Cooking or cookery is the art or practice of preparing food for consumption with the use of heat. Cooking techniques and ingredients vary widely across the world, reflecting unique environmental, economic, and cultural traditions. Cooks themselves also vary widely in skill and training. Cooking can also occur through chemical reactions without the presence of heat, most notably as in Ceviche, a traditional South American dish where fish is cooked with the acids in lemon or lime juice. Sushi also utilizes a similar chemical reaction between fish and the acidic content of rice glazed with vinegar.Preparing food with heat or fire is an activity unique to humans, and some scientists believe the advent of cooking played an important role in human evolution. Most anthropologists believe that cooking fires first developed around 250,000 years ago. The development of agriculture, commerce and transportation between civilizations in different regions offered cooks many new ingredients. New inventions and technologies, such as pottery for holding and boiling water, expanded cooking techniques. Some modern cooks apply advanced scientific techniques to food preparation.HistoryThere is no clear archaeological evidence when food was first cooked. Most anthropologists believe that cooking fires began only about 250,000 years ago, when hearths started appearing. Phylogenetic analysis by Chris Organ, Charles Nunn, Zarin Machanda, and Richard Wrangham suggests that cooking may have been invented as far back as 1.8 million to 2.3 million years ago. Other researchers believe that cooking was invented as recently as 40,000 or 10,000 years ago. Evidence of fire is inconclusive, as wildfires started by lightning-strikes are still common in East Africa and other wild areas, and it is difficult to determine when fire was first used for cooking, as opposed to just being used for warmth or for keeping predators away.