Thiocythera

Thiocythera is a fictional world designed to resemble what planet Venus may have been like in the distant past when it likely had liquid water and possibly life.

Optional: Skipping the Nerdy Chemistry Stuff

The Master Reaction

This reaction produces thiocarbohydrates (thiocarbs), including tetrathioglucose which is favored for the way its atoms are arranged. Glucose is unique among sugars for its molecules having essentially a pancake shape, and tetrathioglucose is the same way. Other simple sugars have molecules with atoms above or below the flat part, which is less stable, especially if they are bulky sulfur atoms. Glucose, including its polymers, is the most abundant carbohydrate on Earth, and thioglucose is pretty much the only thiocarb on Thiocythera. Thioglucose also forms polymers, which I will call thiostarches although their molecular structure is not the same as that of starch. Thiostarches form the membranes of cells on Thiocythera as well as the fibrous structural materials of sedentary organisms.

Thiocytheran life is still not limited to just one kind of sugar, however. For one thing, thioglucose, like ordinary glucose, occurs in "left handed" (L-glucose) and "right handed" (D-glucose) isomers, compounds whose molecules are made of the same parts in different configurations. Earthly life uses only D-glucose, but Thiocytheran life uses both D- and L- isomers of thioglucose. Also, thioglucose molecules can differ by which atoms are oxygens and which are sulfur. Together this adds up to 30 possible isomers of thioglucose. But not all 30 can be used by cells. Thiocarbs differ from regular sugars in that they are not soluble in water. Therefore, the atom sticking out of the very end of the molecule, where normal glucose has an oxygen atom, has to be sulfur instead, so that it can be oxidized to a sulfonyl acid group, which makes the compound soluble. Also, the atom that forms part of the molecule's ring has to be oxygen, otherwise the ring would be kinked and therefore less stable. So out of the 30 possible isomers of thioglucose, only 8 are useful to Thiocytheran organisms.

Glucose molecules have a puckered hexagonal shape where the oxygens can stick out sideways (equatorial) or up or down (polar). The carbon atoms are numbered 1 through 6, and the oxygen on atom 1 can be either polar or equatorial, while the others are always equatorial. (The equatorial atoms are the reason for thioglucose's relative stability.) When the oxygen on atom 1 is in the equatorial position, the molecule is called beta-glucose. On Earth, cellulose is made of beta-glucose units linked together by their 1- and 4- oxygen atoms. This makes long straight strands that can cling together into a tough, durable sheet. By contrast, our starches are made partly of amylose, which is also bonded on atoms 1 and 4, but with the subunits being alpha-glucose, having atom 1 in the polar position. The result is a strand that curls up into a coil. Starches also have branches where a glucose subunit, already bonded on its 1- and 4- atoms, can bond another subunit on its 6- atom and begin a new chain.

Thiocytheran thiocarbs don't have as much oxygen in them as our carbohydrates do, so even in the beta form they can't stick together with hydrogen bonds and make strong sheets like cellulose can. So there's not much evolutionary pressure to prefer beta thioglucose subunits, and the alpha and beta forms get randomly jumbled together or arranged in repeating sequences. The thiocarb strands still have different physical properties, and they're not the same as their earthly analogues. Sulfur forms its bonds at close to a 90° angle, so while cellulose makes nice straight strands, its thio equivalent (beta-thiostarch) makes a zigzag-shaped molecule. Thiostarches cannot curl up like amylose because the sulfur atoms are too bulky. Also, thiostarches cannot branch on atom number 6 like our starches can, because that's the atom that becomes the soluble acid group. (Even though the acid is reduced back to a thiol when the polymer is built, the units have to be acidic while the enzyme is bonding them together.) Since atoms 1 and 4 can be either oxygen or sulfur, both water and hydrogen sulfide are released by the production of thiostarch. If atom 2 or atom 3 is an oxygen, it can link to other molecules, such as groups that allow the thiostarch to perform specialized functions, including cross-linking with other thiostarch strands for strength.

