About 4 or 5 million years ago, major changes in the Earth's climate brought about grassland regions where giant mammals could graze. This is important because it provided ancestral humans with their first real opportunity to eat meat. The earliest recorded evidence of human meat-eating is from about 2.6 million years ago, where marks on animal bones discovered correspond to the sort of sharpened stone tools that humans had recently begun to manufacture. By about 2 million years ago, they had begun engaging in primitive forms of hunting. Not having any weapons at this point, their main tactic was ambushing animals to kill them. the advent of fire would eventually lead to cooking, and it is believed that by about 250 000 years ago that humans had begun to cook their meat.