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Appendix B

Jopian Life

          Jopian life originated and evolved within the mid-tropospheric circulation of Jopitar's belts. Habitable regions, mainly between 500 and 620 kilurets depth, form giant tori that girdle the planet, and collectively comprise the Jopian biotorus system. Here hot convective plumes rich in thermal energy and vital raw materials regularly percolate up from deep below. These plumes, which were even more active and widespread early in the planet's history, expand outward and cool as they rise. They vary in size up to a several hundred kilurets across at mid levels, where they tend to break up into distinct streams, or rivers. Clusters of life-sustaining hot springs and fountains are spun off along the river peripheries.
          The first Jopian entity that could properly be called life, commonly known as Alphabios, probably developed from silico-organic residues some two bevujopes (3.2 billion Earth years) ago, around upwelling plume currents near the base of the habitable range, where ambient temperatures hovered near 600 nevlu (300° Celsius higher than the boiling point of water on Earth) and the pressure exceeded 100 times that on Earth's surface. Outside the plume environment, silicon was found in significant quantities only below the silicate (SiO2) cloud tops, some 370 kilurets deeper and more than 280 nevlu warmer. But inside the plume river currents, temperatures were high enough that appreciable amounts of silicon were lifted through the 620 kiluret depth mark, mainly in the form of silane (SiH4). Temperatures at this level were low enough in the cooler surrounding fluids for complex organic compounds to develop there.
          Even the most primitive organisms required an energy source and a supply of raw materials to survive. Energy ultimately derived from the same abyssal heat that powered the deep atmospheric circulation. The silane and other chemical species present in the scalding plumes and associated springs were not in equilibrium with the mixture in the milder surroundings. This disequilibrium drove chemical reactions along the perimeters of the hot flows, which became the basis for metabolic processes.
          A majority of the silane was pyrolyzed to tightly-bound silicon dioxide, which precipitated back into the depths. But a fraction reacted with water to form polymers of hydrated silicon dioxide, which in turn combined with carbon compounds to form durable silicone polymers (characterized by a silicon-oxygen backbone with carbon-based organic side groups). Some of the silicones tended to form ultrathin sheets. These folded into themselves, creating pockets that collected pure buoyant hydrogen, and providing protected surfaces and spaces for self-perpetuating polymers and chemical systems to evolve. Carbon (from methane), nitrogen (from ammonia), oxygen (from water), sulfur (from hydrogen sulfide), plus a variety of trace elements were involved in this chemistry. The silicone structures stabilized complex molecules from heat degradation, and provided a skeleton from which living organisms could evolve. Silicones have been a fundamental structural material for Jopian life ever since.
          Organic materials caught in the main flow of the mid-tropospheric circulation experienced lethal variations in temperature and pressure over the course of a complete cycle. But objects with neutral buoyancy that lingered outside major updrafts and downdrafts experienced a comparatively benign range of conditions, allowing evolutionary forces to work. Some of these longer-lived structures had a tendency to grow, and shed pieces of themselves. Originally (before a genetic code was established), these "offspring" were not perfect copies structurally, though they were functionally equivalent. They soon outnumbered and choked out their non-reproducing cousins. The very plumes that supplied raw materials did introduce a degree of instability to the life zone, limiting the time any of these free-floating objects remained before being dragged into the general circulation. Alphabios was the first cell-like biochemical entity that managed to reproduce at a rate faster than that of its removal by stray currents.
          The most persistent membrane systems eventually assumed a regular, double layered, spherical form, surrounding and supported by a bubble of pure hydrogen, with diameters (averaged over a typical growth cycle) of 200 to 400 micrurets, much larger than typical unicellular organisms on Earth. The hydrogen was generated by simple chemical reactions within the membranes, and provided buoyancy. The mean size of these primitive cells was determined by the requirement of neutral buoyancy: larger ones floated too far upward, away from the silane sources; smaller ones sank too deep, and degenerated in the escalating temperatures. Since growing cells tended to become more buoyant, reproduction became linked to and triggered by a minimum ambient pressure, corresponding to a maximum diameter.
