But new modeling suggests that the ancient coastline was not uniformly icebound. Instead, North America’s west coast appears to have been a complex fractal of microenvironments, including some ice-free zones along the mainland, and even more on offshore islands, which were outside the ice sheet’s reach. A coastal people would have known to look for bountiful ecologies where major rivers came to the sea. Ancient bear bones have been found near some Ice Age estuaries, suggesting rich food webs capable of supporting apex predators.

At the largest rivers, small groups might have splintered off, rowing inland where the waters were calm, and hiking along the banks of the more violent stretches. Zigzagging into the continent, through river valley after river valley, they likely would have feasted on salmon runs that had never before been fished by humans.

It’s not clear how far inland these explorers could have pushed, but archaeologists have discovered a tantalizing cave site 200 miles into the Oregon interior, where they found human feces that date back 14,300 years, when the Pacific Northwest was likely inaccessible by any inland route. At another site farther northeast, in Idaho, archaeologists dug up hearthstones, charred and cracked by campfires set more than 15,000 years ago. This location predates all Clovis sites by centuries, and it too was almost certainly inaccessible from the interior, but connects to the coast by way of the Salmon, Snake, and Columbia Rivers.