  When the early humans who would become the Native Americans entered America they came from a land bridge that streched from Modern day Russia to Alaska. From that point they began to spread to the South and East. It takes time to cover that distance on foot and they weren't just looking for that continent. They were focused on survival so their goal wasn't to just get to South America. Eventually they began migrating so they could follow the animals that they needed to survive and it led them towards south America. Most Native Americans would either continue to head south or they would begin to head East and they set up in the land that would become the southern US.