Antihumanism is a relatively new philosophy, which emerged along with critique of industrialism in the modern era, particularly after WWI. Antihumanism gathered strength from the nihilism of post-1960s counterculture and is now evolving rapidly, so there's already a wide spectrum. At the moderate end are mainstream novelists such as Kurt Vonnegut ("Breakfast of Champions"), Margaret Atwood ("Oryx and Crake"), and Paul Theroux ("O-Zone"). All of these books contain antihuman concepts and observations, though their authors probably wouldn't use the word. There are also many antihuman movies, too many to list, but at a minimum "Eraserhead" by David Lynch, "THX-1138" by George Lucas, and "Soylent Green" should be mentioned. At the extreme end are actual organizations such as The Church of Euthanasia, VHEMT (Voluntary Human Extinction Movement), and GLF (Gaia Liberation Front).
In the sciences antihumanism is usually expressed by paleontologists and biologists, and increasingly by climate scientists. Some current examples are Jeremy Jackson and Kevin Anderson (see links below). Edward O. Wilson is best known for his work on biological diversity, but he was also the first biologist to seriously propose that intelligence snuffs itself out, and that this solves Fermi's paradox: we don't receive messages from the stars because by the time an alien life form has enough power to transmit that far, it's already on the threshold of annihilating itself, and the odds of its brief blaze of glory lining up with ours are infinitesimal. This is closely related to the view that life (particularly human life) creates short-term order at the cost of accelerating the entropy of its environment, in stark contrast to the idealistic Gaia theory. For example paleontologist Peter Ward's "Medea Hypothesis" demolishes the notion that life is self-regulating, and compares life to a drunk stumbling around in a darkened room.
Antihumanism can be usefully contrasted with humanism. Humanism derives from the ancient Greek notion that man is the measure of all things, and that without human existence nothing would have value. Concealed within this is the assumption that only humans experience value. This assumption has no basis in biology, but is nonetheless one of the pillars of modern civilization, because it provides justification for extermination of other species. The denial of intrinsic value to non-human life is the essence of speciesism, and is closely related to the dogma of dominion, i.e. that it's man's destiny to subjugate all other living things (a concept that Edward O. Wilson attacked in his "Consilience").
Beyond humanism is transhumanism, sometimes known as futurism or extropianism. This is the belief that not only is man the measure of all things, but the only part of him that matters is his mind, and the sooner his mind is freed from the limitations of biology the better. The moderate form is life extension and cryogenics, while the extreme form is downloading human intelligence into robots and conquering outer space, like the Daleks on Dr. Who. Famous transhumanists include Ray Kurzweil, and Stephen Hawking who recently stated that humanity's only hope is to escape to other planets before we destroy this one. Antihumanists regard transhumanists as archenemies, due to their flagrant unconcern for non-humans. From the antihuman point of view, transhumanism bears a striking resemblance to Christianity. Both are escapist, characterized by unshakable belief that humans belong somewhere else, i.e. Heaven/Outer Space. Both express contempt for biology, e.g. Catholic repression of sexuality, and transhumanist use of derogatory terms such as meatspace. Both are motivated by fear of death, and presumably of life too, since one engenders the other (literally via natural selection). Both reject the limits of existence on earth, and promulgate a fantasy that justifies exceeding those limits. The danger isn't that the fantasy will be realized, but that deluded people will make earth unsuitable for life far sooner than would have otherwise been the case.
Unlike mere misanthropy, antihumanism is distinguished by reverence for non-human life. Biological diversity is considered an axiomatic value, and contrasted with the ugliness and sterility of human monoculture. Earth is described as a "wrecked planet" (Kurt Vonnegut), and various measures are called for to prevent further damage, the most obvious being drastic reduction or elimination of the human population. The pre-human fecundity of earth is idolized, and provides a reference for demonstrating impoverishment of ecosystems. This relates to the shifting baseline syndrome posited by Jeremy Jackson and others, in which each successive generation wrongly assumes the degree of biological diversity they observe was also seen in previous generations.
The central paradox of antihumanism is that humans evolved, and are therefore no more or less natural than any other living thing. Stephen Jay Gould argued convincingly that evolution doesn't converge on anything except fitness for conditions: there are no good or bad organisms, just ones that survive, and mostly ones that don't. Richard Dawkins went even further and described organisms as mere transport for genes, in which case the DNA we share with all other eukaryotes is the winner, regardless of what humans do. One proposed resolution is that humans are malignant life, as argued by A. Kent MacDougall in "Humans As Cancer". This sidesteps the problem however, because cancer is also natural, and closely related to viruses. The higher-order question is ethical: why is malignancy bad, and from what point of view is its badness determined?
The paradox of human naturalness could possibly be resolved by arguing that sentience is not intelligence but the ability to feel pain and pleasure. What distinguishes humans from other primates is the existential suffering that results from self-knowledge, particularly fear of death. Since humans have such capacity for suffering, we should have equally developed empathy, but instead we succumb to corruption, creating hellish conditions for humans and non-humans alike. Thus despite our naturalness, humans can and should be blamed for wrecking the planet, precisely because we're capable of feeling remorse for having done so. If we're unable to reform ourselves, as seems increasingly to be the case, we should have the decency to step aside and give other organisms a chance. Apes might re-evolve back into us, but they might not, and either way it won't be our fault.
Dan Miller: "A REALLY Inconvenient Truth"
Peter Ward (The Medea Hypothesis):
Professor Kevin Anderson, Tyndall Centre for Climate Research - Climate Change: Going Beyond Dangerous
Brave New Ocean - Jeremy Jackson
Church of Euthanasia
The Voluntary Human Extinction Movement