Forms were different physical stages assumed by the Forerunners over the course of a lifetime. The Forerunners were able to mutate into various forms through an artificially-induced transformation. Manipular, or Form Zero, was the first and simplest form, assumed by Forerunners at birth.[1] It was noted that Manipulars bore a striking resemblance to humans; however, this would change with higher forms.[2] The original anatomy of a Forerunner would also be distorted when mutating to higher forms, most of which lacked some natural abilities possessed by Manipulars, including the ability to smile.[3] After a mutation, small changes or corrections could be made, but these were mostly cosmetic.[4]
In each mutation, a Forerunner would acquire the genetic imprint of their mentor, typically the individual's father. However, warriors sometimes performed mutations in battlefield conditions, where the imprint would be supplied by one's commander.[4] The form one would mutate into was dictated by their rate, as well as the person who performed the mutation. For example, Warrior-Servants were noticeably taller, more rugged and more muscular than Builders, whose forms were more refined.[5] Aside outward appearances, there were also differences in mental and neurological functions.[3] Higher forms of Warrior-Servants, for example, had extraordinary mental abilities; they could absorb massive amounts of information from one another,[6] share sensory input with thousands of warriors during battle,[7] or splinter their perception of time into several "streams."[8]
By roughly 100,000 BCE, a new, more natural mutation pattern had been developed. Instead of being subjected to multiple mutations over several centuries, one would undergo a series of changes over a single year. This would result in less inherent physical or mental distortions. Most Forerunners mutated according to this new pattern came from Builder families, with a few Warrior-Servants among them.[4]