The presence of photosynthetic organisms making thiocarbs means other organisms are going around eating them, just as earthly animals eat plants and consume the carbohydrates that the plants produce by photosynthesizing. Like earthly animals burning carbohydrates, mobile Thiocytheran organisms burn thiocarbs, reversing the master reaction and emitting carbon dioxide and hydrogen sulfide. Thiocythera's atmosphere is largely carbon dioxide, ranging from about 30% at its lowest levels soon after photosynthesizing life appeared, to more than 90% by the time all Thiocytheran life ended. But the reactions involving sulfur cause an atmospheric sulfur cycle, which is not the same as the sulfur cycle we have on Earth; instead, sulfur is constantly being interconverted between sulfur dioxide and hydrogen sulfide, with plant-like organisms producing the former and animal-like entities releasing the latter. Each lifeform breathes the other's output, and the two gases remain well balanced. Unnecessary to say, Thiocythera's atmosphere would be very stinky and toxic to us earthlings, not to mention suffocating since it lacks even the faintest trace of free oxygen. But this lack of oxygen allows Thiocytheran life to use compounds that would not be stable in our atmosphere.

The Amino Acids

Some of the amino acids of Thiocytheran life are familiar to us because we have them here on Earth as well. The only difference is our amino acids are all L-amino acids, while theirs are all D-amino acids. Just like how glucose has D- and L- isomers, the amino acids do too. Thiocytheran amino acid molecules are mirror images of ours. The only exception is glycine, which is achiral, meaning it is symmetrical and does not have mirror image isomers. Life on both planets includes glycine. Some of their amino acids are unlike anything we have on Earth. Thiocythera's atmosphere and biosphere contained considerably more sulfur than Earth's, so sulfur is comparatively overrepresented in their amino acids. It also had more phosphorus and chlorine, so we find those elements in their aminos but not in ours. Here is a table of the Thiocytheran amino acids used by life on that planet to form proteins:

If this list seems long, it is — there are 24 amino acids in Thiocytheran proteins, compared to only 20 in earthly proteins. Many of the amino acids have special functions.

Cysteine in earthly proteins forms disulfide bonds, which cross-link strands together for structural stability, and contribute to the strength of proteins like keratin and collagen. On Thiocythera, by contrast, the lack of free oxygen means cysteine side chains form monosulfide bonds; instead of joining the two sulfur atoms together like we do, they use a process that keeps only one sulfur atom in the cross-link and releases the other one as hydrogen sulfide. Thiocytheran lifeforms can also cross-link a cysteine with a serine, producing the same type of link and releasing a molecule of water instead of hydrogen sulfide.

Aspartaldehyde and glutamaldehyde are capable of cross-linking with ornithine, by forming what's called a Schiff base, where the oxygen of the aldehyde is replaced by the nitrogen of the ornithine. Water is released by this reaction, and the nitrogen atom holds onto a hydrogen keeping a positive charge (it's protonated). Schiff bases are stabilized by the acidic environment of a Thiocytheran cell, but the linkage is easily reversible, which helps make proteins that are more elastic and less rigid than with cysteine cross links.

Isohistidine resembles earthly histidine, except it has its two nitrogen atoms adjacent on the five-membered ring instead of separated by a carbon like we have. Isohistidine can perform all the same protein-pump actions as histidine, but its lower pKa (stronger alkalinity) means it can function in acidic conditions where ordinary histidine would react to the acid by becoming negatively charged.

Phornithine, or phosphornithine, has a basic (i.e. alkaline) side chain like ornithine, but is a weaker base and is more hydrophobic, and can be used to bring local pH control to the interior parts of proteins.

Letter Code	Name	Also present on Earth?
J	Adhesine	No.
A	Alanine	Yes (mirrored).
L	Aspartaldehyde	No (unstable in oxygen).
D	Aspartic acid	Yes (mirrored).
E	Cysteic acid	Known (mirrored), but not used in proteins.
C	Cysteine	Yes (mirrored).
Y	Furanalanine	No.
R	Glutamaldehyde	No (unstable in oxygen).
Q	Glutamine	Yes (mirrored).
G	Glycine	Yes.
B	Homoalanine	No.
K	Isohistidine	No.
N	Norleucine	Known (mirrored), but not used in proteins.
V	Norvaline	Known (mirrored), but not used in proteins.
O	Ornithine	Yes (mirrored), but not used in proteins.
P	Phornithine	No.
S	Serine	Yes (mirrored).
U	Swombhicine	No.
I	Swombhiline	No (slowly degrades in oxygen).
W	Swombhine	No.
H	Thioglutamic acid	No.
F	Thiophenalanine	No.
T	Trichloroalanine	No.
M	Trimethylphornithine	No.

