Jor-El was a great scientist, arguably the last great scientific mind of the planet Krypton. Born under the dim sun of his homeworld, he had only legends and stories about his people's former glory, tens of thousands of years previously.
Jor-El came of age on a planet trapped in its death throes. Krypton was suffering from centuries of cellular degradation in their genetic stock, from overuse of cloning technology combined with the damage of the radioactive rocks in orbit. In short, they were a race that was facing extinction and doing so with an almost bored weariness. Jor-El's experiments and attempts to repair his own genetic code were proving to reach the limit of their effectiveness. He was healthier than many Kryptonians, but had only borrowed more time.
Jor-El enacted a dangerous gambit. He found and repaired one of the very last hyperspace ships, a craft so old it was functionally derelict. With it, he flew off in search of the last hope of Krypton: DNA from the long-lost colony of Khera, cousins to Krypton.
His path took him somewhat furthur afield than he intended. Jor-El found himself orbiting the planet of Earth. Whether by fate or destiny or a curse or luck, he picked up traces of Kheran technology on the surface after several days of orbit. Jor-El was a brilliant scientist, but an inexpert aviator, and promptly crashed his shuttle into the Libyan Desert on his descent. Midway through the process of repairing his ship, he was attacked by an armored Panzer battalion that were under orders from Rommel himself to retrieve the alien vessel.
Jor-El's shock and confusion was so great that at first he didn't realize what had happened when he snapped the handcuffs with a twist of his wrist. One of the Germans drew a gun and shot at him-- a ballistic weapon, crude beyond imagination-- and the impact barely stung.
Empowered under the light of a yellow sun as no Kryptonian had been in generations, he began single-handedly fending off the entire battalion of soldiers. He was relieved to find he wasn't alone, as a black-clad figure stealthily dismantled them from their rear flank. The two met in the middle, and there Jor-El met Azzuri of Wakanda.
Their trip would bring them east, first, trying to avoid the Axis reinforcements. In Tunisia, the duo met another unlikely pair: Carter and Sheila Hall, aka Hawkman and Hawkgirl, fresh from their own encounters with the Axis invaders. The four resolved to learn more about this war that clearly was overtaking the world, and with Carter and Sheila explaining the dangers of this new German military machine, Azzuri and Jor-El agreed to lend their abilities to the Allied cause. Agreeing it was best to keep his origins disguised, Jor-El simply adopted the moniker the terrified Germans had given him: Ubermensch. In English: 'Superman'.
Jor-El came to regard this strange, diverse group as the family he'd never known. The stalwart Captain America, the bristling competency of Diana of Themyscira-- the Justice Defenders were alive with purpose, something Krypton had lacked for many years.
Jor-El stayed with the Defenders to the bitter end, until the Nazi Moon Base was scoured and Adolph Hitler killed. The farewells were long and tearful, and Jor-El made one last stop with Azzuri before he went home: a return to Wakanda. The Kheran heart-shaped herb had for centuries influenced the DNA of the hereditary noble family, and this DNA was crucial to repairing the damage done to the Kryptonians. He repaired his ship and settled in for the long hyperspace sleep back to Krypton.
He arrived in the midst of disaster. Their star had abruptly started to enter a pre-nova stage. What should have taken a thousand years might bring down his world in mere decades. Intense and unpredictable solar winds drove chunks of Kryptonite down from above. The Kryptonians were forced to abandon their domes and disappear into caves underground.
Jor-El worked furiously, aided by his family. The sterility plague unleashed by Oa affected both male and female chromosomes, and would require a complete rebuild of both genetic sequences. His brother Zor-El and his brother's wife Alura had a daughter: Kara Zor-El. Thirteen years later, he and his wife Lara conceived a child-- Kal-El.
Jor-El and his family spent their remaining days working virtually without rest, converting the hyperspace ship into a pair of escape pods. The timing would have to be perfect. Too soon, and the children might crash into the Kryptonite field. Too late, and... too late wasn't an option. The death of their sun was imminent.
The first detonation of their sun sent massive earthquakes rippling through Krypton. The initial blast of plasma scorched the far side of the planet in seconds, but blew away the mineral cloud in the atmosphere.
Jor-El and his family launched the two lifeboats into the sky, plugged into the only coordinates Jor-El could conceive of as a safe haven for the children: Earth. The last thing any Kryptonian alive would see in the night sky was the last hope of Krypton, rocketing away to a distant, backwards world on the edge of nowhere.