Trees on Chorus use a purplish pigment for photosynthesis
The Halogenics are a hypothetical clade or group of clades, now believed to be extinct. They seeded a large number of chlorine worlds about 780 million years ago. Their existence was first postulated by Irene Habelsein-Io of the Tycho City Institute of Xenopaleology in 1744 a.t. to explain the identical biochemical pathways and genetic codes of the life forms on the first two chlorine worlds discovered by Terragens: Tytalus and Outland. This theory was further supported by the discovery of additional chlorine worlds in widely separated parts of Terragen space. These worlds usually orbit K stars, and every one discovered to date exhibits the same details of biochemistry. All in all, 356 such chlorine worlds and former chlorine worlds (in some cases the biosphere is extinct) have been reported to the HIE. Despite their common origin, the multicellular species on the chlorine worlds differ markedly in their basic body plans, but convergence analysis has identified the original life forms. They appear to have been a group of related single-celled organisms: one species comparable to Terragen eukaryotes in complexity and fourteen to seventeen species that were of prokaryotic complexity. These in turn all use the same genetic code. They also have the same set of biochemical pathways, though the "prokaryotes", like their Terragen equivalents, are a very diverse group compared to the "eukaryotes". It appears from available evidence that the Halogenics gengineered distinct multicellular forms (the equivalents of plants, animals, and fungi) at each site based on the eukaryotic ancestor. Of course in the 780 million years since evolution has obscured and elaborated upon the original forms. Whether the founding organisms of the chlorine worlds were representatives from a naturally evolved biosphere on an ancestral chlorine world, or whether they were modified from more conventional oxygen-based biota, or whether they were entirely neogen, cannot be proved given the available evidence. Transapient researchers tend towards the third of these options, based on a characteristic that that may be translated as "design style". The concept and its supporting arguments are not entirely comprehensible to beings of less than S1 grade, however, and in any case not all transapient investigators support the idea.
The nature and origin of the Halogenics themselves remains unknown. Some details of the techniques they used suggest that they had dry nanotech, and that they had specialized evolution-maintenance AIs. There is no evidence that they developed or maintained a wormhole network, and it is quite possible that they never achieved the levels of technology known now in the Terragen sphere, and in particular any technology that requires transapient intelligence to create. All that they have done could in principle have been accomplished by self-replicating Neumann turingrade AI entities using solar sails or simple amat drives, sent out from a single originating stellar system. Such technology could easily have blanketed the entire galaxy in chlorine worlds in only a few million years. The most recent of Halogenic interventions, not long before their apparent demise, suggest control of some form of artificial nucleosynthesis. This suggests to some that their technology advanced over the course of their work, but it may simply be that they only turned techniques to such worlds once the most suitable had already been seeded. The physical form of the Halogenics, or their creators, is a matter of speculation. Some believe that they were AIs, or vecs, or oxygen-breathing bionts. Others believe that they were a naturally evolved chlorine breathing species. Still others assert that they must have been a chlorine breathing species of artificial origin. Available evidence is not sufficient to distinguish between these hypotheses.
Based on the dates at which the various chlorine worlds were established, and based on their spread across the galactic disk, the Halogenics must have seeded every possible chlorine world within a region at least 25,000 light years across. They were apparently active for no more than one million years before they vanished. During that time they were extraordinarily thorough, and developed any candidate system within their sphere of influence. Some of their more marginal choices (worlds around stars that later left the main sequence, worlds near flare stars, worlds too near the inner or outer edges of their stars' life-zones, worlds too small to retain an atmosphere for more than 100 million years, and so on) have not survived into the present age. It is apparent that they carried out operations on a time scale longer than that of modern Terragen civilization, and had plans for a very long future. For instance, they did not attempt to create habitable environments on worlds that would have required artificial support to retain a biosphere, or that would have lost an atmosphere on the time scale of a million years or less. All of their projects, even the most marginal, lasted for tens of millions of years. On the other hand, they apparently did not expect to simply leave their worlds unattended for nearly a billion years afterwards. What happened to them is unknown, but they seem to have vanished or moved to another location quite suddenly after completing their work. Their only legacy is the chlorine worlds themselves. They apparently did not engage in any other works of megascale engineering. Faint traces of computronium nodes, amat farms, and orbital habitats remain where they were active, but either the Halogenics themselves or some other agencies since their demise or departure seem to have taken great care to remove all but the subtlest traces of artificial activity.