          Metabolic processes took place in airy scaffolding that occupied the narrow space between the inner and outer membranes. Unlike Earth life, which evolved in a liquid environment, the free volume was not filled with liquid, but a comparatively low-density supercritical fluid – neither gas nor liquid, but possessing characteristics of both. Because the atmosphere of Jopitar was (and still is) chemically reducing, with essentially no free oxygen, Earth-like aerobic metabolism was not possible for the early life forms. Methane and water (rather than carbon dioxide and water, as on modern Earth) were typically combined to manufacture foodstuffs, stabilized within a silico-organic matrix. Hydrogen was released in the process. Energy was subsequently reclaimed from food stores by controlled reaction with molecular hydrogen, regenerating methane and water as byproducts.
          The scions of Alphabios evolved in multitudinous ways throughout the prolonged primordial period, driven by the twin forces of natural selection and mutation. More efficient energy production and transfer reactions developed. Metabolic energy yields approached those of anaerobic Earth bacteria. Several distinct genetic codes for the transmission and processing of structural and functional information evolved and competed for some time. Eventually the so-called SNA scheme, based on sequences of six distinct code molecules on closed loops of silicon-oxygen polymer, became dominant. Vestiges of the other ancient genetic systems remain in the reproductive elements of cellular organelles in a variety of organisms. It is interesting that the genetic codes and biochemistry that spontaneously arose on Jopitar are all based on mixed chains of silicon, oxygen and carbon, while those on rocky terrestrial worlds (such as the DNA-based genetic code of Earth life) tend to be carbon based. This difference can be explained by the markedly lower temperature and pressure conditions under which life on terrestrial planets generally develops.
          The earliest SNA-based cells were probably spherically symmetric, with uniformly distributed interior elements surrounding a central hydrogen bubble. The space between inner and outer membranes gradually reorganized, as the bodies developed a more stable and efficient top-bottom asymmetry in Jopitar's strong gravity. Most of the heavier functional and genetic cellular machinery gathered together along the bottom, which tended to hang in a downward position. The inter-membrane space expanded in this region, supported by a network of fine microtubules, which also guided the movement of biomolecules. The hydrogen bubble moved to the top of the cell, and became more balloon-like. The dual membranes in this area pressed together, forming a thin, tough, semi-elastic envelope that secured the hydrogen.
          By this time the most successful microorganisms had assembled most of their genetic material into a single circular loop of SNA, which was anchored at a dedicated site to a propitious patch of inner membrane near the bottom of the cell. These microbes reproduced by simple fission. During this process the main SNA loop would replicate, the cell pinch inward and divide through the (vertical) replication plane, and each daughter receive a single identical copy of the original SNA. Most cells also harbored so-called plasmods – short loops of secondary genetic material, analogous to plasmids in Earth bacteria. The plasmods typically replicated independently of the main SNA loop, and were distributed randomly between daughter cells during cell division.
          Although this mode of reproduction could lead to the widespread dissemination of a favorable mutation through a new cell line, it was unable to bring together favorable traits arising independently in different lines. Yet cells would occasionally collide. Some had spotty coatings that caused them to temporarily stick together. The earliest microbes developed a tendency to exchange genetic material through transient openings in their outer membranes, and then separate, a process analogous to conjugation in Earth bacteria. While this mechanism was not strictly sexual, it did allow favorable genetic mutations to be shared. Microbes with this ability readily became preponderant. Many evolved hairlike extensions, analogous to bacterial pili, that facilitated the process. A subset of these structures subsequently evolved into sillia, movable appendages analogous to cilia and flagella, with which to swivel around and manipulate other cells. Acquired genetic material was either carried by the host as a new plasmod, or integrated into the main SNA loop or one of the preexisting plasmods.
          Primeval microorganisms also developed an ability to enter an inactive state, and form a durable endospore, when confronted with hostile conditions. A microbe could be carried into an inhospitable region by a strong plume current or downdraft. Alternatively, the entire local plume complex could fail. To this yad, a typical plume waxes and wanes in strength over a period of jopes, until it ultimately dissipates, stranding any resident microbe colonies. Dormant cells revive when they drift into favorable environs. Microbes caught in a strong updraft, and carried into the upper-tropospheric circulation, can even be blown as spores into an adjoining belt, and thus spread across the entire planet.
