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Human Biology

HOME BIOLOGYContent warning: nudity, reproductive descriptions PLANET CULTURE

Lifespan: 130 years
Average Adult weight: 80 kg
Average Adult height: 170 cm
Visual range: red through violet
Diet: Oppuortunistic omnivores, focus on grains

Humans are descended from tree-dwelling omnivores that evolved a bipedal gait and became plains-dwelling hunter-gatherers. They bear live young and have a two-sex reproductive system similar to centaurs, with an inseminator and gestator sex. Do to the Pleistocene extinction event, they only have one remaining natural species in their genus. However, their fixation on genetic modification has created new human species, colloquially referred to as GMH (genetically modified humans).

Most of these species were originally created by the entertainment industry, for show; and by scientific communities, to create humans hardier to the conditions of space travel and newly terraformed planets. “Humans” are thus described as a genus of animals, like avians. Although many GMH are unique and reproductively incompatible with any other humans, some GMH are created in compatible groups large enough to form stable reproductive populations, referred to as clades. Some of these GMH clades are also genetically unisex, unlike typ unisex humans, who undergo elective surgery to produce both eggs and sperm.

Major clades:

Typ humans

Typ humans are the largest human genepool, and the genepool that includes non-modified humans (which you are, presumably). Barring natural variation, most sexually mature humans either produce eggs and have a womb, or produce sperm. However, there are a fair number of typ humans who choose to under go elective surgery as adults to become unisex, i.e. able to produce both sperm and eggs. Surgically unisex humans vary a lot in morphology depending on the aesthetic preference of the individual, but they tend to have a blend of the typical physiological traits associated with the basal sexes.

Unisex humans are a distinct category from intersex humans; who are born with chromosome patterns, gonads, or genitals that don't allign with the typical two categories of the bisex system. Like any other typ, intersex humans are not unisex unless they undergo surgical transition.

Humans’ self modification does not end with their sexual traits. Many wealthy humans will commission natal labs to alter their children's genetics in ways that don’t interfere with their offspring’s ability to reproduce with other typ humans. Originally novel artificial traits such as patterned or vividly colored skin, pointed ears, fur-like body hair, yellow-red-purple spectrum irises, and purple-blue-green spectrum hair color have become endemic to portions of the human population and can be inherited through normal genetic means.

Tailed spacers

Average adult height (bipedal): 80-150cm
Average adult weight: 40-60 kg

Tailed spacers are the oldest successful clade of GMH, and are also known as tailers ("spacer" being a generic term for a person who doesn’t live on a planet). In modern day RttS there are about 2 billion living tailed spacers, and they’re the most common kind of claded GMH, though unique GMH outnumber them. Tailers are generally very gregarious and co-communicative, many living in tight communities onboard space stations. The largest ground-based community of tailers is on Luna, where they outnumber typ humans 20 to 1.

They were engineered a couple hundred years ago as part of a Martian vanity project seeking to create the ‘ideal’ human worker for life in micro/low gravity environments, and start populating the outer solar system for Jovian hydrogen mining. Their older English name is “Simians,” which is not considered PC any more… (Generally, referring to GMH as the non-human animal they most closely resemble is considered rude. Do not drop the ‘girl’ in ‘catgirl’ unless you are looking for a fight).

The primary pigment in their skin is selenomelanin, a pheomelanin (reddish pigment) derivative that’s effective at blocking most forms of low level cosmic radiation, which can increase cancer risks with long term exposure. It expresses much more uniformly over their body than melanin in a typ human, and can even be found lining their organs. Their skin tone ranges from dark red to purple-brown to near-black depending on the amount of eumelanin (black-brown) they have in addition to selenomelanin; and their hair and fur ranges in color between strawberry blond to red to black. Tailed spacers also have genes that help prevent muscle and bone density loss from life in microgravity… but, modern gene therapy regimens have made it so that most typ humans, especially in space-dwelling polities, now also have these genes.

