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The Zoo Hypothesis is an academic speculation regarding the assumed behavior of technically advanced alien civilizations.
The Zoo Hypothesis states that highly advanced extraterrestrial life exists yet does not contact life on Earth in order to allow for its natural evolution and development. It thus seeks to explain the apparent absence of extraterrestrial life despite its generally accepted plausibility and hence the reasonable expectation of its existence (Fermi Paradox).
Aliens might, for example, choose to allow contact once the human race has passed certain technological, political, or ethical standards. They might withhold contact until humans force contact upon them, possibly by sending a spacecraft to planets they inhabit. Alternatively, a reluctance to initiate contact could reflect a sensible desire to minimize risk. An alien society with advanced remote-sensing technologies may conclude that direct contact with neighbors confers added risks to oneself without an added benefit.
The Zoo Hypothesis assumes that aliens have great reverence for independent, natural evolution and development. A variation on this concept was introduced into public awareness in the Star Trek television series as the Prime Directive.
These ideas are perhaps most plausible if there is a relatively universal cultural or legal policy among a plurality of extraterrestrial civilizations necessitating isolation with respect to civilizations at Earth-like stages of development. In a Universe without a hegemonic power, random single civilizations with independent principals would make contact. This makes a crowded Universe with clearly defined rules seem more plausible.
If there is a plurality of alien cultures, however, this theory may break down under the uniformity of motive concept because it would take just a single extraterrestrial civilization to decide to act contrary to the imperative within our range of detection for it to be abrogated, and the probability of such a violation increases with the number of civilizations. This idea, however, becomes more plausible if all civilizations tend to evolve similar cultural standards and values with regard to contact much like convergent evolution on Earth has independently evolved eyes on numerous occasions, or all civilizations follow the lead of some particularly distinguished civilization . . . the first civilization.
With this in mind, a modified Zoo Hypothesis becomes a more appealing answer to the Fermi Paradox. The time between the emergence of the first civilization within the Milky Way and all subsequent civilizations could be enormous. Monte Carlo simulation shows the first few inter-arrival times between emergent civilizations would be similar in length to geologic epochs on Earth. Just what could a civilization do with a ten-million, one-hundred-million, or half-billion-year head start?
Even if this first grand civilization is long gone, their initial legacy could live on in the form of a passed-down tradition. Beyond this, it does not even have to be the first civilization, but simply the first to spread its doctrine and control over a large volume of the galaxy. If just one civilization gained this hegemony in the distant past, it could form an unbroken chain of taboo against rapacious colonization in favour of non-interference in those civilizations that follow. The uniformity of motive concept previously mentioned would become moot in such a situation.
If the oldest civilization still present in the Milky Way has, for example, a 100-million-year time advantage over the next oldest civilization, then it is conceivable that they could be in the singular position of being able to control, monitor, influence or isolate the emergence of every civilization that follows within their sphere of influence. This is analogous to what happens on Earth within our own civilization on a daily basis, in that everyone born on this planet is born into a pre-existing system of familial associations, customs, traditions and laws that were already long established before our birth and which we have little or no control over.
Appearance in fiction 
- In Olaf Stapledon's 1937 novel Star Maker, great care is taken by the Symbiont race to keep its existence hidden from "pre-utopian" primitives, "lest they should lose their independence of mind". It is only when such worlds become utopian-level space travellers that the Symbionts make contact and bring the young utopia to an equal footing.
- Arthur C. Clarke's The Sentinel (first published in 1951) and its later novel adaptation 2001: A Space Odyssey (1968) feature a beacon which is activated when the human race discovers it on the moon. An alien race has apparently visited us in the distant past.
- In Childhood's End, a novel by Arthur C. Clarke published in 1953, the alien cultures had been observing and registering the Earth's evolution and human history for thousands (perhaps millions) of years. At the beginning of the book, when mankind is about to achieve spaceflight, the aliens reveal their existence and quickly end the arms race, colonialism, racial segregation and the Cold War.
- In Star Trek (1966), the Federation (including humans) has a strict Prime Directive policy of nonintervention with less technologically advanced cultures which the Federation encounters. The threshold of inclusion is the independent technological development of faster-than-light propulsion. In the show's canon the Vulcan race limited their encounters to observation until humans made their first warp flight, after which they initiated first contact.
- In Julian May's 1987 novel Intervention, the five alien races of the Galactic Milieu keep the Earth under surveillance, but do not intervene until humans demonstrate mental and ethical maturity through a paranormal prayer of peace.
- In Robert J. Sawyer's SF novel Calculating God (2000), Hollus, a scientist from an advanced alien civilization, denies that her government is operating under the prime directive.
- In "Star Wars: Return of the Jedi", Endor is described as the Sanctuary Moon due to its protected status. The Galactic Republic set it up as a nature reserve until such time that the native species attained the ability of space travel.
- Ball, John A. (Jul 1973). "The Zoo Hypothesis". Icarus 19 (3): 347–349. Bibcode:1973Icar...19..347B. doi:10.1016/0019-1035(73)90111-5.
- Soter, S. (2005). Astrobiol. Mag. 17 Oct "SETI and the Cosmic Quarantine Hypothesis"
- Crawford, I.A., "Where are They? Maybe we are alone in the galaxy after all", Scientific American, July 2000, 38–43, (2000).
- Kozmik, Z.; Ruzickova, J.; Jonasova, K.; Matsumoto, Y.; Vopalensky, P.; Kozmikova, I.; Strnad, H.; Kawamura, S. et al. (Jul 2008). "Assembly of the cnidarian camera-type eye from vertebrate-like components". Proceedings of the National Academy of Sciences of the United States of America 105 (26): 8989–8993.
- Bracewell, R. (1982). Pre-emption of the Galaxy by the First AdvancedCivilization, Pergmon Press, Oxford.
- Kardashev scale Kardashev, N.S. (1964). Soviet Astronomy. 8, 217
- Hair, T. W. (2011). "Temporal dispersion of the emergence of intelligence: An inter-arrival time analysis". International Journal of Astrobiology 10 (2): 131. doi:10.1017/S1473550411000024.