The process of brewing beer was most likely first discovered by humans heating water and grain, since the "heated mixture would have produced a major improvement in the digestibility and nutrition gained from grass grains" (Sinclair 27). It was also more palatable that chewing on the grains themselves.
If the gruel was left to sit for a day after heating, yeast would settle on the gruel and found it a excellent environment for growth. The yeast would then eat the oxygen in the mixture, digest the sugar molecules, and create ethanol. Despite not knowing how this action occurred, the resulting beer-gruel was both nutritious and mood-altering, making it a useful addition to the human diet.
The process of heating water and grain also served to help sterilize the water. "Temperature required to denature proteins in grain would also denature proteins in many disease microbes living in water....Those people who heated their gruel and avoided the direct consumption of water would have lessened their exposure to diseases carried in water" (Sinclair 27).
As a potable substitute for water, beer played a crucial part in ancient Egypt and Mesopotamia society across all social classes. In Egypt "all sections of the community drank beer, from the Pharaoh downwards, and it was a product that was inextricably woven into the fabric of daily existence" (Hornsey 35). In Mesopotamia it played a similar role and was "interlinked with mythology, religion and medicine" (Hornsey 77).
Beer production was of course not limited to Egypt and Mesopotamia as it was present in the areas surrounding the fertile crescent and as far away as Central and South America. While not all grain producing civilizations brewed beer, a great many did take advantage of its ability to ferment.