Broad consensus exists that humans initially migrated into the Americas via a land bridge that temporarily formed between Siberia and Alaska. But experts believe their southward progress was then blocked by the Cordilleran Ice Sheet, one of two continental ice sheets that covered much of northern North America during the Quaternary glacial cycles. Genetic evidence indicates that migrations south of the ice sheet began roughly 16,000 years ago, but the path the nomads took around the retreating ice remains uncertain.
According to one hypothesis, the earliest migrants traveled along an interior corridor that opened between the receding Cordilleran and another, even larger ice sheet to the east. But recent evidence indicates this route may not have hosted enough resources for humans to cross it until about 13,000 years ago. A competing hypothesis proposes that the first humans migrated south along British Columbia’s west coast. However, it’s unclear exactly when this route became viable.
Darvill et al. present a dating chronology that provides new geological constraints on the timing of Cordilleran Ice Sheet retreat. The researchers gathered 32 samples from seven locales along British Columbia’s central coast, a crucial portion of the proposed coastal migration route. The team used beryllium-10 cosmogenic nuclide exposure dating to estimate the length of time rocks have been exposed on Earth’s surface since the last deglaciation.
Their results include data from bedrock, moraine, and glacial erratic samples and indicate that the ice sheet’s westernmost margin began retreating about 18,100 years ago. This timing is earlier than previously thought and earlier than the time at which the ice sheet’s southern lobes reached their maximum extents. Because the new chronology suggests that at least some portions of British Columbia’s west coast were ice free well before the interior corridor became viable and also more closely coincides with the available genetic evidence, this study bolsters the argument that the first humans migrated into the Americas along a coastal route. (Geophysical Research Letters, https://doi.org/10.1029/2018GL079419, 2018)
—Terri Cook, Freelance Writer
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