In fish, sharks and amphibians, a long vascular system extending from the gills supplies blood to the heart.

In fish, sharks and amphibians, a long vascular system extending from the gills supplies blood to the heart. 

By contrast, in mammals and birds, this ancestral vasculature exists only during the embryo stage and later transforms into adult coronary arteries which are wrapped around the heart. This evolutionary change is likely connected to the transition of vertebrates from water to land, which led to the loss of gills," explained researcher Hiroki Higashiyama from the Department of Physiological Chemistry and Metabolism at the Graduate School of Medicine at the University of Tokyo.

Amphibians sit in between fish and mammals on the evolutionary tree and so may offer some insight as to how one type of heart evolved into the other. "As an example, we saw that the transient blood vessels observed in mice embryos bore a striking resemblance to the ancestral vascular system found in amphibians," said Higashiyama. "As the mouse's coronary arteries form, these ancestral vessels disappear, but they act as essential starting points for creating the new coronary arteries.

The results appeared to show that the structure of the human heart originated from a common amniote ancestor. When this animal transitioned from being a water dweller to living on land, an important development took place which involved significant remodeling of ancestral blood vessels to form the coronary arteries we have today.