Swombhiline is somewhat unstable due to having a double bond right at the end of its side chain. It likes to link together its terminal CH₂ with other swombhiline molecules, and proteins with several inward facing swombhilines can form a central ring of carbon atoms that holds the protein's shape very tightly. Swombhine and swombhicine can also participate in this ring lock phenomenon. When the protein is broken down, the rings and their metabolites become waste products. Swombho- amino acids form cross-links in thiostarches. One of the most successful clades of life on the planet uses predominantly elastic thiostarch having swombho- cross links, which makes for a spongy, springy texture. The breakdown of unreacted swombho- amino acids releases odoriferous compounds that lend a mushroom-like quality to Thiocytheran life. All of this leads to a fungus-like appearance and smell of many of the planet's lifeforms. In fact, all three swombho- amino acids are named after the Indo-European root for "spongy" or "mushroom".

Adhesine changes its hydrogen bonding properties depending on pH. Organisms that incorporate this amino acid into their exterior proteins can go sticky or non-sticky at will. Early lifeforms used this capability to temporarily stick to silicate minerals on the seafloor. When multicellular life first ventured onto land, selectively adhesive proteins made it easier to navigate the sandy shores. Ultimately, some lifeforms would even begin to coat themselves in silica, and symbiotes with adhesive proteins in their feet could climb the silica by engaging sticky mode for enhanced grip.

Thiophenalanine is the closest thing on Thiocythera to earthly phenylalanine. Benzene rings aren't very common on Thiocythera, and rings of 6 carbon atoms are generally treated as waste products and excreted by cells. Thiophene is a compound with very similar physical and chemical properties to benzene (it even smells like benzene, which is unusual for a sulfur compound), so its inclusion in lieu of a benzene ring makes for an amino acid with very similar properties to phenylalanine. Similarly, furanalanine has some properties similar to tyrosine, except without the ability to donate hydrogen bonds.

Trichloroalanine functions similarly to earthly proline, but with a less pronounced effect. Proline forms a kink in the protein strand, and in the case of an alpha helix, causes a bend on the helix. Thiocytheran alpha helices (which coil in the opposite direction from ours) can be slightly kinked by a trichloroalanine. The bulky chlorine atoms push against neighboring side chains. It is common to find trichloroalanine being used to form curved helices consisting of a TTXTTXX motif (a repeating sequence of amino acid subunits, where X means any amino acid), and coiled coil protein segments are possible with TTXXTTXX and TTXTTX motifs, these two patterns coiling in opposite directions.

Trimethylphornithine is a powerful methylating agent. It is stable only at low pH so it usually has to be assembled from plain phornithine by dedicated enzymes at the time and place where the protein is being built, at a ribosome-like structure that converts genetic information into protein strands. Typically trimethylphornithine occurs in enzymes where the presence of a ligand causes the enzyme to contract, bringing a deprotonated (i.e. without the normal positive charge) ornithine near the trimethylphornithine side chain, and causing it to deposit a methyl group onto the ligand in return for the release of a proton. The spent enzyme can then be recharged by a long complicated process involving methylation enzymes similar to those at the ribosome.

Genetic Material

Thiocytheran life does not use DNA like we do. Instead, the nucleotides are formed from dimers of the amino acids. In order to accomplish this, a molecule of serine and a molecule of another amino acid are joined head-to-tail to form a six membered ring. (In chemistry terms, this ring is internally redoxed, with release of a water molecule, to form a pyrazine ring, with the serine's -CH₂OH at the 2- position, a hydroxy group at the 3- position, and the other side chain at the 5- position. The 2-CH₂OH and 3-OH groups are joined to their neighbors by sulfate groups, forming a strand.) Which amino acid participates alongside the serine determines which "letter" of the genetic code is formed. There are six nucleotides in total, arbitrarily called A, B, C, D, E, and F. The pyrazine rings stack together for added stability. Two strands fit together head to tail in a double helix much like DNA does. Like with DNA, the sequences are complementary and are reversed with respect to each other. The nucleotides pair as follows: A with B, C with D, E with E, and F with F.

Nucleotides A and B are made from glutamaldehyde and ornithine, respectively. The amine and aldehyde groups form a strong hydrogen bond, but do not form a Schiff base, as the helix keeps the two sufficiently separated. Nucleotides C and D are formed from phornithine and thioglutamic acid. S-acids are stronger than plain carboxylic acids and can protonate phospino groups; although contact between B and D nucleotides is possible, according to HSAB theory phosphorus and sulfur are more "soft" than nitrogen and oxygen, so C and D prefer to bond with each other rather than D with B. E is made from glutamine; two E nucleotides form a pair of hydrogen bonds between each side chain's NH and the other side chain's O. Finally, nucleotide F is made from thiophenalanine and binds to another F via pi stacking.