          Alphabios and its close descendants passively maintained a hospitable depth in the atmosphere by their innate buoyancy. A cell might be lifted several kilurets while feeding in the rising currents near a fountain, but would drop back to its equilibrium depth after being thrown clear. If a microbe were caught in a weak downdraft, its inherent buoyancy would limit the maximum depth and temperature it would endure. Because most species maintained the same inter-membrane spacing throughout life, however, cells did naturally become more buoyant as they grew. Conversely, cells abruptly became less buoyant during division, when they shed bubble hydrogen. Microbes consequently tended to cyclically rise and sink through the ambient air over the course of a life cycle. This was beneficial, since nutrients needed for growth were more plentiful at greater depths, while reproduction was more successful in the cooler temperatures higher up. But these organisms otherwise helplessly relied on the currents to bring nutrients to them, without sweeping them away. Life had no chance for significant further development until mechanisms evolved that actively drew creatures toward favorable environments, and away from inhospitable ones.
          A crucial step occurred when microbes acquired an ability to adjust their natural buoyancies, mainly by modifying the volumes of their hydrogen bubbles. Soon changes in buoyancy were actively linked to various cell needs. For example, the normal increase in buoyancy might be retarded while a young cell was feeding, and only augmented later as the cell matured and prepared to reproduce. A cell might expand or deflate its hydrogen bubble if conditions became too hot or cold, respectively, regardless of its state of maturation. Tiny temperature sensors evolved in the outer cell membrane, together with pressure sensors along the inner membrane, which facilitated the new behaviors.
          In parallel with these developments, many species acquired an ability to glide horizontally during vertical movement. The more advanced of the silliated microbes sported a pair of flattened sillia on opposite sides of the cell equator, for efficiently turning around their vertical axes. The disposition of these sillia strongly affected overall motion as a cell rose or fell through the surrounding air. Various lines randomly developed tendencies to extend the sillia in different ways during ascent or descent. Some patterns (for example, ones that caused a cell to spin) were detrimental, and quickly weeded from the population. Others were beneficial, and favored by natural selection. In particular, many microbes developed a tendency to "face" nearby hot springs, with opposing sillia on either side laterally, and to extend the sillia outward during vertical motion. The direction to the springs could be now be sensed, as certain types of the newly evolved temperature sensors also naturally served as primitive infrared detectors. The most successful of these creatures tended to angle the paddle-like sillia downward in the direction of the springs. This practice became common, as it would cause maturing microbes to glide away from hot springs into cooler surroundings as they ascended through the ambient air. Following division, daughter cells would slip back toward the springs as they descended.
          Soon microbes evolved that combined these various abilities, in order to approach, withdraw, or maintain distance from a hot spring as they rose or sank in any combination. For example, a microbe could angle its sillia upward in the direction of a spring while increasing its buoyancy, and thus glide toward the spring even as it ascended. These changes became coupled with more flexible propensities to approach warm, nutrient-rich upwellings when hungry, and to seek higher, cooler environments when mature, while maintaining a given range of temperature and atmospheric pressure. Depth control was accomplished now by exploiting the currents (e.g., backing away from a surging fountain when pressure fell below a critical value), in addition to adjusting cell buoyancy.
          Microbes that acquired these new instincts attained a survival advantage over their cousins. They would seek out food, rather than passively wait for it to come to them. While living continued to be risky, the likelihood of being swept away was reduced, so the cell types grew progressively more abundant. A larger, richer habitat opened to them.
          The risk remained comparatively high near the borders of the atmospheric belts. Here the weak prevailing latitudinal currents could sweep a microbe over the top of the local convection cell, into the general subsidence of the adjacent zone. Nourishing springs were rare in the zones, and an active organism would starve even if it managed to maintain a hospitable depth. Microbes that preferred currents flowing toward the central latitude of the resident belt would have suffered fewer losses along the borders. While an ability to sense planetary latitude was not attainable by the microscopic creatures, various cell lines accomplished the next best thing, when they randomly incorporated (rare) iron atoms into heat sensory molecules, and acquired an ability to sense Jopitar's normally south-north magnetic field. Instincts subsequently evolved to prefer springs in the locally appropriate southerly or northerly direction. Yet a rigid directional instinct was not advantageous. When a cell type spread from one side of a belt to the other, an originally adaptive directional preference would have become maladaptive. In addition, the magnetic field of Jopitar is not completely stable, and periodically reverses. A small but regular mutation rate in the directional preference was thus beneficial. Every new generation harbored a small proportion of (expendable) individuals with a previously maladaptive response, ready to exploit changes in local conditions.
