The Guardian has a really interesting article on prehistoric tools found in the Amazonian jungle. That alone would make for a good find, however, these tools were made and used by chimps, not humans, and point to a "chimpanzee stone age" 4,300 years ago. They suspect that the idea of using stone tools might have even been passed on to both humans and chimps from a common relative even further back, and are hoping to find evidence of older stone tool use in the region.