Out-of-place biota

The history of life in the Milky Way galaxy is complex and riddled with irregularities. The most prominent of these anomalies is out-of-place biota, the unexplained presence of organisms on worlds where said organisms clearly did not originate. This phenomenon is related to, though more specific than, classical panspermia: while the panspermia hypothesis is concerned with the proliferation of microorganisms which may serve as precursors to complex life throughout space, out-of-place biota involves complex life with a clear lineage to an offworld evolutionary chain. This implies said life could not have been displaced incidentally on meteors and the like, as commonly postulated in panspermia, and would require deliberate action by technologically advanced beings to implement. Evidence gathered from different sites suggests that the ecosystem relocations may have spanned tens, potentially hundreds of millions of years, with the most recent known examples occurring between 100,000 and 200,000 years ago.

This phenomenon, which often goes by the name biodisplacement, forms its own field of study within astro- and xenobiology and astropaleontology, known as metatopetic astrobiology, or metatopetics, from the Greek metatópisi ("displacement"). It is an ever-evolving and controversial field of science, and competing schools of thought emerge and die as rapidly as new examples of the phenomenon are encountered. Most recently, the discovery of the Forerunners, the firing of the Halo Array and the Conservation Measure has shed light into some aspects of the phenomenon, though older instances of biodisplacement remain a matter of heated debate. Interactions with the Covenant meta-civilization and limited access to their worlds have also added their own pieces to the puzzle, revealing that the phenomenon to be much more widespread than humanity once thought.

Overview
Humanity first recorded the phenomenon with terrestrial biota in the early 24th century with the first on-site surveys of exoplanets, with the earliest known example being the incontrovertibly terrestrial-related biosphere of Tau Ceti's Strauss' World. Early on, some scientists suspected that Earth may have been used to seed all or most life in the galaxy, or at least Sol's local neighborhood. As understanding of the phenomenon improved, it became clear it was much more far-reaching than Earth.

While initially controversial and subject to heated debate, the preferred explanation for out-of-place biota within the scientific community is the Ancestral Displacement Theory, which posits that out-of-place organisms and ecosystems have been moved from their original evolutionary environments by one or more spacefaring civilizations (commonly referred to in the scientific community as "precursors" or "priors" ) in the distant past, perhaps for the purposes of terraforming (the Cosmic Gardeners hypothesis), preservation of unique life in the event of cosmic disasters (the Conservation hypothesis) or experimentation (the Galactic Petri Dish hypothesis). The displaced life has often diverged considerably from its terrestrial ancestors due to the passage of time and the different evolutionary pressures of alien worlds, coupled with potential genetic modification to facilitate adaptation or for other reasons. While most theories avoid undue conjecture as to the nature or motivations of the civilization responsible for this transplantation, they nevertheless pose an intriguing enigma that has been the subject of intense speculation for generations.

As centuries of research have failed to discover material artifacts, foreign organisms, or other direct and incontrovertible evidence of ancient visitors on Earth or the Sol system, it can be concluded that the priors must have deliberately avoided leaving any traces of themselves. It can be postulated that they may have wished to leave the planet of origin (in this case Earth) as undisturbed as possible so as to allow the evolution of life to take its natural course there. If they did leave any indications of their visits, they either covered them very well, or those traces were so minute as to be completely erased by the passage of time. Even prior to the discovery of Forerunner artifacts during the Human-Covenant War, mainstream scientists agreed that there were some unexplained irregularities on Earth; a handful of geological aberrations in Antarctica, Siberia and Eastern Africa, out-of-place rubble in the Yucatan peninsula, and odd metals found under the former ice sheets of Greenland. However, while potential signs of ancient visitors, these were never held to be conclusive proof thereof.

To date, no sapient species are known to have been displaced from their homeworld to thrive elsewhere, though since the Human-Covenant War, the Kig-Yar have been brought forth by some researchers as a potential example of a heavily modified and uplifted long-lost lineage of terrestrial dinosaurs due to their striking similarities. Some have also noted the Jiralhanae's superficial similarity to terrestrial apes, though subsequent genetic analysis has shown they are unrelated.

The phenomenon has also been well documented by the Covenant since their early spacefaring history. Early Sangheili explorers in particular reacted to the presence of Sanghelios life on other worlds much in the same way as humans did to terrestrial biota, i.e. viewing it as a sign of their own exceptionalism. With time, however, perspectives into the phenomenon would come to emphasize the role of the Forerunners' "divine providence" in seeding the galaxy with life. Some of the more commonly encountered examples of transplanted fauna found on multiple Covenant worlds include the rangmejo, the thorn beast and the koi-like oachi. The Sharquoi are so far the only known example of semi-sapient or protosapient being found on multiple worlds without native space travel.

