The argument doesn’t present any necessary conditions for the truth of the conclusion. The premises tell us specific ways in which taste can help humans stay healthy, but these specific ways aren’t necessary for the conclusion to be true.

This possibility doesn’t affect the argument. Whether people “associate” foods more with smells doesn’t reveal anything about the purpose of the ability to distinguish different tastes.

This possibility doesn’t weaken the argument. Foods might change taste after being cooked, and that might reveal something about the healthfulness of the food after being cooked.

The argument doesn’t make any assumptions about the range of foods eaten by humans. The argument is simply about the purpose of distinguishing tastes. We know early humans could distinguish between certain tastes; the range of their diet doesn’t affect this.

Taste helping to detect healthfulness of foods is a partial explanation of human’s ability to distinguish tastes. But the author assumes it’s a complete explanation. This overlooks the possibility there could be other reasons humans can distinguish tastes.