Passage Summary: A researcher argues that guinea pigs aren't actually related to other rodents because their DNA is so different from rats and mice, even though they look like rodents.

Conclusion: Guinea pigs do not share a common ancestor with rats and mice, despite their similar physical appearances.

Reasoning: Genetic comparisons show that the differences between guinea pigs and mice are as significant as the differences between mice and entirely different orders of mammals.

Analysis: The researcher's argument relies on the assumption that a high degree of genetic divergence necessarily implies a different evolutionary origin. To weaken this, we should look for an answer that suggests genetic differences can be misleading. For instance, if guinea pigs underwent a period of extremely rapid evolution, they could be very genetically distinct from their cousins despite sharing the same ancestor. Alternatively, if the 'nonrodent' mammals used for comparison are actually more closely related to rodents than previously thought, the researcher's benchmark for 'great difference' would be undermined.