Out-of-place biota

Out-of-place biota is the unexplained presence of organisms on worlds where said organisms clearly did not originate. 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 presence of flora and fauna that unambiguously share a common ancestor with Earth counterparts on Strauss' World in the Tau Ceti system. It is similar 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.

While initially controversial and subject to heated debate, the most likely explanation the scientific community has since come to favor 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 Garden hypothesis), preservation of unique life in the event of cosmic disasters (the Conservation hypothesis) or experimentation (the Galactic Petri Dish hypothesis). It has further been postulated that said civilization(s) may have wished to avoid direct interference on the planet of origin (in this case Earth) to allow the evolution of life to take its natural course there.

The phenomenon remained one of the strongest cases 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 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).

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. The phenomenon also inspired art and entertainment; the discovery of "lost colonies" of humans in particular was a staple of 24th- and 25th-century science fiction.

The phenomenon has also been well documented by the Covenant since their early spacefaring history. The Covenant universally attributed ancestral displacement to the Forerunners and their "divine providence", and the early Sangheili 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. 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.