One change was revealed in the Prism Pentad and then put in the Revised edition that in the world’s Blue Age, when the oceans covered the planet and the sun was blue colored, the halfling-creator race fought against something that was destroying the life-creating source of the oceans so used the Pristine Tower to drain energy from the sun to power a counter measure and destroy this ‘brown tide”. It worked and turned the sun yellow and drained the oceans and the halflings-creators then used their life-shaping magic to create all the standard D&D races and monsters in what is called the Green Age which lore says was typically supposedly base-standard D&D.
Thousands of years pass and one really bad dude in the Green Age discovered the past of the world with the halflings life-shapers ruled an ocean-covering planet and wished to Make Athas Ocean Again and restore halfling life-shaper rule to the world. Starts recruiting humans (because AD&D rules say have unlimited potential) to wage genocidal war to exterminate non-humans (the leaders of the war each get a specific race to target). The really bad dude discovers arcane magic and sees that it is powered by life. Teaches the method that does not drain life to most, but secretly teaches a more powerful style that drains life permanently to the leaders of his war of genocide. The setting lore calls this the Cleansing Wars, it lasts decades.
Eventually the champions of the Cleansing War learn the bad dude’s true plan was to destroy the world including them and they teamed up and betrayed him. Imprisoning the bad due in a planar prison. Used the Pristine Tower again, this time changing the sun into the current start of a dark crimson. The defiling method spread beyond the champions, and Athas slowly wasted away.