As the human scientific community learned of the Forerunners as a distinct and clearly-defined civilization during the Human-Covenant War, this also made the Forerunners popular to human xenobiologists as the most likely solution to the ancestral displacement conundrum. However, as understanding of the Forerunners has grown in the post-war era, some scientists have expressed increasing skepticism at this interpretation, as some of the displaced biota must have diverged from its origins millions of years before the earliest known evidence of Forerunner activity in the galaxy. Since the Forerunners and their activities have become relatively common knowledge in the scientific community, studies of biodisplacement are now divided between the irregularities introduced by the firing of the Halo Array and the Conservation Measure, and earlier interference, Forerunner or otherwise.

Cultural impact
The phenomenon remained one of the most incontrovertible pieces of evidence for the existence of extrasolar intelligent life prior to the contact with the Covenant. While the existence of out-of-place biota has largely passed into the realm of mundane everyday knowledge as of the 26th century, it became a cultural sensation at the time of its discovery and remained at the forefront of public discourse for over a century, as human societies and religions dealt with its impact. Evidence for "ancestral meddling" grew by the day with more and more irregularities (real or imagined) being discovered as humanity's reach expanded throughout space. Early examples in particular were often subject to intense scrutiny after several hoaxes came to light, most famously a half-baked attempt by a terraforming corporation to draw interest to Roswell. Regardless, the discovery of terrestrial life on other worlds reinforced the human exceptionalist beliefs common at the time, with some even positing that an ancient civilization had paved the way for humanity's spread into the cosmos (less optimistic commentators asked the question of what payment might said civilization exact in return). The phenomenon also inspired art and entertainment, and popular staples of 24th- and 25th-century science fiction were directly influenced by it: encounters with "lost colonies" of humans, long-displaced advanced human civilizations, uplifted terrestrial fauna, or speculative depictions of the ancient civilization responsible for the transplantation.

Fringe theories
Out-of-place biota has fueled numerous conspiracy theories and cults, including the infamous Other Homeworld Theory. Its discovery also marked a resurgent interest in 's 21st-century, which contemplated the existence of divergent branches of humanity around other stars, or even terrestrial humans themselves having originally evolved elsewhere; however, the latter speculation has always been widely rejected as unscientific due to the overwhelming evidence to the contrary. However, the relatively contemporaneous discovery of the — a peculiar discontinuity in the fossil records of Earth and other worlds during the Late Pleistocene — led to some theories questioning the validity of the fossil record itself. The most fanciful theories speculate that Earth's entire biosphere was transplanted from elsewhere around 100,000 years ago (the "Fake Earth" hypothesis). Like the Other Homeworld Theory, the Fake Earth hypothesis is not widely discussed in mainstream science, as it is effectively unfalsifiable: while it is theoretically possible that an ancient civilization terraformed Earth and made its ecology appear billions of years old so perfectly that human science cannot tell the difference, such claims cannot be proven or disproven and, as such, are pointless to seriously discuss. Advocacy for a Fake Earth is often politically motivated, and has become relatively popular on worlds seeking independence from the United Earth Government; in some fringe communities, it has even been taught in schools, albeit often as an "alternative" to the mainstream scientific interpretation, known contemptuously in some colonial circles as "Earther science".

Another theory, which even enjoyed limited exposure in mainstream science for a short time, postulated that the "priors" were never aliens, but a long-forgotten spacefaring branch of humanity, or, alternatively, an older and since-forgotten terrestrial sapient species. Though its subvariants go by many names, this idea is commonly called the Ancient Human (or Terran) Starfarers hypothesis. It is rejected by mainstream science as it fails to explain the lack of evidence for such a civilization having existed on Earth and/or the Sol system; even a modest spacefaring culture, and certainly one as long-lived and advanced as one capable of transplanting entire ecologies over millions of years, would leave extensive evidence of their presence on their homeworld and -system. Some proponents try to explain this absence with the "Landscaping hypothesis", the claim that said civilization went to great lengths to cover up their prior activities from future scientists. Though even the legitimate scientific community widely accepts that the ancient alien visitors must have taken care to hide any evidence of their activities, the extent of cover-up required in the Ancient Terran Starfarers hypothesis makes it highly implausible, especially as it is difficult to explain why a civilization native to Earth would go through with such a massive cover-up. While proponents of the theory have often claimed to uncover evidence for ancient occupation, such evidence is widely disputed, and the theory in general has been discredited by the abundance of oft-crude hoaxes and poor research methodologies.