          Significant differences ultimately arose between coexisting cell types. One group continued to specialize in utilizing raw materials bubbling up from the depths, and grew progressively more proficient at the chemosynthesis and storage of beneficial complex organic substances, in particular higher-energy food stores. A breakthrough occurred when some members of this group began to perform photosynthesis, harnessing the net flux of infrared radiation from the hot plumes and springs. All these cell lines ultimately led to today's chemosynthors and photosynthors, the Jopian version of chemosynthetic and photosynthetic bacteria. Both early synthor types remained dependent on the plumes and springs as a source of silicon, and maintained instincts to feed near them at the greatest tolerable depths, where nutrients were most abundant, but then ride the peripheral currents to higher levels to reproduce. Synthors continue to thrive at the deepest levels, where they remain microscopic and unicellular to this yad.
          Another group of microbes took advantage of the advances made by the synthors. Originating through a variation in the gene-swapping mechanism, these microorganisms would attach to the synthor cells, but then secrete enzymes to immobilize and digest them, absorbing needed complex substances through small openings in their outer cell membranes. The synthor cells were identified as prey by distinct genetic markers. The predators evolved to feed at the upper reaches of the synthor domain, where conditions were more hospitable and the synthors most ripe, then ride the currents to still higher levels to reproduce themselves. This overall shift to more moderate temperatures and pressures permitted more complex structures to evolve.
          The subsequent developments of the synthors and the predators were interwoven, as each group forced the evolution of the other. Synthors with tougher walls were more difficult to consume, so they became more plentiful. Separate microscopic lines emerged that sported fine silicic bristles to ward off predators, or secreted selective poisons against predatory cells. The predators in turn evolved more effective coatings and antidotes to the synthor toxins, and adapted sillia to seizing their prey. They also developed new chemical sensors for detecting synthors, but continued to rely on heat sensors to locate warm upwellings, and the swarms of synthor microbes invariably found nearby.
          New predatory lines emerged that fed on other predators. These were able to occupy progressively higher, more hospitable levels in the atmosphere. A hierarchical ecosystem emerged, spanning depths from the original 620 kilurets up to roughly 500 kilurets, or temperatures from 600 nevlu down to 500 nevlu. Silicon requirements diminished at higher levels, as lower temperatures allowed more versatile carbon atoms to replace an increasing fraction of the silicon in structural materials. Complexity likewise tended to increase, even as the density of microbes decreased, since predators at any depth remained dependent on life lower down for both silicon and ordinary food.
          In many of the higher-level predators, one or more plasmods grew comparable in size and importance to the ancestral primary SNA loop, and the distinction between these loops disappeared. Maintaining the number and configuration of all the major genetic loops during cell division became critical to producing viable offspring. As cells grew more complex, the overall organization of cell structure and activity similarly became more important. One by one, the ability of the original SNA loop to selectively attach to the inner membrane near the bottom of the cell was transferred to the new loops. These became anchored to a cluster of short tubular protrusions from a common patch of inner membrane, which folded inward, and eventually formed a protective sac around the loops. This region expanded somewhat, and became functionally equivalent to the nucleus of a eucaryotic Earth cell. Mutosis, a process analogous to mitosis in Earth life, emerged in which nuclear SNA replication and cell division were controlled in an orderly fashion.
          Haphazard transference of genetic material through the outer membrane was no longer an optimal method for the nucleated microbes to attain novel genetic characteristics. A primitive mode of sexual reproduction subsequently evolved among the nucleated predators. At first, these cells were normally haploid, and maintained a single copy of each distinct type of SNA loop. Yet when nucleated cells collided and stuck together, they would sometimes fuse. If the cells were not closely related, the two nuclei would typically compete for control, until either the fused cell died, or one nucleus overpowered the other, and appropriated all cellular resources. If the cells were closely related, with homologous (not necessarily identical) SNA loops, the fused cell would functionally become diploid. The two nuclei would be drawn to a common site, where the homologous SNA loops would be guided to adjacent anchor points. Cell unions that were able to maintain a diploid status and still divide by mutosis were often more viable than their haploid brethren (especially during times of stress), and flourished.
          Sharing of genetic material by fusion alone was self-limiting. Further fusions of diploid cells rarely produced viable offspring, due to space limitations within the combined nucleus. Yet the successful diploid cells inherited their nuclear mechanisms from haploid predecessors. Originally, the mutotic apparatus interpreted the existence of paired identical SNA loops as a trigger for cell division. Under certain conditions, in particular when nutrients were abundant, the same apparatus now interpreted the existence of paired homologous loops as a trigger. In this event, a diploid cell would divide into two haploid cells. Just prior to division, the homologous loops would align, and confused ancestral SNA repair mechanisms would randomly swap homologous gene sequences. The two daughter cells would again be haploid, but now with unique combinations of genetic characteristics.
