The idea that early Americans had moved southward along the Pacific Coast, using boats or coastal terrain to navigate around the edge of the glacial ice, explained how people could have been in South America before the interior corridor was usable.

The kelp highway hypothesis gave the coastal migration a specific ecological logic: the kelp ecosystems of the North Pacific coast were among the most productive on earth, providing the marine resources that a maritime people could live on while moving southward.

But the direct evidence for coastal migration was limited, partly because the coastline of the relevant period was now largely underwater.

At the same time, the morphological debate about ancient American skulls was generating headlines that went beyond mainstream archaeological opinion.