A professor brings his students to a museum housing "The Hall of the Last Ones," a gathering of endangered species. He shows them animals like the last elephant and the last fox, explaining why science is unable to help them. He also unveils a terrifying example of cross-breeding gone mysteriously wrong.
Finally, he comes upon a dead subject whose demise saddens him. He explains to his young students that when he and his fellow aliens conquered the Earth, they tried their best to preserve all the animal species.
This subject was the last human being.
He allows himself a moment of sadness, then suggests they all have lunch.