          This mixing process was advantageous, but had one critical problem. Destructive genetic crossovers frequently occurred between paired homologous loops, creating nonviable segments of SNA. This was rectified when the SNA loops opened up at their anchor points, uncurled, and became anchored SNA strands. Now crossovers resulted in additional constructive mixing of genetic material. Other facilitating mechanisms rapidly evolved. Muosis, a process analogous to meiosis in Earth life, emerged in which diploid cells would mix genetic content between homologous SNA strands, then divide into haploid cells.
          Many cell types now developed a diploid-haploid life cycle. These microbes would normally live in a diploid state, growing and dividing by mutosis, until changing conditions prompted them to divide by muosis into haploid cells. The trigger might be a sudden abundance of nutrients, if the cells were able to grow and multiply more rapidly in the haploid state under these conditions, and so better exploit the situation. The haploid cells would be prompted to fuse back into more resilient diploid cells, when conditions returned to normal. Alternatively, the trigger might be a severe deterioration of conditions. In this case muosis would produce inactive haploid spores. These would reactivate, and multiply as haploid cells, or fuse into diploid cells, when conditions improved.
          Nucleated microbes with the ability and propensity to reproduce sexually with compatible cousins soon became common. Many lost the ability to divide asexually more than some fixed number of times. When these species later generated multicellular life forms, constraints on cell division would be largely responsible for the aging process, and restrict both the lifespan and the maximum size of an individual organism.
          Some of the nucleated predators acquired an ability to ingest smaller cells by phagocytosis. In this process, a portion of a predator's lower outer membrane first invaginated to enclose a prey, and then engulfed it whole. The prey cells were usually digested, but occasionally survived intact, and some formed endosymbiotic relationships with their hosts. The predator microbes were already accommodated to harboring a discrete nucleus, and typically affixed internalized cells to their inner membranes. Many of the captured cells evolved over time into specialized organelles. Their obsolete hydrogen sacs withered away, along with any capacity for independent existence. The organelles maintained a restricted ability to reproduce within their hosts, and copies were divided between daughter cells during cell division.
          One family of advanced predatory hosts established an endosymbiotic association with wayward photosynthors, and subsequently specialized in photosynthesizing high-energy food stores. An ample flux of infrared radiation was available even at the peak of their range, both from the ascending hot rivers and springs, and (to a lesser extent) from the warmer air directly below, so these microbes achieved a large degree of independence from life at deeper levels. They still needed to ingest (more primitive) microbes to obtain silicon, which became rapidly depleted as the currents rose. Silicon was required in particular prior to cell division, in order to manufacture additional structural material and chemical stabilization elements. These hybrid photosynthetic-predatory microorganisms, which share characteristics with Earth plants and animals alike, led the way to the modern Jopian version of plants. A second group of advanced predators subsequently specialized in feeding on the energy-rich plants, spawning today's animals. Both plant and animal populations swelled, but remained loosely tied to the hot plumes and springs.
          About half a bevujope ago, some silliated plant cells acquired a tendency to stick together following division, to form colonies. The most successful configuration initially was a simple spherical shell, with sillia pointed outward. While these groupings by no means displaced the solitary one-celled life forms, there were numerous advantages to a colonial existence. It was comparatively difficult for a unicellular predator to phagocytize colony cells. In particular, the cells of a colony protected each other by reducing the exposed surface area vulnerable to attack. Individual cells of a colony could be sacrificed, without jeopardizing the organism as a whole.
          The plant colonies evolved quickly. Most began filling the interior space with pure hydrogen, providing extra buoyancy. This in turn permitted affiliated cells to shrink their individual hydrogen bubbles, and so become more compact and less puffy. These organisms soon developed a top-bottom asymmetry, much as happened with their single-celled predecessors. Cells along the bottom hemisphere specialized in food production, processing, and other functions entailing bulk. Cells covering the top reformed into a thin, flexible skin for the internal hydrogen cavity, which now swelled balloon-like above the rest of the colony, stabilizing it vertically.
