Human extinction is the hypothetical end of the human species due to either natural causes such as population decline from sub-replacement fertility, an asteroid impact, large-scale volcanism, or to anthropogenic (human) causes. For the latter, some of the many possible contributors include climate change, global nuclear annihilation, biological warfare, and ecological collapse. Other scenarios center on emerging technologies, such as advanced artificial intelligence, biotechnology, or self-replicating nanobots. The scientific consensus is that there is a relatively low risk of near-term human extinction due to natural causes. The likelihood of human extinction through humankind's own activities, however, is a current area of research and debate. History of thought Early history of thinking about human extinction Before the 18th and 19th centuries, the possibility that humans or other organisms could become extinct was viewed with scepticism. It contradicted the principle of plenitude, a doctrine that all possible things exist. The principle traces back to Aristotle, and was an important tenet of Christian theology. Ancient Western philosophers such as Plato, Aristotle, and Lucretius wrote of the end of humankind only as part of a cycle of renewal. Later philosophers such as Al-Ghazali, William of Ockham, and Gerolamo Cardano expanded the study of logic and probability and began discussing abstract possible worlds, including a world without humans. Physicist Edmond Halley stated that the extinction of the human race may be beneficial to the future of the world. The notion that species can become extinct gained scientific acceptance during the Age of Enlightenment in the 17th and 18th centuries, and by 1800 Georges Cuvier had identified 23 extinct prehistoric species. The doctrine was further gradually undermined by evidence from the natural sciences, particular the discovery of fossil evidence of species that appeared to no longer exist, and the development of theories of evolution. In On the Origin of Species, Darwin discussed the extinction of species as a natural process and core component of natural selection. Notably, Darwin was skeptical of the possibility of sudden extinctions, viewing it as a gradual process. He held that the abrupt disappearances of species from the fossil record were not evidence of catastrophic extinctions, but rather were a function of unrecognised gaps in the record. As the possibility of extinction became more widely established in the sciences, so did the prospect of human extinction. In the 19th century, human extinction became a popular topic in science (e.g., Thomas Robert Malthus's