The Octopi emerged on a mostly tropical, water-based world and slowly evolved over hundreds of millennia. A large number of more primitive lifeforms also existed and evolved during this time, most at sea, but some on land. It rained a lot, so land creature's development was hampered. The marine life was a dangerous one, wrought with many perils, from tiny to massive. The Octopi, consisting of seven species, were generally rather small, and so learned to work together for defense of the bigger creatures. Through this repeated interaction, they achieved a form of primitive communication. Dye sacs emerged and they could signal a mate, confuse prey, or flee. Color-changing cells emerged, and they could blend in with their surroundings or bedazzle a suitor.
However, other marine life were vying for the top of the food chain also, and fighting intensified. Some Sharks and Whales also learned to cooperate, and began hunting and defending in packs (pods.) This quickly decimated the Octopi (and to a lesser extent, Shark numbers), while most whales (the largest creatures) were unaffected.
Natural Selection eliminated the weak, slow, and less intelligent. Most of the Octopi were destroyed, except for the fast and highly adaptable Veined Octopus. The sharks were more fortunate (being larger), and Starlight Sharks emerged as their apex predator. Whale population skyrocketed, with the K.O. species becoming the apex predator.
However, while the Starlight Sharks and K.O. Whales used might to duke it out, the Veined Octopus used subtlety. It blended in. It watched from the shadows, always learning. Their numbers slowly rebounded as their camouflage, creativity, and intelligence increased. Their size and strength increased. They began hunting larger prey in packs. They learned to live near rocky shore, where sharks and whales had difficulty catching them. They learned to use the broken shells of fruits and other creatures as armor. They even learned to go further up the shore for short periods when hunting parties came in to avoid them altogether. And when the occasional predator did make it on shore and became beached, the Veined Octopus' sheer numbers and venomous bite ensured that it would be an easy meal.
All of this evolution had profound ramifications for the Veiny Octopus. They developed small air sacs for breathing oxygen to lengthen the amount of time they could remain out of water. They learned to take prey on land to feed. Their on-land eyesight improved. They learned cooperation and leadership. Eventually they learned to build above-water "dens" or "gardens" by piling rocks into a bowl-shape, and using ink, saliva, and mud as concrete. They brought water up to the den and left it there, for a quick rehydration when needed. They learned to use rocks and pointy things as weapons.
Sentience occurred when they learned to divert the freshwater runoff to their above-water dens. They did this to provide an almost limitless supply of oxygen-rich water outside of the dangerous oceans. They formed societies and primitive machines, created freshwater reservoirs, and discovered fire (which only worked above-water, how weird is that?) Despite being able to live partially on land, they didn't enjoy it much, as traversing dry land for long periods was impossible. They learned to stock their reservoirs with creatures from the sea, for an almost-unlimited food supply. (Until their population skyrocketed and all of the food was eaten. Those that didn't die of starvation fled to the sea to hunt, but were killed by sharks and whales. This is the legend of the "Great Slaughter.")
Fast-forward 30 millennia, and the Veiny Octopus can spend about 1/4 of it's time in air, and still lacks a natural skeleton. They have perfected thin, carbon-nanotube protective armor for water-use, so no longer worry about ocean predators. Underwater, they have built large cities and "floorscrapers" (underwater skyscrapers) a thousand arms' deep, protected from the rest of the ocean by electrical shock technology. For land use, they created exoskeletal rigging, allowing them to be about as strong and mobile as a human, but with less stamina. They have refined uranium and created nuclear weapons and reactors, but found these far too dangerous and polluting, so sunk all the materials to the bottom of the Mariner Trench. They have made submersibles, automobiles, and aircraft, but found little use for them other than military might. They have created rockets and sent Octopi to the moon, however it is very costly as water is difficult to get out of orbit. The next furthest planet in the solar system is used for mining and manufacturing (to keep the pollution there) but it has little liquid water and is rather cold, so is otherwise uninhabitable.
Modern families are allowed to have one or sometimes two offspring per solar cycle, as determined by the Unified Nations Government. This is to keep population levels steady. Every youngster goes through schooling, and an aptitude test is given to show where they can contribute the most. Post-education is a requirement, but the course(s) are up to the individual. A fine balance is maintained between food supply, healthcare, and population, as not to repeat the Great Slaughter. Healthcare is very advanced, with current research going into adding collapsible bone-like muscle structures to their physiology. Currency has been eliminated for a life-long trade system. (Everyone does something useful and productive each day, otherwise "rights" are revoked.)
Commonly Useful Science and Technologies:
- Hydration suit, available remote water source.
- Protective (water) armor, or method thereof.
- Rapid water transportation system (fluid pipes?)
- Waste reclamation, re-processing, bio-degradation.
- Water filtration, aeration, purification, reduction of pollutants.
- "Cockpit" may be water-filled and fitted with previous.
- Water-traversing vehicles.
- Land-traversing (mechanical,cyborg) exoskeleton.
- Land-traversing transportation/service/farming/excavation vehicles.
- Land-based facilities that cannot operate underwater (Air filtration, generator, solar power, etc.)
- Water-based facilities that cannot operate in air (Water filtration, high pressures, fusion reactor?)
Most of these names are fictitious of course. But this is an interesting read about the real Veined Octopus: http://greatmysterypublishing.blogspot.com/2012/08/the-amazing-octopus-did-you-know-they.html