Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska, is known to be 11,200 years old. Researchers reasoned that, since glaciers prevented human migration south from the Alaska–Siberia land bridge between 18,000 and 11,000 years ago, humans must have come to the Americas more than 18,000 years ago. The argument above relies on which of the following assumptions?The argument says humans were already in Colorado 11,200 years ago, but migration south from the Alaska-Siberia land bridge was blocked from 18,000 to 11,000 years ago. So the researchers conclude that humans must have arrived before 18,000 years ago.
The key assumption is that humans who reached the Americas before 11,000 years ago had to come through that land bridge route, not by some
different route.
A. The earliest controlled use of fire on the American continents occurred around 11,200 years ago.
This is not required. The argument only needs the Colorado hearth to show human presence by 11,200 years ago.
B. Any humans who came to the Americas more than 11,000 years ago came via the Alaska-Siberia land bridge.
This is correct. If humans could have come to the Americas by some route other than the land bridge, then the glacier blockage of that route would not prove that they arrived before 18,000 years ago.
C. The Alaska-Siberia land bridge was the result of the uptake of seawater by the continental glaciers.
This is irrelevant. The cause of the land bridge does not affect the migration timing argument.
D. Early human inhabitants of the Americas were hunters whose diet consisted primarily of meat, rather than gatherers who subsisted on fruit and seeds.
This is irrelevant. The argument is about when humans arrived, not what they ate.
E. Early humans tended to migrate to warmer climates, even those who were accustomed to living in cold, harsh climates.
This is not required. The argument depends on route and timing, not on general climate preferences.
Answer: (B)