The Thiocytheran genetic code is not based on a fixed codon length like ours is. Earthly DNA has codons made of three nucleotides, 64 possible codons, each one representing an amino acid (or a signal to end the protein strand). On Thiocythera, the codons can be as small as two nucleotides but are usually 5 to 7 nucleotides in length, sometimes longer. One may even occasionally find strange repeating patterns like EDABABABABABABABABABCF representing a single codon, in this case a synonym of EDABCF in which the number of AB units happens to be arbitrary for purposes of protein coding.

Multicellular Life

The first colonies of cells on Thiocythera were microbial mats. The acidity of the planet's atmosphere meant that plenty of minerals were available; the seas contained carbonic and sulfurous acids as a result of absorbing atmospheric gases, and this acidity caused many metal ores to dissolve, meaning the water was enriched with sodium, potassium, calcium, magnesium, iron, copper, zinc, manganese, cobalt, chromium, nickel, vanadium, titanium, lithium, strontium, yttrium, and lanthanides. The presence of so many heavy metals meant the water had an olive-green hue, which was especially noticeable when looking into the depths. Photosynthesizing life had a strong evolutionary pressure to leave the water, and it did; microbial mats colonized the land, becoming mounds of brilliant color, which I will refer to as land corals. Iron based photosynthesis made for vermillion land corals; nickel made teal; cobalt a deep indigo-violet; manganese magenta. The microbes were able to extract silica from the sand, forming a silicate exoskeleton around each cell to conserve water. As cells underwent division and expanded the surface, new cells would migrate to the top and old cells would die from lack of light, producing a pumice-like layer of fused spent exoskeletons beneath the surface.

Life on Thiocythera incorporated more mineral components than Earthly life has. Besides using heavy metal ions for photosynthesis (earthly chlorophyll uses only magnesium for this), there were also organisms that had silicon microcrystals for seeing light, and organs that incorporated ferromagnetic metals and could receive (and emit) low frequency radio waves. The background sound on this page is a representation of what the surface of Thiocythera would have sounded like in a combination of normal sound and audio-frequency radio waves, two ranges that many of the lifeforms could perceive, in their case as separate but parallel senses. Ambient wind noise can be heard on the audio side, while the radio side reveals various atmospheric effects including sferic waves resulting from distant lightning strikes.

Migrating onto land held another advantage for life — space. At the peak of optimal conditions for life, the planet was about 30% covered in shallow seas, with a few deep ocean trenches. The remainder of the surface was a rocky desert. Life in the seas began to form into blobs of undifferentiated cells. These were not related to land corals; in fact, they weren't photosynthesizing, but rather fed on photosynthetic microbes. The earliest blobs were gelatinous in appearance, translucent to transparent, and made mostly of protein. They evolved the ability to convert photosynthetic molecules from their food into photoreceptors, thereby allowing them to sense light and even distinguish colors to a limited degree. A new lineage of eye-bearing blobs developed and became highly successful, hunting photosynthetic organisms by sight.

One type of blob that became very widespread and abundant was a taxon that used bioluminescence to signal to others of its species. These blinkers, as I will call them, emitted flashes of blue, violet, or ultraviolet light and could distinguish at least 4 primary colors including two kinds of ultraviolet.

Other types of eye-bearing blob evolved into a bewildering array of jellyfish shapes, worm shapes, mushroom shapes, some with wispy tentacles, some with segments, some with stinging venom, and one taxon even characterized by a segmented body plan and thousands of fine tendrils dangling from each segment. The tendrils were great for filtering the water and bringing food to the beings' mouths.

One species of mushroom-shaped blob evolved to root itself to the seafloor, forming a tough fibrous thiostarchy stalk tens of meters long, and culminating in a huge umbrella-like formation near the water surface. It would filter the water for food using gill-like structures under its canopy. The stalk was able to twist very freely, causing the elaborate ornate canopy to turn in one direction for a while and then stop and turn in the other direction. I will call this a carousel chandelier because that best describes what it looked like. The canopy made a great hiding place for blinkers, who already had a predator hunting them: the shark dart, an opaque beige-gray entity with a streamlined shape ending in a very sharp pointed tip. A powerful swimmer, the shark dart would spear blinkers and then suck their gelatinous innards through slits along the outside of the terminal spike. Carousel chandeliers evolved a stinging defense against the shark darts, and blinkers, immune to the sting, took advantage of this, providing the chandelier with light that attracted food. This symbiosis lasted more than a hundred million Thiocytheran years, long past the extinction of the shark darts.

