Episode 3: First Step of Evolution

From the rise of phototrophic organisms, which cause the oxygen level in the atmosphere to rise, Aerobic Respiration emerges soon after. Over time, aerobic respiration-organisms become more complex, developing colonial behavior, and eventually, a multicellular organism is born.

First Multicellular Organism
With the rising oxygen level, aerobic microbes' potential reaches its fullest. One group of the heterotrophic microbes cluster together into a colony. This increased size of the colony give them an extreme edge against other microbes; they no longer falls prey to other microbes while feeding on other microbes. This make the colonial microbes' population explode in no time, becoming widespread throughout the whole ocean.

Within these heterotrophic, colonial microbes, some have gone even further, developing tissue-like layers with the outer layer composing of dermal-like cells an collar cells. While the inside is filled with gelatinous fluid where amoebocytes move inside. With this development, the Bulbozoa ("Round Animal") was born. Being the first true multicellular organism. They float about in the ocean currents, taking up oxygen and nutrients directly from the surrounding water.

The emergence of the bulbozoa changes the ocean in a massive way, however, it returns to a stale state again within millions of years, until an event occurs, which triggers evolutionary arm races among the population in the early sea, the "Multicellular Predation".

Radial Symmetry
Later in the history, some of the bulbozoa become more rounded and somewhat heavy, causing them to easily sink to the ocean floor. While rolling around on the seafloor at first, which causes some of them to fall into a deep trench where food is scarce, some group eventually able to stick themselves to a hard surface like rock by developing a pad with sticky surface. In favor of the natural selection to this development, this group was able to develop further, and become a sessile organism.

The change in lifestyle has changed their morphology even further. Becoming sessile means reduced access to nutrients floating in the currents, in which some group was able to develop a trait that become the signature trait of the clade. A ring of six long and flat appendages that look like a leaf. The choanocytes, instead of covering the whole body thoroughly, now concentrate on the the appendages, which helps greatly in feeding on planktons. As they become fully sessile, the method of sticking to a hard surface has changed, which is first done by the juveniles resulted from a fertilization instead, although they can still reproduce asexually like their ancestor. This can sometimes result in a colony of the local population.

As mentioned above, this clade has developed a form of sexual reproduction. An individual can produce both type of gamete, in which it is released from the center of the appendage ring. They have also developed a strategy of "Broadcast Spawning", where every full moon, the local colonies release enormous amounts of eggs and sperms into open water. When a sperm and an egg meet, which only has a small chance, fertilization occurs and the egg develops in to a small larva. The larvae are small enough to stay afloat in the water current. The larvae's appendages, or "feeding arms", are capable of beating to a limited degree, granting them some degree of movement. Once the larvae reach a point in their life cycle, they will settle on the seabed and metamorphose into adulthood.

With its ring of leaf-like appendages that act as its mouth, this clade is dubbed Phyllostoma ("Leaf Mouth").

Bilateral Symmetry
Around the same time, another group of bulbozoa, which has more cylindrical and lightweight body, start to have greatly increased. Their lightweight body makes them easily carried by water current. Their long, cylindrical body, which has developed into true tissues, as well as the more advance nervous system, makes them able to control the direction they are moving to a degree by wiggling their body. This clade also develop a pair of eye spot on one side of their body. Eventually, an important trait has occurred, which practically changes the future of this clade forever, that is the "Cephalization".

Their central nervous system becomes more concentrated toward the end where the eye spots are located, as well as developing a nerve ganglia. The posterior side of the body developed into a long, flat tail for swimming even more effectively. With the developed front end and back end, a single directional movement becomes possible. When the creatures swim through water, it face water filled with oxygen and plenty of planktons, which promotes the development of a "digestive tract" that consists of a deep cavity at the front connected to the "cheeks", as well as an opening just under the base of the tail, creating a cannel where water flow through the creature. This cannel, which the choanocytes lining the inside and thin layer of tissue lining the cheek openings, makes filtering planktons and absorbing oxygen easier, which in turn increases the upper limit of the creature body, allowing them to grow bigger. They also developed three pairs of fins used for stabilizing and turning.

Like their cousin, these creatures has also developed a method of sexual reproduction. However, each individual can only produce one type of gamete. When two individual of the opposite sex meet, the sexual organ activates and produces the gamete, which is then released into the water through the same opening as the digestive waste. When the egg is fertilized and developed, the juvenile, which looks like a smaller version of the creature, has to take care of themselves since the first moment.

As their guts develop further, and become true gut, where digestive enzyme is released from the cells lining its guts, enabling of digestion of larger matters, while cells that line the later part absorbs the nutrient back into the body. Due to the enlarged body, the self-movement of the amoebocytes, the cells that transport various chemical throughout the body, become not efficient enough. To deal with this problem, the creature develops a hollow organ that automatically beat on itself with several openings with valves. These valves only allow a one-directional flow of the hemocoel within this organ, and from the activity of this organ create a flow within the hemocoel, increasing the transportation efficiency of the amoebocytes throughout the body, which now move along the flow instead of on its own. This organ essentially becomes creature's "heart".

This clade is dubbed Hexapterida ("Six Fins") due to their characteristic three pairs of fins.

To Evolve Further
As their guts has developed further, the creature is able to specialize for specific kind of food sources and branches out.

One group specializes in feeding on clouds of planktons in the ocean. The mouth part widens even further to increase the amount of water intake, essentially increasing the amount of planktons it can consume. And with the increased nutrition intake, it causes a feedback effect, causing the creature to increase in size even further, becoming a giant of the sea. Its giant size, however, making swimming with its small tail become less efficient. Being a filter feeder, however, meaning that it doesn't require as much speed as other type of pelagic dwellers, thus its six fins enlarge to aid the tail, which also enlarged, in swimming. Its massive fins, in proportion to its already gigantic size, generate a powerful thrust in a single, combined beat. Its body also become more hydrodynamic to not hinder the creature.

While the adults are essentially immune to predation, the juveniles, however, are not. Due to their smaller size, the juveniles can fall prey to a predator that has just emerge around the same time as them. To protect itself, the creatures evolved a hard, chitinous exoskeleton its whole body, except for the fins and the tail. The juvenile also secretes a substance from their skin that binds with calcium ion in the water, making the exoskeleton as hard as rock. The juvenile also has a very high growth rate, reaching half the adult's size in around two years.

This group is called Aspidotitania ("Shield Titan") for their whole body armor and gigantic size.

Another group specializes in feeding on larger organisms. The widen mouth allows them to feed on creatures smaller than themselves. Such diets give them large amounts of energy in one go, causing them to enlarge as well, but not to the degree of their cousins. The lifestyle also changes according to the new traits. To be able to obtain food, which is smaller organisms, the creature evolves a larger, fluked tail which gives the creature powerful acceleration for it to be able to chase down prey. It also evolved larger fins to be able to control the movement more easily when accelerating, as well as a streamline body to reduce resistance from water. To catch its prey effectively, its mouth is able to close down to prevent prey from escaping, as well as special muscle lining the digestive tract that move the prey from the throat into the stomach to be digested. Further evolution resulted in the mouth part elongates and lined with small ridges to hold down onto its prey more easily before swallowing.

Similar to its cousins, its juveniles are vulnerable to predation, but from same species themselves. However, its lifestyle makes evolving hard shells unaffordable as it need the speed and flexibility to chase down preys. Instead, the juveniles use their speed and smaller size to quickly hide in rock formations.

The present of jaw in this group earns its name Gnathopterida ("Jawed Fin")