We can examine the physiology of animals and how they develop to see evolutionary pathways. It is known from embryological, biochemical and fossil evidence that fish evolved into amphibians then to reptiles and then into mammals. Some reptiles also evolved into birds. By examining the way the heart and circulation system develops in all these animals we can see that they are related to each other in just the way that the other evidence would suggest. In the following diagrams the heart itself has been shown in a different position to how it actually appears in the body. This is to make the pictures clearer. For a representation of the mammalian circulation and a human heart go here so that the way it actually appears can be compared.

This picture is the heart system seen in vertebrate embryos and in adult fish. It is a single circulation, in fish the blood goes to the gills via the arterial arches 1-6 (but often no.1 and sometimes no.2 disappear) and then round the body.

This is the heart in a tadpole. The heart has started to develop into two sides, and the first two arterial arches have disappeared. The splitting of the heart into two halves is seen in all terrestrial vertebrates.

Here in the adult frog we can see the amphibian system. Arterial arch no. 3 constricts to become the vessels for the head (carotid arteries/veins), no, 4 becomes the circulation for the body (systemic arteries/veins) , no. 5 disappears and no. 6 becomes the circulation that goes to the lungs (pulmonary arteries/veins). The heart becomes separated into two halves. The way the side arches disappear is seen in all terrestrial vertebrates.

In the crocodile (a representative reptile, there is some variation in reptile circulatory systems) the two blood systems of oxygenated/deoxygenated blood are separate. There is only a small hole between the two sides of the heart. The left systemic arch (no. 4) carries deoxygenated blood. Notice that the left systemic arch (no. 4) comes from the right ventricle. This is different to mammals, as will be pointed out below. The hole in the heart is seen in human embryos but it normally closes when the child is born.

This is the circulation in a bird. It is very similar to the crocodiles except that the left systemic arch has disappeared. All the blood that goes to the body is oxygenated. This is much more efficient and is a pattern that is basically the same in mammals, but with some important differences..

Here is the mammalian system. It too is efficient, in that only oxygenated blood goes to the body and the seperation of oxygenated and deoxygenated blood is complete. In mammals it is the right systemic arch that has disappeared. Notice, though that the left systemic arch comes from the left ventricle. This means that the circulation of the mammals must have developed from something totally different to the circulation of the crocodiles and birds. When thinking about the evolution of the mammals, this fact must be taken into consideration.