They developed. They encountered the Flood. When destroying the Flood they ended up razing Forerunner worlds to cleanse them of the parasite (thus invoking their wrath) and sacrificed a large portion of their population to infect the infection. Then the Forerunners hit back unaware of the Flood and just thinking they'd been attacked. Humans were weakened, numbers depleted. Forerunners won, their lifeworkers devolved the ancient humans who were then exiled to Earth. Later on a few were taken to the Ark by the Librarian for preservation, and were reseeded back on Earth post-firing of the Halo array. Natural evolution took place, regular human history happened, and eventually space travel was developed and there they are in 255X.