The emergence of mammals led to various modifications within the digestive system in order to accommodate the changes in diet. While there are differences in the digestive system among different animals as a result of evolution, many of the structures themselves have evolved very little since the earliest fish that inhabited the Earth.
The digestive system is made up of the alimentary canal, oral structures and digestive glands. The principal parts of the digestive system, in addition to oral structures, are pharynx, esophagus, stomach, small intestines and large intestines.
The primary function of the digestive system is to provide the body with nutrition. For mammals, evolution has played a vital role here and led to a range of modifications in both the mechanical and chemical digestive procedures.
Food enters the mammalian mouth through the oral cavity, which includes the lips, cheeks, tongue, teeth, palate and salivary glands. The evolution of a secondary palate allows for dual mastication and breathing. This feature first appeared in reptiles, from where it has since evolved into the mammalian secondary palate.
The tongue in mammals has also evolved extensively and plays a more prominent role in digestion. It is more muscular and allows for greater manipulation of food within the oral cavity, and is the place where taste receptors in the body are focused.
Specialization of teeth is a common feature, and this was an important aspect of evolution. The teeth can be seen to be specialized in accordance with the diet of the animal to best carry out the job of tearing and chewing food.
To help with the process of chewing and swallowing, salivary glands in the mammalian oral cavity are activated as soon as the food enters the mouth. In some animals, the saliva is composed of salivary amylase, which further helps digestion by breaking down starch into maltose.
Upper Digestive System
From the mouth, the food passes through esophagus which is surrounded by muscles to help the movement of food to the stomach. The mammalian stomach is specialized in accordance with the diet of the animal, and secretes mucus and digestive enzymes for the chemical breakdown of food.
Lower Digestive System
The food passes from the stomach into the small intestine, which is highly evolved in mammals and the longest part of the alimentary canal. It is where most of the enzymatic digestion, and almost all of the absorption of nutrients is carried out.
An illustration of differences in the small intestine brought about by evolution would be the variations in the size of the cecum. Cecum contains bacteria for the breakdown of plant material and can be seen to be significantly reduced in mammals which rely upon a carnivorous diet.
The small intestine opens up into the large intestine, which plays an important role in mammals for the conservation of water, and also stores the feces till its release through the anus.
There are a number of accessory organs which have evolved in mammals and play a vital role in the digestive system. Prominent among these are liver, which secretes digestive solutions that contain bile salts; gall bladder, which is responsible for storing the bile secreted by the liver; and pancreas, which secrete digestive enzymes.