The Votans happened. Their home system was destroyed so they thought they'd set up residence on Earth. It all started out polite enough, with ambassadors and negotiations and talks of maybe giving them part of South America to redecorate like home. But the talks broke down and it turned into an alien invasion instead. Terraforming devices were used on the Earth, unplanned and indiscriminately. It's like somebody took two different jigsaw puzzles and tried to make the pieces fit together. Some habitats are like old Earth, some are like the Votan worlds, and some have ended up a mix of both as species spread and adapted.