Augustus is a large J-Class gas giant with aerial life. It is the 4th planet of the 1st star in the Tolkien binary system, home to 13 planets.
The atmosphere of Augustus is different than most gas giants in that it's made of oxygen, not the typical helium and hydrogen of most J-Class worlds. However, there are bands of hydrogen within the planet's atmosphere. Unusual flying creatures have adapted to life there, but life originally didn't exist in Augustus' atmosphere.
Life came from the neighboring planet Nocoma, a warm E-Class world with basic unicellular life. Billions of years ago, Nocoma was struck by an asteroid which wiped out all life save for some hardy microorganisms that eventually made it to Augustus where they began to evolve in their new environment.
The bacteria became multicellular organisms. Them majority of life on the planet appear similar to terrestrial whales or dolphins, but also have a slightly bug-like appearance. They breath in the small amount of helium in the planet's atmosphere, and use it in large bladders to stay afloat. Some also have wings, though it is hard to fly through the dense atmosphere. Some life has adapted harder exoskeletons and have migrated to the lower regions of the gas giant where the pressure is so high that the gas turns into a liquid of oxygen and helium. The creatures 'swim' through the honey-like substance, and produce bioluminescence.
Discovery by Humans
A human ship discovered the planet in the 73rd century. They colonized it and built a floating research lab there to learn about the strange creatures that lived there. During that time Theodore Musk, the descendent of Elon Musk, decided to build a large city in the clouds, connected to other cities.
The new human colonists left the flying creatures in peace, although some were tamed to help build by transporting supplies to construction sites. The project was finished in the early 74th century. Billions arrived at the planet and started up a civilization there.
However, disaster almost struck the natives Augustians. It wasn't discovered until human colonies were set up but carbon dioxide was toxic to the hydrogen-helium based lifeforms that lived in the clouds. Mass die-offs occurred as carbon dioxide built up. The human colonists discovered that a gas giant with similar properties existed orbiting the other star in the Tolkein system. Large teams captured all the creatures they could find and transported them to the planet (named Peram), where they lived away from humans. They started to recover, and humans continued to colonize the mega-cites that floated on the planet.
With their transport to Peram, the creatures began to slowly evolve. The Augustians soon established something akin to a tribal system. As the humans continued to expand their floating cities on Augustus, the flying creatures evolved without them knowing. They then began to peer out at the stars and wonder what was out there. But they were stuck floating with no land to build cities on ando materials to build floating cities.
Then, a mutation occurred. An Augustian was born with a harder exoskeleton then the normal creatures, and could store the helium for long trips instead of constantly having to breath in more. The creature was fascinated by the stars, attempted to fly out of the planet's atmosphere and succeeded. He told his companions about the discoveries he had made. The creature eventually had 3 children, and they had carried down that gene of strength exoskeletons and respiratory endurance. He took his young out for flights around the planet. Eventually, they grew up and passed on their genes to other creatures. The genome spread throughout the planet.
The creatures were no longer bound to the atmosphere. They could experience the cosmos at will. But they still couldn't go that far out, and needed to return to their planet for more helium. Then one named Aydo. flew to one of the planet's moons, a temperate S-Class world. She was the first of their kind to go to a different stellar object. But she did not make it back in time to get more helium. Aydo was remembered as a very strong figure in their culture.
After humans had pretty much covered Augustus's surface with mega-cities, they decided to return to the Augustinians and check on them. They were amazed to see how they had evolved in such a short period of time. Humans called them Perambulantes, Latin for 'skywalker.' The humans helped the perambulantes by building them a large city to live in the atmosphere. The Perambulantes had the intelligence of 21st century humans at this time and formed a peace with humans, but under one rule: the humans should not interfere with their evolution beyond this point.
Thus, the perambulantes were left alone to live on the city. They used the technology in the city to invent their first rockets, and were able to fly to Peram's largest moon, the same moon that Aydo had discovered and died on. The Perambulantes built a lunar base there. Their technology continued to improve, and eventually they colonized a few other planets in their half of the binary system.