Paleolithic humans had digestive tracts identical to those of modern humans. In both paleolithic humans and modern humans, digestion was meant to be a one-way process.
The process of digestion begins in the mouth. Chewing breaks food down into smaller, digestible pieces. Salivation moistens, lubricates, and liquifies food so it can begin its journey down the throat. The mouth releases an enzyme called amylase that begins breaking down complex carbohydrates into the simple carbohydrates the body can use. And in waves of coordinated contractions called peristalsis, the chewed, moistened, liquified mass of food begins to travel down the stomach.
The stomach breaks down the proteins and fibers in food with acid. It begins releasing acid just as soon as there is a taste sensation in the mouth. When you eat something especially tasty, your stomach begins producing acid even before the food reaches your throat. The hydrochloric acid in your stomach kills both friendly and unfriendly bacteria, leaving just a few to survive the passage down to the intestines, and begins to break down fats with an enzyme called lipase. Proteins are not completely digested in the stomach, but they are unfolded into a form that can be further digested at the next stop, the small intestine.
When digested food leaves the stomach, it arrives at the small intestine. The “small” intestine is actually about 20 feet (a little more than 6 meters) long. In the first section of the small intestine, known as the duodenum, the pancreas releases bicarbonate to neutralize stomach acid. The pancreas also releases still more amylase for digesting carbohydrates, lipase for breaking down fats, and elastase and proteases for further digestion of proteins into amino acids. The liver releases bile to help dissolve fatty acids so they can be transported into the bloodstream, and the pancreas also release dexoyribonuclease and ribonuclease to break down the DNA and RNA in food.
The second section of the small intestine, the jejunum, is the first place nutrients are actually absorbed into the body. The surface of the jejunum is covered with tiny, finger-like projections that are just a millimeter (about 1/25 of an inch) high. These “fingers” in the jejunum are known as villi. In this brush-like border of the digestive tract, completely digested nutrients trickle into the bloodstream. Amino acids, sugars, and water-soluble vitamins enter the bloodstream here.
The third and final section of the small intestine, the ileum, transports water and fat. The ileum extracts fatty acids from the bile salts and sends the bile salts back to the liver for recycling.
The residual material leaving the small intestine next travels to the large intestine, also known as the colon. The colon can absorb water and electrolytes for the use of the body, but its primary function is to house trillions and trillions of bacteria that break down fibers, create some fat-soluble vitamins, and add bulk to the stool. The colon stores the stool until the bowels move and the colon is emptied by the process of defecation.
If you have GERD, digestion is an uncomfortable process largely because the food that is supposed to go down the digestive tract tends to come back up. And the reason it comes back up is gas.