The first complex life to colonize the land was a worm-like organism, just an unassuming featureless creature with only a set of hair-thin antennae for smelling with. Like most of Thiocythera's lifeforms, it was based on radial symmetry, so while it had no back or belly or left or right sides, it could crawl along on any of its 6 poorly defined sides, its nearly circular cross section flexing to conform to the ground. But each side had its own eye and its own antenna. Later descendants of this "inchworm" would include taxa with more or fewer sides, ranging from 5 to 9. The microbes it had hunted in the seas were not available on land, so it adapted to graze on land corals. Later descendants of the inchworm included the segmented worm, which had evolved segments independently of its distant marine relatives. Segworms were mostly 8-sided, and the first one to three segments bore eyes and antennae. Segworms were also able to extract silicon from the silicates in the land corals they ate, and could suspend a silicon nanocrystal between a ring of proteins and use it as a photoreceptor. In order to detect light, silicon must be "doped", meaning the two halves of the crystal must have impurities that produce an excess of electrons in one half and a deficit in the other half. Segworms actually accomplished doping their silicon using phosphorus and variously boron, aluminum, or zinc. This was a huge development because their ancestors' eyes, adapted for the narrow range of light wavelengths present in the seas, could not take advantage of the full spectrum of light available on land. But the silicon nanocrystals could.

Land corals, already threatened by inchworms, were now being ravaged by segworms, vicious hungry creatures with selective-adhesion proteins, acute vision sensitive to a wide range from infrared all the way down to UV-B, an advanced sense of smell, and in many species, rudimentary legs with claws for digging silicate exoskeletons open. Evolutionary pressure was intense. Fortunately, life mutates often and evolves quickly on Thiocythera, since the lack of free oxygen means there is no ozone layer to protect life from UV-C rays. There is also no magnetic field to help protect from radiation from space, although the thick atmosphere absorbs most charged particles. Land coral cells began to group together inside their silicate exoskeletons, thickening the silica into a branching structure that protected the life inside, even at the cost of making growth more difficult. The result was a beautiful array of what I will call glassy shrubs, resembling a Chihuly-esque garden of transparent sculptures, in all the same vibrant colors as land corals bore. Segworms could still feed on glassy shrubs, but did not devastate them, and actually offered a benefit because they could transmit spores from one shrub to another, increasing the diversity of the shrub's cells and allowing it to absorb more light since not all cells were using the same wavelengths. Thus a symbiosis developed between the shrubs and the worms. The shrubs became multicolored, and from segworms evolved a specialized clade of entities I will call caterpillars, that were best adapted for glassy shrub symbiosis and had advanced color vision for seeing all the shrubs' hues. Inchworms could not compete and mostly died out with the land corals, though a few species of inchworms persisted as scavengers.

One group that descended from segworms was the verticals, consisting of three stacked segments roughly equivalent to head, thorax, and abdomen. The head segment bore eyes around its circumference; the thorax segment had wispy tendrils for smelling the air, and the abdomen segment bore legs that the entities walked with. Verticals were the first to incorporate large amounts of swombho-cross-linked elastic thiostarch in their tissues, giving them a spongy, springy, mushroom-like appearance and smell. The first verticals were scavengers that used their tendrils to find food and could run equally well in any direction. Once food was located, they'd flop down onto it and crawl over it like worms, eating through a mouth in the top of the head. But later species had evolved multiple mouths on the sides of the head, each one a vertical slit between two of the eyes. Some even had only one slit mouth, and therefore had a front and a back. Verticals had only minor importance at first, but would come to dominate the planet much later.

A type of jellyfish-like organism gained the ability to photosynthesize when it happened to absorb a manganese-based photosynthetic microbe. While previously these jellyfish things would grow rooted to the seafloor and produce numbers of free swimming medusae that would look for a new place to take root, the new photosynthetic medusa found it more advantageous to stay in one spot and stack upward. The result was a tower of purple transparent discs sticking out of the water. At first the stacks would just grow, becoming looser near the top, until a disc broke free and swam along looking for a new place to take root and start the cycle again. But a novel kind of mutation happened. A microbial infection caused one of the discs near the top to branch out into a shape like a bunch of radiating fronds, similar to the pathology known as witch's broom of earthly plants. The result was a purple entity that resembled a palm. The infectious microbe became part of the palm, forming a symbiosis that enabled the combined organism to reproduce, and palms soon evolved seed-like spores with long wispy tails that could be carried on the wind and take root where they land.

