Foster: Many species of extremely large North American mammals became extinct during the last ice age, which was also the time of the first human migration to North America.
Conclusion
The extinctions of super large North American mammals during the last ice age was caused by the dramatic shift to a harsher climate.
Evidence
The climate changed so rapidly that the species could not adapt.
Evaluate
Since Foster and Fisch are trying to Solve the Causal Mystery of what caused the extinction of the large NA mammals at the end of the last ice age, we can use our Explain Curious Fact mindset.
Curious Fact Why did super large mammals go extinct at the end of the last ice age?
Fisch's Explanation The dramatic shift to a harsher climate did it. The climate changed more rapidly than these large species could adapt to it, so they eventually died off from challenges dealing with this new climate.
Goal
We either want an answer to rule out an alternate explanation for the extinctions (such as ruling out that the idea that the presence of humans caused the extinctions), or more likely we want an idea that bolsters the plausibility of the author's explanation.
The most common way to Strengthen an Explain Curious Fact argument is to provide an answer that shows . So we might see something like,