Unprecedented! Inconceivable!
Never before had so much changed because of one new magic spell.
And yet, the discovery of one single spell put the various "human"* races on a very different trajectory than imaginable.
"Reverse Gravity" had been centuries in the making. It required an in depth understanding of the mechanics behind gravity, it's relationship to mass, and the math behind it's range of effect. It was the team of Mordenkainen and Bigby of Oerth who first cracked the secrets of Reverse Gravity. Once they had the concepts of gravity correct, it was only a matter of math to discover how high of a level the magus would need to be to maintain the spell's duration long enough for someone to escape the gravity well of their planet. Eventually, Reverse Gravity was expanded so it could be cast at 9th level, lasting longer and taking fewer casts to break the grip of gravity. A few modifications to Otiluks' sphere spells, and Mordenkainen and Bigby were ascending to the cold darkness of Wildspace together in a bubble of air.
But as the Council would later discover, things on various worlds in the Radiant Triangle tended to happen simultaneously. Elminster of Toril had discovered the spell in his explorations, as did Raistlin Majere of Krynn. In fact, wizards on half a dozen planets in the Radiant Triangle were ascending the stars in various ways, all utilizing the simple method of reversing gravity within a certain area of effect. And it was inevitable they would meet up. The Council of Wizards was started by a handful of almost godlike wizards, floating in a bubble of oxygen in the cold depths of space.
To their amazement, the original Council discovered that all their worlds contained the same mixture of sentient, human life forms. Elves on Krynn were mostly the same as the Elves of Oerth, the dwarves of every world in the Radiant Triangle were pretty much the same. Most amazingly, the languages tended to be mostly the same. Some percentage of words would be different, but the most common words like "the" or "and" were identical, from groundling race to groundling race.
This could be no coincidence, they thought. So they formed the Council as a way to control the access between space and their homeworlds while trying to discover the link that made so many of the races so similar to one another. Then, the answer seemed to present itself as if from nowhere. An alien race that called themselves the Arcane visited the Council, bearing magic items of great power, and claiming a shared heritage with the "human" races of the Triangle. They sold magic items called Helms for exorbitant amounts of gold, which enabled a craft to fly in space. It handled all of the magic necessary, and only required a spellcaster to exert all their spell slots for the day into it. For centuries, the Council took the Arcane at their word, and worked hand in hand with them to promote space travel and commerce between the Triangle planets and the rest of the known spheres.
But the answers the Council sought were not forthcoming; the Arcane would never completely answer any questions or submit to enough testing to fulfill their investigation. While they certainly looked humanoid, beyond the blue skin, there seemed to be enough differences that many of the council began to doubt. Once the doubt began amongst the Council, it was only a short time before they started to work on their own version of Spelljamming helms, and the other technology to replace the Arcane.
The solution began on Ebberon, where a team of mages and artificers succeeded in creating the first element rod, and began fitting ships with them. The element rods were an elegant solution but they still did not generate enough thrust to escape Eberron's gravity. But at least, once they got the ship out of the gravity well, the elemental rods would allow the ship to move. Next the Fraternal Order of Wizards on the planet Cael made available certain wizards and mechaniks that had developed interesting combinations of magic and technology, and a lot of their works went into the first running versions of a human powered spelljammer. In time, the combined efforts of mages, artificers and mechaniks all over the Golden Triangle resulted in a system that no longer needed the expensive Arcane Helms. Some of those old helms still remain, and work just fine. But the issues with helms have never changed; spelljamming with an Arcane Helm takes all the helmsmans spell energy. He cannot cast spells when sitting in the helm, and when his 8 hours are over, he has no spells memorized any more. But, some people opt for that because the Helm does most of the work.
The Arcane took the hint, and grew distant from the Council, eventually ceasing all contact with them or the human races. Who they were selling things to after that, could only be guessed. In half a century, the Council had perfected the technology of rune plates and trapped elementals. Meanwhile, a variety of races, most notably the dwarves, took the matter into their own hands, developing non magical methods. The Council was somewhat concerned; the use of arcane magic to travel the stars had been their method of control, but it became clear, they would always keep the majority of space travel under their direct and/or indirect control.
At present, the Council still controls 75% of the lift offs from the worlds of the Triangle, through the control of the spell that started it all, Reverse Gravity. A Council wizard lives in every port city, and authorizes and performs the lift off spell for payment or barter. Some newer research is threatening to replace the Reverse Gravity spell with other tech that can provide lift off from a decent sized planet. Of course, at some point, all those technologies are based on magic, and the Council can always find a way to throttle their competition. The Arcane have moved on, dealing with other races in other spheres, who knows where. The various alien races in the known spheres, such as the Scro, the Ilithid, the Neogi, the Beholders, etc etc watch "humankind" carefully. Council involvement has promoted humankinds' discovery of the stars in a quick enough fashion as to be able to stand against those races if necessary. Of course, their alliance with the Elven Territories doesn't hurt.
* "Human" , or "Humankind" is often used to describe all the various demi human races in a single term. One can assume the term applies to humans, dwarves, elves, gnomes, and halflings simultaneously.