Passage Summary: We survived because our ancestors were willing to sacrifice for their families; since that sacrifice is a form of altruism, our ancestors must have been altruistic.

Conclusion: Human ancestors possessed at least some degree of altruistic motivation.

Reasoning: Human survival depended on ancestors sacrificing themselves for the sake of their relatives, and such sacrifice is defined as a type of altruism.

Analysis: This argument uses a valid deductive structure: it identifies a specific behavior (kin sacrifice) as a necessary condition for an outcome (survival), then notes that this behavior belongs to a broader category (altruism). To find a parallel, look for an argument that concludes a general trait must have existed because a specific sub-type of that trait was necessary for a known result.