Much of their body has been altered for their intended microgravity lifestyle. Tailed spacers have a bi-quad bauplan, hyperflexible lumbar spine, thumbed feet, and a muscular prehensile tail for microgravity maneuverability. Moving through spaces in microgravity is usually done head-first, which is easier for bi-quads. The tail and feet allow them to form a stable tripod grip on surfaces, and their flexible spine allows them a huge range of motion for their arms and upper body while gripping to a surface. Their supraorbital, genal, and carpal whiskers give them a greater spatial awareness in cramped quarters and detect airflow in spacecraft compartments. Their diminutive height means less fuel mass needed to push them through space, less food and water weight needed to sustain them, and less room needed on spacecraft to keep them sane. Tailed spacers are also much more efficient at retaining water than typ humans (they have very concentrated urine) and their metabolism adjusts based on food intake, so the less food they eat the more they slow down. There is a breaking point where they don’t have enough energy to move and go into torpor (or starve) but overall they can get away with eating much less than a WT human while remaining longterm healthy and functional.

Tailed spacers are genetically unisex, having a uterus and internal gonads that can produce both eggs and sperm. Click here to see an illustrated diagram of their reproductive system. The original reasoning was that they would be much more genetically stable at smaller population sizes– tailers were engineered before first contact with bug ferrets and the introduction of wormhole FTL, when Martian political powers were throwing around the idea of multi-generational interstellar ships. Most tailed spacers identify as some kind of non-binary gender, but it’s not an overwhelming majority… some, like Shyam, choose a binary gender, and others still might choose to transition away from unisex. They also have a much higher rate of polyamory than typ humans, but it's unclear if it's an innate difference or a cultural one.

Magpie Bridge

Average adult height (bipedal): 140cm
Adult weight: 25-40 kg

Magpie Bridge is a winged GMH project from a Chinese genetics firm. The starting population of the clade was about 60 individual corvid-human mixes with a physiological emphasis on fast and efficient flight. Min was the first born of the second generation.

Unique GMH:

Unique GMH are technically non-claded, or each belong to a clade of one. They are generally not reproductively compatible with any other humans, though they can still have genetically similar children with a partner through a natal lab. Mammalian traits are the most common mix-ins for aesthetic alterations, as the genetics are the easiest to mesh together. Guiomar is an example of a unique GMH.

GMH Glossary:

Catgirls to anthros

Catgirls is a broad classification that ironically includes non-cats and non-girls. Generally, however, a catgirl GMH appears typ human other than their ears and tail, which frequently resemble a house cats' but may be another animal's (such as a dog or rabbit). GMH are considered anthros when they remain bipedal and generally human shaped, but the non-humanoid traits are more extensive, including full coats of fur or scales, horns, unusual dentition, non-humanoid feet, and digitigrade legs.


Bi-quad GMH can walk on either two or four legs. Their body plan usually has arms and legs of roughly the same length, long torsos, and long necks with cervical attachment further back on the skull than a typ human. The term sphinx mostly refers to cat based bi-quads.


Taurs are hexapodal humans who generally locomote on their rear four legs. GMH taurs are not to be confused with centaur aliens, though horse based GMH taurs are frequently also referred to as centaurs, just to keep things confusing.


Winged GMH are hexapods (generally bipedal) with limbs they use for flight. They are frequently small in stature with thin legs and barrel chests. Most larger winged GMH have trouble taking off from flat ground.


Mermaids are GMH with a bauplan optimized for aquatic movement, and frequently gills. Tailed mermaids tend to have short rear limbs and heavy muscular tails, which gives them a raptor stance. Mermaids with a short tail or no tail (sometimes called selkies) have wide paddle-like feet and are usually bi-quads. Mermaids can also be taurs.


Dragons are a loose group of GMH with mixed reptilian, avian, and mammalian traits. Some are anthros, some are bi-quad, some are taurs, and many are winged.