Sustained flight appeared on the planet for the first time. Surprisingly, flight had not been attempted by any lifeform before. A species of vertical evolved with a detached membrane connected to the top of the head segment. The membrane was connected at 5 evenly spaced points, alternating with the 5 evenly spaced eyes. In the space between the head and the membrane, the organism could adjust the temperature by emitting heat that would inflate the membrane with buoyant atmospheric gases. The other two segments became merged into the bottom of the head, and the 10 stubby legs would evolve into claw-tipped toes capable of picking things up and carrying them away. This was the first in a lineage I will call air balloons, although they more resembled an apple pie with a small parachute on top. Air balloons evolved the strategy of gliding over water, unfolding their toes to capture aquatic lifeforms as food.

Global Warming

As time went on, Thiocythera experienced continuous volcanism. Since it did not have tectonic plates, a volcano might appear anywhere the crust was sufficiently thin, and volcanic gases slowly filled the atmosphere. In the short term this was good news for life, since volcanic gas contains carbon dioxide, sulfur dioxide, and hydrogen sulfide, all of which were involved in the planet's biological sulfur cycle. But over time, these volcanic gases accumulated faster than the lifeforms could use them. The gases increased global temperatures via the greenhouse effect. Seas began to evaporate, making the atmosphere humid and resulting in the formation of a planet-wide cloud deck. The skies became overcast with acidic clouds; the seas were reduced to lakes and ponds, and daylight was reduced to a sulfurous yellow glow. Most of the planet's surface became deserts. The days were also getting longer due to a slowing of the planet's rotation, probably due to tidal forces from the planet's close proximity to its star. A day on Thiocythera came to last about 60 hours and continued to lengthen.

Under these warm muggy skies, lifeforms had to conserve water. Many organisms evolved a strategy of using pores lined with hygroscopic salts to soak up as much humidity from the atmosphere as possible during the early morning hours, then closing off their pores for the long day to minimize evaporation. Such entities could be seen looking puffy and globular in the morning, and noticeably thin with a shriveled texture by late afternoon. Some organisms instead sequestered their water in underground organs or reservoirs. One example was the trapdoor, a creature that somewhat resembled an upside-down octopus. Its 7 arms extended outward from a central bulge that was completely buried in the ground save a mouth with sharp teeth growing out of where the lips would be, and gear-shaped grinding mechanisms down inside the mouth. The arms were camouflaged against the rocky ground, even down to having a rough coarse texture. But millions of microscopic hairs sensed the motion of anything passing over them. If a wandering lifeform happened upon one of the arms, the limb snapped into action, engaging an adhesine protein to trap the prey, and quickly carrying — almost throwing — the hapless victim into the trapdoor's gaping mouth.

A few unrelated organisms independently evolved to switch from photosynthesis to thermosynthesis; these could also live underground, away from the worst of the heat and away from creatures that would eat them. Others adapted more efficient light gathering and water sequestering methods. Descendants of the glassy shrubs came to possess a silicone-like exoskeleton made of clear flexible thiostarches, instead of the silicate minerals of their ancestors. By combining all of the photosynthetic colors together, they maximized their harvesting of daylight, and by living close to the shore with roots anchored in the sands under the water, they could stay hydrated all the time. These entities I will call ivy tubes formed ground cover in various shades of dark brown, dark red, and black. Amid the ivy tubes scurried tiny segworms and caterpillars, as well as multi-legged descendants of some of the sea creatures that had been forced onto land by the vanishing water.

A type of tripod-like vertical, disc shaped with three spindly legs and an arm for picking up things off the ground, waded through the ivy tubes eating the little bug-like creatures. Tripods retained radial symmetry of their eight eyes and could see all around in 360° view, with parallax, however their mouths had migrated down from the top of their single segment to the bottom, and one of their four limbs had to become specialized as a grasping appendage. Additional eyes on the underside of the disc scanned the ground for things to eat. Tripods were fast runners, but had to always be ready to sprint away from a predator. Squatting down to excrete waste could undermine their agility, so they solved that problem by evolving a tubular anus that launched the waste like a projectile. They would aim outside the ivy tubes so as to not contaminate their food source. The waste fertilized the bare ground enabling the growth of tall fungus-like lifeforms, resembling upright pinkish wrinkled sausages, descendants of microbial mats that adapted to land when the seas shrank.