          Small islands of cells in the lower hemisphere adapted to snaring and digesting prey microbes (needed for silicon). These eventually formed small pits, lined with sticky silliated cells, in the outer colony surface. Other cell clusters specialized in photosynthesis or reproduction. Loose networks of helper cells developed inside the primary cell layer, forming channels both for distributing nutriments, and (later) for transporting waste products to the digestive pits for expulsion.
          Colony movement was originally accomplished through the collective actions of the individual member cells. As a common internal hydrogen balloon became established and cells specialized in various functions, this changed. A separate mechanism evolved for controlling the balloon volume, and thereby initiating overall vertical motion. In larger plant species, the sillia of cells along the equator lengthened and interconnected, to form a series of short paddles responsible for turning a colony around its vertical axis. Adjacent cells specialized as infrared sensors, to locate nearby heat sources. Most of these colonies concurrently developed a front-back asymmetry, in which the photosynthetic sites became more efficiently clustered on a front side, which was actively oriented toward nearby hot springs, and to a lesser extent in the bottom polar region, which utilized excess infrared flux from below. Many organisms ultimately acquired an ability to move sideways, by angling opposing equatorial paddles as they rose or sank through the surrounding air. All these modes of movement were similar to those of the plants' one-celled ancestors. Although they tended to be slow and cumbersome, only limited mobility was needed, since the plant colonies produced most of their own food.
          In the more complex plants, the interior helper cells gradually engendered a complete second layer of cells, lining the hydrogen cavity. On top, the new cells joined the original layer to form a stronger, more proficient balloon envelope. Lower down, the inner cells specialized in chemosynthesis and storage. As the plants became more complex, their energy demands grew. Because only anaerobic respiration was possible, development of higher-energy foods was essential. As with more primitive creatures at warmer depths, these were stabilized during storage within silico-organic matrices. Energy yields about half that of aerobic respiration on Earth were ultimately achieved, which proved sufficient for more complex multicellular creatures to evolve.
          Animal colonies probably appeared shortly after the first multicellular plants. They may have acquired at least some of the requisite genetic blueprint directly from ingested plants, although the extent of such a transfer is speculative. Some may have even been rogue plant colonies that shed their photosynthetic capabilities. Like the plants, the first animal colonies were spherical shells comprising a single layer of silliated cells. This cooperative arrangement was again more resistant to attack by other predators, and offered an enhanced ability to entrap one-celled prey. These forms were also much more successful than their unicellular relatives at dispatching the early plant (and genetically distinct animal) colonies. A primitive predator colony would press flat against and attach to a prey colony, then collectively remove and consume targeted cells inside the contact zone.
          Because stable vertical orientation promoted survival, the animal colonies quickly developed top-bottom asymmetries. Unlike the plant colonies, which generated plentiful excess hydrogen by photosynthesis and converted the central cavity into a common topside hydrogen balloon, most multicelled animals initially continued to rely on the internal hydrogen bubbles of their individual member cells for buoyancy. To achieve vertical stability, the set point of each cell's bubble became dependent on the cell's vertical location within the colony. Bubbles along the top (dorsal) surface were inflated more, and bubbles along the bottom (ventral) surface less, than for an isolated cell.
          Most of the early animal colonies also acquired front-back asymmetries, more efficient at ensnaring prey. The front (anterior) end specialized in predation. A concave depression developed here, which deepened into a pit, lined with silliated cells that drew quarry into a sticky trap. Species that fed on other multicellular organisms evolved rings of raspy spurs just inside the pit opening, used to firmly attach to these prey and rip away cells. Excess nutrients ingested by pit cells were passed on to the free space at a colony's core, and made available to other colony cells (hydrogen and other inorganics were initially absorbed directly from the atmosphere). Indigestible debris was periodically expelled from the pit. A small pocket of cells at the rear (posterior) end soon became responsible for reproduction. This primitive organ could release haploid gametes, which drifted away in search of mates.
          In the more advanced animal colonies, the digestive pit deepened further and elongated, folding inward and forming an analogue to the endoderm of Earth animals. The protected interior pit cells could now focus even more on digestion. The external cell layer took on defensive and sensory roles, analogous to an ectoderm. Cells encircling the forward orifice specialized in detecting prey by chemical means. Small spots, typically one on either side laterally, further adapted to detecting the infrared signatures of other organisms, and the warm springs that supported abundant life.
