The boundaries of the ethical have traditionally coincided with the boundaries of humanity. This, however, is no longer the case. Scientific developments, such as genetic engineering, stem-cell research, cloning, the Human Genome Project, new paleontological evidence, and the rise of neuropsychology call into question the very notion of human being and thus require a new conceptual map for ethical judgment. The contours of this map may be seen to emerge in works of science fiction (SF), which not only vividly dramatize the implications and consequences of new technologies and discoveries, but also exert a powerful influence on culture, creating a feedback loop of images and ideas. This essay focuses on three SF topoi: the human/animal evolutionary boundary; non-biological subjectivity (AI); and the human/alien interaction. It explores each of these topoi in a selection of SF texts, including novels by H. G. Wells, Olaf Stapledon, Stephen Baxter, William Gibson, Stanislaw Lem and others, showing how the boundaries of humanity are expanded and then exploded through the radical subversion of the tenets of liberal humanism.