The ivy tubes had an effect on the ecosystem, since they were transporting cool water over the ground, even if the water remained entirely enclosed. The resulting cooler ground made an attractive place for all kinds of organisms, including a clade of small verticals that had become sedentary. These verticals began their lives as tiny, fully mobile creatures, but would look for a cool, well-lit place to settle and put down literal roots, the abdominal segment growing into a branched subterranean network of water-seeking mineral-uptaking fibers. The head segment would then grow out into a flat or domed cap, the thorax became a fibrous stalk, and the whole creature took on the appearance of a mushroom, complete with gills, a spongy texture, and a musty smell due to metabolism of swombho- amino acids. Some species were photosynthetic, taking in daylight through a brown or dark gray upper surface of their cap. Others used thermosynthesis, absorbing ambient heat through their gills. Some were both. But their requirement for the contradictory conditions of good light and lower temperatures meant the ivy tubes were the perfect place for them.

Soon, these mushrooms had evolved to be completely sedentary, dropping their spores under their caps, and some even glowed at night from under their caps in order to nourish their spores with additional light. The resulting mushroom forests each became a miniature ecosystem shielded from the worst of the day's heat. Small "bugs", descendants of the multi-legged creatures the tripods once ate, evolved to hop or fly from cap to cap, eating a powdery secretion emitted by the mushrooms and pollinating them. Others scurried around the forest floor, scavenging whatever happened to be laying around. Some bugs were even bioluminescent themselves, reusing molecules from fallen mushrooms. As the last of the ponds evaporated or were subsumed by ivy tubes, the mushroom forests became marshes, cool shady wet oases containing the last of the planet's liquid water.

Outside of the marshes, the remainder of the planet became one big desert. Some verticals were successful there that could thermosynthesize, running among the rocks and dust to maximize absorption of heat from the searing ground. Thermosynthesizing mushrooms took to the deserts and evolved into tall spindly forms, first with a red or brown photosynthetic cap, but later developing a hollow beige head that stored buoyant gases. These tether balloons, as I call them, towered over the desert on tough fibrous stalks of cysteine-cross-linked rigid thiostarch, and could reach several hundred feet, or more than a hundred meters, in height. A type of inchworm evolved that I will call a balloon worm, which used the undersides of the balloons as safe mating grounds, and would spend the night piled up in and out of a torpor, followed by climbing down the stalk in the daytime to search for another tether balloon, thus spreading the balloons' genes around as well. Spores as big as bean bags would grow around the equator of each tremendous balloon, eventually detaching one by one and parachuting to the ground where each spore would take root and grow into a new tether balloon.

Predatory verticals evolved that had a single head segment, no thorax, and a squished up abdomen sporting dozens of crab-like legs. The mouth extended horizontally under the eyes, and wrapped around more than half of the head. Multiple rows of sharp teeth protruded from the top and bottom of the mouth, and instead of a throat or esophagus, the whole thing opened into a stomach area with a drain at the bottom that led to the anus. These fearsome creatures resembled a cross between a spider and a hamburger. I guess I could call them burger crabs. Most species became nocturnal, evolving a dark blue-gray coloration that camouflaged them in the shadows. The new threat of nocturnal predation meant other verticals had no choice but to hide at night.

In the final stages of the hot overcast era, three separate species of verticals, under pressure of avoiding predation, developed a degree of technological civilization. The first of these were the Membranous Ones. They were of a subtly mottled dark bluish gray coloration, taller than a human, with a modified six-sided symmetry that gave them a front and a back. The head segment was conical with three faces separated by membranes. Each face had two eyes and a mouth. The back of the head was covered with a hood-like membrane. The membranes extended from the head all the way past a tall, roughly hourglass-shaped thoracic segment, to the bottom segment which was more pear shaped. Each membrane was elastic and prehensile and could function as a limb. The thoracic segment also bore filamentous antennae for smelling. From the bottom segment extended six octopus-like tentacles, each tapering to a point. The Membranous Ones developed a civilization roughly equivalent in technological complexity to our ancient Egypt. They built shelters out of stone and used a symbolic motif resembling a citrus slice or a bivalve shell with radiating spokes.

Around the time of the appearance of the first burger crabs, the planet became too hot to support ivy tubes and they went extinct, causing the marsh ecosystems to collapse, and ultimately resulting in the extinction of the Membranous Ones. Rainfall, already uncommon, became much more scarce.

