One of the hominids finds an animal carcass and recalls the black monolith he previously saw. He makes a connection and realizes that he can use the bone as a tool. He uses the bone to destroy the carcass and eventually uses it to kill other animals for food. The hominids change from herbivores to omnivores and use tools for war.
The Croods use the new tools and domesticate various land mammals. Tools and animal domestication are two qualities that first set humans apart from the rest of the environment, and are some of the reason humans dominate the Earth today.
Eep discovers fire for the first time. Fire allowed humans to fight, stay warm, consume healthier foods with more nutrients, and ultimately survive. Fire is one of the first discoveries that distinguished humans from other animals.