Passage Summary: We haven't tamed any new big animals in ages, even though we probably tried to tame everything that looked useful. Because of this, the animals left in the wild must be either too mean to tame or just useless to us.

Conclusion: Most wild large mammal species remaining today are likely either too difficult to domesticate or not worth the effort.

Reasoning: All currently domesticated large mammals were tamed thousands of years ago, and humans have likely tried to domesticate every wild species that seemed useful since then.

Analysis: The argument assumes that our ancestors were successful whenever they encountered a 'worthwhile' and 'tameable' animal. If there were species that were easy to tame and very useful, but humans simply failed to domesticate them due to bad luck or geographic isolation, the conclusion would be weakened. Therefore, the argument requires the assumption that if a species was both worth domesticating and capable of being domesticated, humans would have successfully done so by now.