A few million Thiocytheran years after the end of the Membranous Ones' civilization came the Curious Ones. These were uniform beige with an oblate head segment having 8 eyes alternating with 8 thread-like antennae that waved around in the winds and ambient currents. The thorax bore 8 relatively featureless tentacles, 6 of which were prehensile and quite dextrous. The abdomen segment was elongated in one direction, forming a backside, with the eight stubby legs arranged as four pairs along the bottom, giving the entity a centaur-like appearance. The two thoracic appendages located directly over the abdominal extension were less developed than the other six. The mouth was a vertical slit located between the two frontmost eyes.

Curious Ones attained a civilization comparable to our ancient Rome. They built long straight roads through the desert to connect their buildings, some of which were tall and narrow and had transparent walls made by fusing sand into something similar to glass. They farmed and ate immature tether balloons from spores gathered during expeditions. When lightning struck a tether balloon, the gas-filled top segment would burst, felling the vast organism onto its side. That made a distinctive sound that the Curious Ones would notice. A party would go off in the direction of the sound to retrieve the balloon segment, which was highly nutritious. They would then grow any spores outside their homes and harvest them when the immature trees got to be a bit taller than the house itself. Curious Ones also kept a creature I will call a chicken, that would perch on roofs at night and peck at the ground during the day. Chickens tasted bad to predators and were mildly toxic, so they could be left outside at night; in the morning they would make beautiful melodic sequences of radio wave frequencies and awaken the Curious Ones.

Hellscape

Continual rising temperatures made things progressively harder for life. The extinction of the tether balloons caused a huge ecological collapse, with nearly all other life soon following, including the Curious Ones.

A few hundred thousand Thiocytheran years after the disappearance of the Curious Ones came the civilization of the Bouncy Ones. They were greenish gray and consisted of 3 mostly featureless segments, fully radially symmetrical, with 6 eyes around the equator of the head segment. The thorax had 6 flap-shaped prehensile appendages. Rather than walking on legs, they bounced like kangaroos on an abdominal segment that resembled a yoga ball. Even when standing still, their highly gelatinous bodies would undulate up and down. Their civilization advanced further than the Curious Ones had, and they reused the Curious Ones' road system. But dwindling food resources and rising temperatures ultimately ended their time on Thiocythera.

Liquid water no longer existed on Thiocythera, except inside lifeforms, and the only way to stay hydrated was to absorb ambient humidity or eat other creatures. Burger crabs resorted to eating each other, and different species quickly evolved distinctive colorful markings so that the adults could avoid eating their own species' young. The skies were black with polysulfides, and the ground felt like an inferno. The ground became a reeking landfill of deceased organisms, piles of rot crawling with burger crabs, the air thick with ammonia and phosphine. Hydrogen was being lost to space, as ultraviolet light split water into hydrogen and oxygen, the latter being quickly used up by oxidation of sulfur. The atmosphere dried out and the cloud deck turned to sulfuric acid.

Burger crabs were the last thing alive on Thiocythera, and their various species were already quickly going extinct when the end happened. Volcanic pressures under the surface had been building for a long time without adequate release, and the pressure finally broke through. Islands of solid rock floated between rivers of lava. The pieces of solid surface melted and capsized, ultimately becoming one vast lava pool that covered the entire planet. All traces of life and civilizations were destroyed. This was the Global Resurfacing Event, or GRE, and because of it, all Thiocytheran surface life was now extinct.

D-alpha-2-O-thiostarch Fairly stiff, owing to dense packing of the subunits.	D-alpha-3-O-thiostarch Stiff with a slight elasticity, especially when wet, from hydrogen bonding between 5 and 3' oxygens.	D-alpha-all-S-thiostarch Tough and rigid due to steric effects of the carbon and sulfur atoms.
D-beta-2-O-thiostarch Good flexibility owing to steric effects of the oxygen atoms relative to the conjoining sulfur.	D-beta-3-O-thiostarch Somewhat elastic due to hydrogen bonding between 5 and 3' oxygen atoms.	D-beta-all-S-thiostarch Bends easily, posable with a waxy-plasticky consistency.
Pure D- and pure L- thiostarches had identical physical properties, although cross-linking with chiral amino acids could cause small differences. But when adjacent subunits had opposite chirality, the flexibility and elasticity of the polymer could increase:
DL-alpha-3-O-thiostarch Elastic and rubbery; the strand tends to curl into a helix.	DL-beta-3-O-thiostarch Elastic and compressible; softens when wet.
The above physical properties were estimated by building physical space-filling models of the thiostarches.
Example of a spongy swombho-linked thiostarch.	Example of a tough cysteine-linked thiostarch.

Thiocythera - a Fictional Venus-like World