Over a million years ago IT WASN'T A CAVE. All of the ancient proto-humans only got turned into fossils because their bodies ended up in the perfect and rare situation where they would be fossilized, and this is where it happened. The actual cause of the formation of the African Rift Valley is still in question let alone this cave system. The first finds were in a limestone QUARRY. Limestone is soft and breaks down quicker than other rocks. That anthropologists have even found three distinct species is still being debated, let alone whether or not they were all pals. It's in the article.