"Doctor, I don't like this." Clara said, grabbing his hand. "Maybe we should go back to the TARDIS."
"Clara, you've seen stranger! Remember Akahten? You weren't frightened by strange looking people then. We didn't go back to the TARDIS then and we shan't now. Look, there's a little shop. I love a little shop, don't you? Lets go buy something. You can do it." The Doctor squeezed her hand to comfort her, then led her through the crowd toward the little shop.
"But this is Earth, and these things aren't humans. I don't like it." Clara continued as the Doctor dragged her along. "They're using human buildings, human inventions, but they're not humans. They've stolen our planet."
"Oh, Clara." The Doctor stopped and turned to face her. "Listen. Over the course of time, the human race turns to the stars because they cannot fit on their planet anymore. They spread throughout the galaxies, and of course, they…intermingle. And are rather prolific. Each of these 'things' has probably got some human in them. Maybe some of them are your friends' so-many-great-grandchildren. You don't need to be scared. They know what humans looked like, and when they look at you and I, they'll think highly of your parentage because you look so purely human. These Earth dwellers will look up to you Clara. You've got nothing to worry about you beautiful human, you." He looked in her eyes in his searching way, serious, for a moment. Then his countenance changed and he grinned like a child. "Now lets go look at the little shop!"