University of Copenhagen researchers Eske Willerslev, Mikkel Pedersen, and their colleagues found that this harsh route only became viable for human migration 12,600 years ago—when the first plants and animals showed up in the region. Meanwhile, archaeologists have ample evidence that people were living in the Americas long before then. “We know conclusively that human groups were in the interior before that date—perhaps as early as 15,000 calibrated radiocarbon years before present—so it is highly unlikely that they came south through the corridor,” said Michael O’Brien, an anthropologist and current academic vice president of Texas A&M University–San Antonio, who wasn’t...