          The bodies of many of the larger animal lines lengthened and narrowed. In most of these species, the digestive pit reconnected with the surface just below the rear end of the body, forming a more efficient, one-way digestive tract, and transforming the anterior opening into a mouth. The free space between the inner and outer cell layers shrank, until nutrients were passed directly from the digestive cells to the outer cells. Eventually a new layer emerged between the other two, analogous to a mesoderm. Cells in this layer initially specialized in food processing and storage, or (later) in material transport. The middle layer ultimately gave rise to a wide variety of internal organs.
          Because they did not manufacture their own food, mobility was much more important to the multicelled animals than it was to the plants. Very early, many adopted a strategy of riding peripheral spring currents to heights well above their neutral buoyancy levels, then slipping out of the updrafts to hunt and feed as they dropped back down. The bodies of these creatures elongated further, and flattened along the base, allowing them to glide toward prey. Many developed billowy wing-like projections along either side, used to catch updrafts and to control the speed of descent. A fine, lightweight bony structure evolved along the body length, providing beneficial rigidity. Sillia of cells at the posterior end lengthened, strengthened and connected to form a primitive, rudder-like tail fin. Smaller control fins also evolved from silliated cells along either side.
          When prey or their habitats were detected, sensors at the front end of these animals would secrete a primitive neurochemical that diffused back toward the tail, stimulating appropriate movement. To induce a suitable dive angle, the front end of the body was made less buoyant than the rear. At first, this was accomplished by transferring hydrogen between the bloated cells along the dorsal surface. In time, many animals developed long, distensible, dorsal buoyancy bladders to perform this function. These internal bladders also allowed body cells to become more compact. The fins guided movement to a target. The tail later evolved mesodermal muscle fibers to provide additional locomotive power. The spine became jointed, and provided an anchor for muscle attachment. Strings of cells connecting the forward sensors and tail eventually specialized at relaying information along the length of an organism, forming a rudimentary nervous system. Eventually, the main longitudinal nerve trunk would be protected inside the spine.
          Interactions among the various microbes and multicelled organisms drove their coevolution. Larger, more complex plants and animals arose, to fill emerging niches in the Jopian ecosystem. The plants developed new strategies to counter the escalating animal threat. The external surface of the hydrogen balloon atop most sizable species became a tough but thin, inedible skin. An array of surface cells lower down focused on protecting the appetizing vulnerable areas covering the bottom hemisphere. These cells typically produced hard but lightweight chitinous secretions, derived from silicic metabolic byproducts, which organized into assemblages of overlapping scales. Yet while the scale system permitted colony growth, it interfered with photosynthesis. Before long, primitive leaves extended out from under the scales, into the open air. Flattened leaf cells drew methane and water directly from the outside atmosphere, and other nutrients from the colony itself, then used infrared photosynthesis to manufacture foodstuffs. These were shuttled along stubby stems into the colony for further processing, distribution and storage. Individual leaves could be sacrificed to browsing animals, without endangering the overall organism.
          Concomitantly with scales and leaves, islands of connective tissue arose in the main plant body between the two primary cell layers, forming an ultralight but strong, foamy supporting matrix, which enhanced the organism's physical integrity. Most of these plants also developed rootes – retractable appendages for capturing passing microbes and acquiring needed nutrients from them. The rootes would typically slide out between scales on the underside of a plant, and dangle downward, to gather ripe, hyperinflated cells rising from far below.
          With successive generations, the scaled armor proved so effective that it crept further and further up the sides of the most successful large plants, until it pinched into the hydrogen balloon. The bulk of the balloon was gradually squeezed outside the main plant body, and transformed into a swollen bladder, connected to the core hydrogen cavity by only a pliant, rubbery neck. During this development, an efficient chemical hydrogen pump evolved in the balloon wall. The pump used infrared radiation to dissociate hydrogen molecules in the surrounding air, then transport the individual hydrogen atoms into the interior. There the hydrogen atoms would reunite into hydrogen molecules, releasing heat and warming the contained gas, providing additional buoyancy. The balloon wall became more insulating, to maintain an elevated internal temperature. Excess cooler hydrogen spilled out of the base of the balloon, into the interior of the main plant body.
          As these plants matured, their external scales tended to stiffen, until they became encased in crusty bark. This bark provided superior defense, but ultimately limited the size of an individual organism. The underlying surface cells that specialized in reproduction released haploid spores or gametes to the outside, or generated buds asexually that bloomed into new globular forms. Originally these broke off to form independent colonies, but species evolved in which they often remained attached by hollow stalks. Soon these plants were forming tiered complexes of interconnected leafy bushes, buoyed by graceful hydrogen bladders.
