Final answer:
Scientists who disagree with the land-bridge migration theory propose alternative routes such as the coastal migration theory and the Solutrean hypothesis.
Step-by-step explanation:
Scientists who disagree with the land-bridge migration theory propose alternative routes that early Americans may have followed. One such theory is the coastal migration theory, which suggests that early humans arrived in the Americas by following the coast of land across Asia and the Bering Land Bridge, down the coast of North America, and all the way to South America. This theory is supported by archaeological sites in South America that predate the North American Clovis sites. Another controversial theory is the Solutrean hypothesis, which argues that the first humans of the Americas descended from the Solutrean culture of Europe and arrived through coastal migration along the ice sheet that spanned the ice age Atlantic. However, genetic studies show evidence of multiple migrations from Asia into the Americas, making the Solutrean hypothesis unlikely.