Precursor

The Precursors were a race of beings preceding and might be hologized by the Forerunner, who believed them to be theoretical "Transsentient" beings, having the ability to travel among galaxies and accelerate the evolution of intelligent life. The Precursors achieved a higher level technological advancement than the Forerunners, being Tier-0, or Transsentience on the Forerunners' Technological Achievement Scale.

History
Not much is known about them, but information that can be found by text from the logs stored in the Terminals around the Ark indicates it is possible that the Forerunner 'Mantle' (Guardians of the Galaxy and all life) was handed down from "legends of the past", the Precursors. It has been presumed that the Precursors viewed the Forerunners as their own legacy, continuing their work after they departed, much in the same way Forerunner lifeworkers (including the greatest lifeworker: the Librarian, or the Lifeshaper) viewed Humanity before the activation of the rings.

The Primordial, a Gravemind that stated itself as the last Precursor, revealed that the Precursors had been ruthlessly killed in a war with the Forerunners millions of years before the events of Halo and that "their answer is at hand" - implying that a proto-Flood form (described as a fine dust, contained in hundreds of glass cylinders) was created as revenge or as a failsafe.

"We meet again, young one. I am the last of those that gave you breath and shape and form, millions of years ago.

''I am the last of those your kind rose up against and ruthlessly destroyed. ''

I am the last Precursor.

And our answer is at hand."

It also may be inferred that the Precursors developed the Flood as a last resort to the Forerunners' uprising, their revenge reaching fruition.

Technology
The basis for all Precursor technology is neural physics. The Precursors were the only known civilization to achieve Tier 0 of the Forerunner Technological Achievement Tiers and to exceed the Forerunner in technological advancement. They can theorecticly accelerate the evolution of intelligent life and travel across galaxies.

According to Bornsteller Makes Eternal Lasting, the Precursors were capable of engineering "orbital bridges" between planets in the same system. These are called unbending filaments.

Trivia

 * The term Precursor means "one that precedes and indicates the approach of another." The term supports that it surpassed the Forerunner.
 * The Latin root word "trans" means "across" or "beyond." "Sentience" is the ability to experience sensation, and is often used to imply sapience, the ability to think. A transsentient being may thus be considered beyond any recognized being and be on the level of godhood.
 * In Halo: The Fall of Reach, Dr. Halsey uses the term Precursor to describe the alien race that created the crystal found on Sigma Octanus IV.
 * There is a possibility that the Precursors may have looked like or been what we call sea scorpions or eurypterids, as described by the Didact in the book Halo: Cryptum.
 * The Precursors' concept of Neural Physics is very similar to the Biotics in Mass Effect and/or the Jedi's belief of The Force in the Star Wars universe or in Star Trek where the traveler states that matter, energy, and thought are one in the same. Overall is the belief of "Mind over Matter" through science.
 * There is a possibility (according to the prisoner) the Flood was created by the Precursors, and would explain plausible extragalactic origins from a possible long term Precursor fail-safe plan, and not from an intergalactic Flood colony. If this is true, then it means that none of the Flood colonies in the Halo series have ever progressed past the interplanetary stage of development.
 * A creature of similar discription is found on the secret, easter egg level in Marathon: Infinity, called "Hat's off to Eight Ninteen". The story tells of a creature nine meters tall, with four arms, and a substance in a transparent cylinder, and an invading race from the stars. To read the original text, click here. (7x117=819)