The leading theory about how early humans arrived on the American continent is that they migrated from Asia to North America over a land bridge known as Beringia. This land bridge existed during the last Ice Age when sea levels were lower, exposing a connection between northeastern Asia (what is now Siberia) and northwestern North America (what is now Alaska). This migration likely occurred around 15,000 to 20,000 years ago, as humans began to populate the Americas.
The other options you've mentioned—such as walking from Africa to South America, sailing from Australia, or canoing from Antarctica—are not supported by the current evidence and are not considered likely pathways for the initial human migrations into the Americas. The Bering Land Bridge theory remains the most widely accepted explanation among archaeologists and anthropologists.