During the last ice age more than 13,000 years ago, rising temperatures began to melt the massive ice sheets that blanketed Canada and the northern United States. Their retreat opened up an ice-free corridor more than 900 miles (1,450 kilometers) long that snaked from Alaska to Montana—a gap that some archaeologists believed was humankind’s highway into the Americas.

But ancient DNA recovered from the bottom of glacial lakes reveals that the corridor didn’t become livable for humans until about 12,600 years ago, centuries after the first Americans are known to have arrived. The finding suggests that the first Americans didn’t come by land; instead, they may have come by sea, perhaps walking—and boating—their way along the Pacific coastline.

“The first human