"I am the last pure human!" so claims Lady Cassandra O'Brien.Δ17 in the second episode of the Doctor Who reboot. In the episode, "The End of the World," the Doctor takes Rose on her first adventure: five billion years into the future where well-to-do beings have been invited to witness the destruction of Earth. One of the prominent guests is Lady Cassandra, who has become little more than a face and stretched skin. Interestingly, Cassandra's claim relies on racial, genetic purity despite her extensive body modification. She rails, "The others mingled. Oh, they call themselves new humans and proto-humans and digi-humans—even human-ish. But you know what I call them? Mongrels!"
Lady Cassandra dramatizes Doctor Who's anxieties over what it means to be (post)human in what Cary Wolfe defines as the “historical moment in which the decentering of the human by its imbrication in technical, medical, informatic, and economic networks is increasingly impossible to ignore” (xv). On the one hand, Cassandra's (and other characters') desire to remain "pure" and not already transformed by technology or radical difference is challenged by the show's embraces a wide range of alien, hybrid, and technologically-enhanced characters and worlds--from the Ood to Star Whales to the Face of Bo, from The Library to Planet Midnight to Other Earth. On the other hand, though, there is clearly a desire to preserve and defend that which is "purely" human, that which is always under threat by technology and things alien. It is no accident that the greatest threat to humanity is the corruption of humanity by the post- and nonhuman--Autons, Cyberman, Daleks, Human-Daleks, Gangers, Smilers, Silurians, the Silence, and the Waters of Mars--to name a few. In fact, humanity's fear of losing itself results in the founding of the Torchwood Institute (and spinoff series) to combat things alien and posthuman.
At the above episode's conclusion, Cassandra is defeated, destroyed by her own posthuman vanity, and Rose laments, "The end of the Earth. It's gone. We were too busy saving ourselves. No one saw it go. All those years, all that history, and no one was even looking." This anxiety over saving the self and the past, present, and future of humanity is a central preoccupation of the series. Even the Doctor himself struggles with his own alien- and human-ness--his two beating hearts--recognizing the ambivalent and radical potential and peril of the posthuman.