“Frogs have a three-chambered heart…”
The first genetic link in the evolution of the heart from three-chambered to four-chambered has been found, illuminating part of the puzzle of how birds and mammals became warm-blooded.
Frogs have a three-chambered heart. It consists of two atria and one ventricle. As the right side of a frog’s heart receives deoxygenated blood from the body, and the left side receives freshly oxygenated blood from the lungs, the two streams of blood mix together in the ventricle, sending out a concoction that is not fully oxygenated to the rest of the frog’s body.
Turtles are a curious transition–they still have three chambers, but a wall, or septum is beginning to form in the single ventricle. This change affords the turtle’s body blood that is slightly richer in oxygen than the frog’s.
Birds and mammals, however, have a fully septated ventricle–a bona fide four-chambered heart. This configuration ensures the separation of low-pressure circulation to the lungs, and high-pressure pumping into the rest of the body.
More, including interview at National Science Foundation