The time-traveler hypothesis, also known as chrononaut UFO, future humans, extratempestrial model and Terminator theory is the proposal that unidentified flying objects are humans traveling from the future using advanced technology. Some notable people have given recent public exposure to the hypothesis, such as retired NASA aerospace engineer Larry Lemke, Wisconsin congressman Mike Gallagher, and American filmmaker Steven Spielberg.

The time-traveler hypothesis is considered extremely implausible by mainstream scholars and is generally regarded as unorthodox even among UFO theorists who argue that UFOs are extraterrestrial spacecraft or interdimensional phenomena.^{[citation needed]}

The notion of time travel from the future to the past is thought to have been introduced for the first time in literature by French botanist and geologist Pierre Boitard in his popular 1861 book Paris avant les hommes (Paris before Men), featuring a man sent back to prehistoric Earth where he interacts with an ape-like ancestor. A few years later, in 1887, Camille Flammarion published Lumen, a novel featuring an alien soul traveling through different worlds and historical periods.

Although traveling back in time is almost-universally considered physically impossible, theoretical research has explored whether it might be possible. It has also been speculated by scientists such as J. Richard Gott, Ronald Mallet and others that humans may someday be able to time travel to the past.

Many authors and researchers have speculated about the future evolution of mankind. In 1888 English author H. G. Wells published his famous science-fiction novella The Time Machine featuring a future evolution of humans and is generally regarded as one of the first modern instances of both time travel and speculative evolution. Other writers on speculative evolution include Edgar Rice Burroughs, British philosopher and science fiction writer Olaf Stapledon with his 1930 "future history" novel Last and First Men, American author Kurt Vonnegut in Galápagos, British writer Stephen Baxter in Evolution, Turkish artist and author Cevdet Mehmet Kösemen in All Tomorrows and others. Climate change, the technological singularity, space travel and genetic enhancement have also been identified as potential factors capable of altering the evolution of humans in the future, leading to different transhuman and posthuman scenarios.

Human beings have reported unknown objects in the skies since antiquity. In the 1890s, a wave of mystery airships sightings was reported throughout the US, followed by Foo fighters and ghost rockets during World War II.

The modern UFO era began in the Summer of 1947, days after the United States announced plans to re-industrialize Germany over strenuous Soviet objection, sparking the Cold War. Amid fears of a Soviet response, civilian pilot Kenneth Arnold reported witnessing a formation of supersonic aircraft. Wartime prohibition of live "man on the street" broadcasts having ended, Arnold's sighting was covered nationwide on radio and print media. In the days and weeks following the Arnold sighting, a nationwide craze swept the nation as over 800 copycat sightings were published in the English-speaking press. 1947 sources speculated that the sightings resulted from novel US or Soviet technology, misidentification of mundane objects, or behavioral effects like mass hysteria; Fringe sources suggested that the sightings were a sign of the end of the world, were angelic or demonic, or were vehicles carrying people from other planets or other "dimensions".

Contactee and Alien Abduction folklore often describe encounters with human or humanoid figures and abductors. In his 1964 article The Nonprevalence of Humanoids George Gaylord Simpson claimed that it is extremely unlikely that there exists any form of extraterrestrial intelligence in the Solar System or elsewhere and that, even if they did exist, it was even more unlikely that they would be humanoids. His article was cited and analysed, alongside evolutionary biologist Theodosius Dobzhansky’s similar view on the implausibility of the evolution of humanoid extraterrestrial life forms, in the famous 2014 collection of essays Archaeology, Anthropology and Interstellar Communication edited by astrobiologist Douglas Vakoch and published by NASA in 2014. In the late 1980s, evolutionary biologist and historian of science Stephen Jay Gould put forth his famous thought experiment that if evolution was rewound and played back again it would likely take a very different course and humans would never evolve, which in addition to his declared view against human-oriented evolutionary teleonomy, helped shaped the established view that if intelligent extraterrestrials did exist, they would likely not resemble humans. In their book Evolving the Alien biologist Jack Cohen and mathematician Ian Stewart assumed a similar position about the iconic representation of the Grey alien from reports of UFO abduction and close encounters, stating that extraterrestrials from an alien world could not have evolved a physiology so similar to humans'.