Passage Summary: Because our ancestors used taste to tell if food was rotten, poisonous, or healthy, the author thinks that's the only reason we have those taste abilities today.

Conclusion: The ability of humans to distinguish the four primary tastes is entirely explained by their historical use in testing food safety and nutrition.

Reasoning: Early humans used sour and bitter tastes to identify spoilage and poison, while sweet and salty tastes helped them identify necessary nutrients.

Analysis: The flaw in this argument is one of over-extrapolation, specifically moving from a partial explanation to a total one. While the author provides a very plausible evolutionary benefit for these tastes, they conclude that this history 'completely explains' why we have them. This ignores the possibility that other biological or environmental factors played a role in the development of these senses. Look for an answer choice that criticizes the author for assuming that a single functional purpose accounts for the entire existence of a complex biological trait.