Since her creation in 1959, Supergirl has been killed, temporarily replaced with non-Kryptonian versions and rebooted. Due to this, her origin has been modified and revised several times. Several common details remain the same, though: Kara Zor-El is Superman's teenager cousin, born to Zor-El—Jor-El's brother and Superman's uncle—and Allura in Argo City, a Kryptonian city that survived the demise of Krypton thanks to an energy bubble-like force-field erected by Zor-El. When Argo City was also being destroyed Kara was sent to Earth where she met her cousin and eventually donned the mantle of Supergirl.
Kara Zor-El was born over a decade and half after Krypton's explosion. At some point her parents found out their nephew Kal-El had survived and was living on Earth. When Kara was fifteen, a meteor shower pierced the dome surrounding Argo City and shredded the lead shielding protecting the Argonians from the radioactive soil. Before Kryptonite poisoning could claim their daughter's life, Zor-El and his wife Allura built a space rocket and sent her to Earth where she was found by her cousin. Superman was thrilled to find out he had family left, but he couldn't take her in, so he sent her to Midvale Orphanage where she adopted Linda Lee's identity as Superman trained her. Eventually Kara was adopted by Fred and Edna Danvers—changing her name to Linda Lee Danvers—and when Kal-El deemed her as ready, her existence was revealed to the world.
Supergirl's origin remained the same from 1959 to 1985, when she was killed during the Crisis on Infinite Earths. However, the original story told in Action Comics #252 was very bare-bones: Argo City was a bunch of houses surrounded by a bubble of air that was hurled away intact by sheer luck, Kara had no surname, her mother was not named, and their family didn't knew about Superman until they were looking for a world to send Kara to. Posterior issues fleshed out and tweaked her origin.
Kara Zor-El was reintroduced to the DC Universe in the 2004 storyline The Supergirl from Krypton. However, her back-story was changed several times until Geoff Johns and Sterling Gates established her definite origin in Action Comics #869 and Supergirl (Volume 5) #35 (part of the Superman: Brainiac and New Krypton events respectively).
In her updated origin, Kara is actually older than Kal-El, being fifteen when Krypton exploded. Argo City was saved thanks to Zor-El researching Brainiac's technology and using his findings to build a force-field around the city. Nonetheless, a Brainiac's android activated and started wrecking the city and killing everyone. Zor-El and Allura shoved Kara inside a space rocket that Zor-El was working on—just in case the city's shields failed—and blasted her into space, asking her to find and look after her baby cousin. Unfortunately, Kara's space rocket got stuck in a Kryptonite asteroid and the pod's emergency systems put her in suspended animation.
Thirty years later, Kara's pod crash-landed in Gotham's bay, where she was found by Superman and Batman. Wonder Woman insisted on taking her in to train her, and Kara spent several weeks in Themyscira until Darkseid abducted her. After being rescued from the Lord of Apokolips, Kara made the choice to become Supergirl.
Supergirl's origin was again modified when DC rebooted its universe in 2011.
Aware that his brother was right and Krypton was doomed, Zor-El took steps to save his daughter, but he never told her about his plans or the planet's impending demise. Moments before the catastrophe he put her to sleep, placed her in a space pod and blasted her into space. Upon reaching the Sol System, her pod orbited the Sun for several decades, in order to fueling her Kryptonian powers, and then it crashed in Siberia.
Kara awoke from her artificially-induced dream to find Krypton was gone for decades, she was stranded in an alien world and her baby cousin was an adult. For all these reasons she was confused and full of grief and anger for a while during which she struggled to adapt to Earth.
- Action Comics #252
- Superboy #80
- Action Comics #285
- Action Comics #309
- Action Comics #310
- Action Comics #334
- Amazing World of Superman #1
- Secret Origins #2
- Action Comics #500
- DC Special Series #19
- The Best of DC #17
- Supergirl (Volume 2) #1
- Superman/Batman: The Supergirl from Krypton
- Supergirl and the Legion of Super-Heroes #17
- Action Comics #850
- Action Comics #869
- Supergirl (Volume 5) #35
- Supergirl: The Last Daughter of Krypton
- Secret Origins (Volume 3) #1
- Supergirl (Volume 6) #0
- Supergirl (Volume 7) #1
- Supergirl (movie)
- Superman The Animated Series: "Little Girl Lost"
- Smallville: "Bizzaro"
- Supergirl: Cosmic Adventures in the 8th Grade #1
- Superman/Batman: Apocalypse
- Superman: Unbound
- Supergirl (TV series): "Pilot"
- DC Super Hero Girls: Hero of the Year
- Supergirl: Being Super (Volume 1)