At first glance, it may seem unusual that so many Thessian residents have such a strong resemblance to Homo sapiens. There is a good reason for this striking frequency—a large number of these species derive from our distant children who, faced with the decline of their world, chose to explore the stars, and slowly lost their way home, eventually coming to be the current Thessian stocks.

In many cases, differentiation was guided only by isolation, as no significant selection pressure is exerted on space-faring species, save for stamina, longevity, and caution around dangerous machinery. On occasion, however, genetic engineering has helped this process along, giving rise to the hereditary information-passing abilities of the Lyrisclensia, the distinctive appearance of the Lilitai, and in at least two cases, the disappearance of the Y chromosome.

Despite their sometimes remarkable ability to proliferate and adapt, post-humans are hardly the only residents of the Thessian cosmos. Their high density is only a consequence of their recognition of one another as distant cousins, and co-settlement on Thet. The planet itself had a wide variety of lifeforms inhabiting it prior to settlement, although none were sentient at that time; since then, the Physarai have evolved from relatively primitive fungal colonies into an intelligent species, and anciently, the world was home to the Tletkettoyi, from whom the word "Thet" ultimately derives.

The Tletkettoyi, in turn, were the ancient enemies of the Ksreskézai, who were masters of many species and worlds throughout many branes, and are best-remembered as the tyrants and aristocrats who created the Lilitai many thousands of years before the modern settling of Thet. The Ksreskézai are now also extinct, but their influence persists across the reach of their former empire.

List

The definitions of appearance given below are not exactly definite. Since the development of phenotype respecification, physical appearances have become a great deal more flexible amongst the more common species.

Name (Plural) Heritage	Description
Cossipa (-ai) Human; evolved	Life span varies. The Cossipai are actually a variety of species, isolated by either captivity or loss of contact with the mainline of the Telai. There may be hundreds of thousands or even millions of different histories of various Cossipian groups, but only a handful contribute substantially to the population of Thet. In general they have a good survivalist streak and display highly cooperative personalities, although this is hardly universally true. With the exception of the technologically adept Hatel, they have not done well economically on Thet, and often side with the Pesene in political matters. No parthenogenesis mechanism is known to be widely-used; most are male or female, although some species have substantial intersex and hermaphrodite birth rates. Lyrisclensian parthenogenesis technology can create a sterile hybrid between most Cossipians and a Lyrisclensian, often with significant magical competence.
Epyeste (-eyi) Off-world	Life span unlimited. Sentient Pesenese robot-suits that rebelled in 6950 ksepo, unwilling to tolerate the rule of the ever-shifting Ksreskézaian Empire. Settled on Makta, occasional trade with the Lilitai.
Hogenem (Hogedep) Off-world	Life span 40–110 tgc. Magic competent. One of the ancient enemies of the Ksreskézai, they are majestic in their own peculiar way, resembling winged snakes in body structure. Their culture heavily blends myth and fact, and as a result their deep scientific understanding often comes with heavily confabulatory language and stories, which makes trading for it difficult (if highly desired) by outsiders. Two sexes.
Khúsak Off-world; extinct	Life span unknown. Magic competent. Little is known about them other than their legal code, which was a guiding beacon in the area later known as the Ksreskézaian Expanse prior to the rise of the Ksreskézai. Eliminated by the Tletkettoyi.
Ksreskézu (-ai) Off-world; extinct	Life span 100–220 tgc. Magic competent. Exoskeleton-bearing quadrupeds evolved from bipeds after their nascent, warlike culture rendered fine motor control unnecessary. Created the Lilitai as servants following fascination with human physical form and mythology to fill in where necessary. Formerly held a vast empire that spanned many branes, but went into slow decline once they had exhausted the resources of those inhabitable branes to which they had access. Eliminated by the Hogedep. Aristocratic and imperialistic, the Lilitai are their legacy. Sexual, with two conventional sexes.
Lilitu (-ai) Human; re-engineered by Ksreskézai	Magic competent. Life span 2000–5000 tgc. Bat-like wings, horns, tail, pointed ears. Romantic, smug, and mercurial, their culture is fundamentally forged afresh out of a pale echo of Ksreskézaian culture, but has changed significantly in response to contact with other human derivatives. Parthenogenic through magic-assisted technology.
Lyrisian Human; self-altered	Magic competent. Life span 150–200 tgc. Partial hereditary communication of knowledge which must be awoken during youth. Entire culture is focused on acquisition and application of scientific knowledge. Parthenogenic through technology. Founding component of the Lyrisclensian culture.
Noctian Off-world; self-altered	Magic competent. Life span 150–400 tgc. Hereditary communication of knowledge similar to Lyrisians. Natural form is quadrupedal resembling an even-toed ungulate similar to a horse, but through their innate magic they can (and often do) adopt near-human form. Assimilated by the Lyrisclensian culture in the 66th millennium LKY. Parthenogenic through magic.
Pesene (-eyi) Off-world	Life span 30–190 tgc. Spider-like quadrupeds that do most of their work with tongues. Significantly smaller than many other species, rendering them frequently misjudged by others, which has had its benefits and drawbacks. Independently achieved space-faring status. To simplify their cultural barriers, they mostly operate larger octopodal robots which can provide more readily-understood language. Self-fertilizing hermaphrodites; semelparity limits life span. Like many species from the Expanse, they were formerly subjects of the Ksreskézai.
Physaru (-ai) Thet natives	Life span 130–250 tgc. Subtle telepathy; ability to detect proteins and ligands at very small concentrations and high specificity in other organisms through touch serves as crude empathy. Similar to a slime mould in nature, but generally taking on a bipedal appearance. Asexual.
Telau (-ai) Human; evolved	Life span 70–130 tgc. Least-changed from the humans that left Earth at the end of time, they retain full sexual reproduction, and represent the most that evolution could squeeze out of the genome given billions of years and no major pressures. As a result, they can expect to live at least twice as long as present-day Terrans, and are quite intelligent, but their prolonged exposure to technology as a facilitatory mechanism of their daily lives has weakened the value of memory. To some extent, they have lost knowledge of science and art in favour of strategy and survival, and are natural-born pilots. As a result, there have been a number of group-splintering events and there may be up to a dozen self-identified independent confederacies on Thet.
Tletketti (Tletkettoyi) Thet natives; extinct	Life span 300–500 tgc. Magic competent. Similar to overgrown trilobites with tentacles. Extensive use of implants and augmentation through technology. Wiped out by the Hogedepi holocaust with the Ksreskézai. Independently invented a comparable magic system, which is what powers magic on present-day Thet. Trinary fusion of gametes between three distinct sexes.