The diaphragm is made of muscles and tendons and it divides the inside of the torso into 2 sections. On one side are the heart and lungs, and on the other side are organs like the stomach and liver.
Mammals are the only animals with a diaphragm, although some reptiles and amphibians share some similar features.
When mammals breathe in, their diaphragm contracts. This increases the volume of their chest cavity, where the lungs sit, and decreases the volume of the abdominal cavity that holds the stomach and liver.
As the volume of the chest cavity increases, the pressure drops until it is lower than the pressure outside the lungs. This creates a suction device, which draws air into the lungs automatically.
The diaphragm helps mammals to breathe quickly and efficiently during intense exercise, for example, when a greyhound or a horse is galloping. Mammals can carry out high levels of activity for a longer period of time than other animals.
The diaphragm also works in reverse. Instead of helping the body to suck in air, it can also help it to eject waste material, such as vomit.
The lower ribs of mammals are much shorter than the lower ribs of other animals. Lizards, for example, have ribs that run right down to their pelvis whereas human ribs stop half way down their body.
The abdominal cavity, that holds the stomach, liver and other organs, would not be able to contract if the ribs were holding it in place. This would mean that the chest cavity couldn't expand, and the suction effect described above wouldn't take place.
The short ribs of mammals also make them more flexible than their ancestors.