          The larger animals were increasingly pestered through this period by tiny single-celled and small, leech-like multi-celled parasites. The external cells adapted by secreting a smooth, protective skin of flexible but tough silicone over all outer body surfaces. Two sillia-lined slits appeared on either side of the mouth, which admitted hydrogen (for respiration) and other needed fluids into the body, and allowed methane, water vapor (respiration by-products) and other waste fluids to be expelled. A variety of internal organs evolved. These included lungs for more efficient exchange of metabolic fluids, a network of channels for more efficient intercellular transport, structures that filtered waste from the transport channels and dumped them into the digestive tract near its terminus, and glands that secreted digestive fluids. An endoskeleton of fine, lightweight bones provided a supportive framework for these organs. Most of the creatures already had internal buoyancy bladders. Yet these remained comparatively small, even as the animals grew in size and complexity. Buoyancy was a much less critical factor than for the multicelled plants, since the animals relied more on active motion to maintain their depth, and could even cling to the plants for support when needed.
          Four broad categories of larger animals evolved: mannavores, herbivores, carnivores, and omnivores. The first group adapted to feeding exclusively on the multitudinous variety of so-called manna (one-celled plants and other microorganisms) that existed by this time. These creatures filtered the manna microbes from the air, and then swallowed them whole. The herbivores adapted to feeding on the multicellular plants, and evolved in parallel with them. These animals typically employed either a pair of opposing silicic ridges (inside the mouth) or a chisel-like beak to bite off leaves, and grind through plant defenses. The carnivores relied on fine quartz teeth to prey on multicellular animals of all types. Their teeth were sharp but comparatively brittle, and must have broken frequently with use, but were readily replaced. Modern thicket serpents (which have been largely eradicated) and ribbon serpents are descended from this group. The omnivores consumed a mix of plants and (generally small) animals.
          Some of the herbivorous prey animals responded to the predator threat by hiding on the same floating bushes that they normally grazed. Many of these established a symbiotic relationship with their hosts. They would eat only what was needed to establish a population of a size and mass the plants could support, and in turn would defend the plants against other, more destructive herbivores. Some of these species evolved social behavior, which promoted the development of a more sophisticated nervous system and central brain. The upper corners of the breathing slits adapted to generating and detecting sounds. This allowed members of a social group to warn each other when a predator or competitor approached, and ultimately to communicate in ever more sophisticated ways.
          Mutations occasionally appeared in some of the larger plants, which resulted in small openings into the central cavity. Some of these would be sloped and fashioned in a way that preserved the internal hydrogen atmosphere. While even these had previously been deleterious, and weeded out by natural selection, now they benefited plants that hosted supportive animals. Access to the central cavity offered these animals more protection against their own predators, and allowed the creatures to more effectively protect the plants from competing herbivores. The resident animals could now feed directly on internal plant stores, sparing the precious external leaves. Many developed more omnivorous eating habits, exploiting in particular the tiny, insect-like creatures that periodically infested the plant cavities.
          Swimming between neighboring plants remained risky for the plant dwellers. Then a new form of plant reproduction emerged, in which an asexual bud would not always break off into an independent plant. Instead it might remain attached to the parent body by a hollow stalk, even as it grew into a new globular form. This allowed resident animals safe access to multiple plants, which in turn indirectly benefited their hosts. The mutation spread rapidly. Soon there were floating thickets, branching complexes of leafy bushes buoyed by graceful hydrogen bladders. Paired appendages appeared in the animal inhabitants, which allowed them to move more efficiently through their extended homes, and fashion comfortable nests. These changes encouraged further brain development. Thicket-based creatures with eight appendages are the ancestors of modern octos.
          The large free-swimming mannavores lagged behind the colony-dwelling herbivores in mental development. They generally remained solitary and non-social, simply growing over time in size and bulk to counter the carnivore threat. By about 16 megujopes ago, they encountered increasingly stiff competition from groups of smaller but more advanced social animals that nested in the plant thickets but fed on manna in addition to the multicellular plants. Herds of these creatures would leave the protection of their nests to feed on manna at nearby hot springs. Over the generations they spent more and more time in free flight, soaring high on the currents out of the serpent habitat after feeding, until finally some broke free of their nests altogether. This new type of mannavore led to the reys.