Antecedent

Antecedent civilisations (or Antecedents) are a broad-sweeping classification, used to describe civilizations which have since gone extinct. The most well-known of such civilisations include the Forerunners, who were responsible for destroying all life in the galaxy with the firing of the Halo Array. As such, the vast majority of known antecedents are those civilisations which fell alongside the Forerunners' empire - though the term has also come to include a number of extinct civilisations whose remains have been found littering the Milky Way unrelated to the Forerunners. The majority of these examples come from the explorative efforts of the Covenant, though a handful are known to humanity.

More recent revelations about the Forerunners now postulates that the Forerunners were far from the first major spacefaring empire to dominate the stars, but rather merely the tip of a large iceberg stretching back billions of years. Evidence of these even older and more ancient interstellar communities is even harder to come by than the already-rare Forerunner relics, though this theory has been proposed as the explanation for a number of aberrations in what are believed to be Forerunner sites and artefact collections.

Antecents have some overlap with the grouping of Ulteriors, though are primarily distinguished by their civilisations being wiped out and known only via their relics.

Pre-array
With evidence increasingly pointing to a galactic starfaring history stretching back hundreds of millions - if not billions - of years, the vast majority of antecedent civilisations existed prior to the firing of the array. The vast majority were likely swept up in the marches of time, with only a handful of a percent expected to ever be recorded in the modern day. The Forerunners were the last in this line of galactic civilisation and as such, the majority of antecedents from this era are those which had contact with - and were recorded by- the Forerunners.
 * Ancient Sangheili civilisation - Prior to the firing of the Halos, the Sangheili were known to have served the Forerunners as subjects within the ecumene. In this role, the Sangheili are believed to have served as levies within the military, paying tithe in return for the Forerunners' protection.
 * Ancient San'Shyuum civilisation - Similarly to the Sangheili, the San'Shyuum are known to have had a rich starfaring history prior the Great Purification. Unlike the Sangheili, evidence recovered from Forerunner databanks suggests the San'Shyuum once held a small interstellar empire in the Milky Way and were seen as rivals to the Forerunners' power, ultimately resulting in a series of wars with the Forerunners eventually seeing the San'Shyuum confined to just their home system under Forerunner rule. During the conflict against the Flood, the San'Shyuum rose up in rebellion against their distracted overlords.
 * Endless - Active in the galaxy around 250,000-160,000 years ago, this civilisation is referenced only a handful of times in Forerunner texts. While information on them is scarce, current theories suggest this civilisation may have once been the dominant galactic power prior to the era of the Forerunners.
 * Forerunners - The most well-known antecedent example, the Forerunners dominated the galaxy in the millennia leading up to the firing of the Halo Array, and left their artefacts scattered across the galaxy to be found by their successors.
 * Precursors - A semi-mythological race mentioned by some Forerunner records, the Precursors were hypothesized by the Forerunners to have lifted earlier Forerunner ancestors from Earth and displaced them across the galaxy millions or billions of years ago.

Post-array
During the reseeding process following the end of the Diluvial War, the Librarian seeded new populations of dozens of sapient species on thousands of worlds across the galaxy. This was done in an effort to ensure that a newly-reintroduced population would not be rendered fully extinct through any of the dozens of Great Filters that could potentially halt the rise of intelligent civilisation - and is a part of the reason for the out-of-place biota phenomenon. As predicted, many of the species that rose up during the following millennia were wiped out through any number of causes both self-inflicted and natural.

The explorations of the Covenant have recorded a small handful of worlds which appeared to suffer such fates, with several of the Covenant's own client species including the Unggoy and the Jiralhanae having come close to joining the list due to overindustrialisation and war.

Humanity has only discovered one such world, though the inevitable expansions of humanity - particularly into the spin-ward regions of the Orion Arm - is expected to slowly reveal more and more lost civilisations to xenoarchaeologists.
 * Jehioi - A civilisation known by the Covenant who established planetary and orbital colonies across space in the Orion and possibly Perseus arms, before falling less than two millennia later. The Jehioi are estimated to have been predominantly active around 40,000BCE.
 * Optem - A species nicknamed the "Looters", this civilisation appears to have been active around 20,000BCE and is known to have raided Forerunner sites for their treasures - hence their nickname. Whether this civilisation corresponds to any of the others known in the galaxy, or whether they remain active, is unclear.
 * Silent Hope - Unlike the others on this list, this antecedent takes the form of a Covenant civilisation. At the time of its settlement in 480BCE, the colony of Silent Hope was viewed as a new cultural nexus in what was then the outer frontier of Covenant settlement. Reaching a population of millions at its height, the world was later entrusted to fend for itself during the waning period of the First Illumination, during which time Covenant civilisation looked inward. By the year 30CE, no confirmed contact had been made with the world for eight decades, and scout missions were sent out to ascertain the planet's status, but only found the world totally abandoned, as though the entire populace had evacuated without packing their belongings or - in some cases - even finishing their meals. No bodies were ever found, and Silent Hope's parent star system became the subject of legend. Even despite the system's ample resources, all that remained in the system was a single mostly-automated helium refinery orbiting the gas giant for refuelling of errant freighters passing through the system.
 * Netherop civilisation - a seemingly-(relatively) primitive civilisation whose ruins were discovered on the eponymous planet. The planet's discovery was relatively late in the 25th century, and included in the Military Survey of Uninhabited Planets as a greenhouse world with little of value to justify infrastructure investment in the Ephyra system. ONI-classified addendums to the report showcased several photographs taken via remote survey probes of the planet's surface, highlighting what appeared to be evidence of city-like structures on the surface, though the near-Venusian surface conditions made practical exploration and research difficult. Netherop was a site of minor interest to the Covenant early in the war against humanity, though the rapid retreat of UNSC forces meant that no major ground exploration was ever conducted. Photographic and video recordings taken by Spartans deployed onto Netherop early in the war have led xenoarchaeologists to estimate that the native civilization on Netherop had at least attained an early industrial level of technology before being wiped out around 30-45,000 years ago.