Why would an amphibian's heart not be suitable for a mammal?

The fish heart evolved before the amphibian heart and is its evolutionary predecessor. The heart's complexity can be traced back to even simpler versions in animals like annelids (earthworm) and other organisms with what is basically a one-chambered heart. The "additional" chambers evolved over time by mutations on the genes that determined the heart's development. Any mutation to the structure of the heart that gave the organism an advantage (additional chambers prevent back flow and greatly improve efficiency of oxygen and heat, which is increasingly necessary to progressively "higher" organisms which have higher metabolic demands for both) increased that organisms chance of reproducing. An interesting testament to the theory that mammals and birds last common ancestor was reptilian is that while we both have 4 chambered hearts, they are oriented in different directions (deoxygenated blood enters our right atrium, but their left atrium). Crocodilians are the only reptiles that also have a four chambered heart, which bares more similarity to those of birds (this model of four chambered heart probably evolved before birds and crocs had their evolutionary split) but with some unique features specific to crocs. If I missed anything or am mistaken, please let me know.

R.DLM The more correct answer is that it reflects the difference between using gills and using lungs to oxygenate the blood. All animals must "expose" their blood to a more oxygen-rich environment in order to create the gradient that moves carbon dioxide out and oxygen in. Because the fish uses the surrounding water to do this, it doesn't need the double system that animals with lungs need. "Double system" means that there is a circulation system to the lungs and to the body, both of which are a closed loop with the heart as the common central point. The physiology is actually quite complex; in fact, it is irreducibly complex. By that, I mean that you just don't randomly mutate and develop another chamber in the heart. To go from the single circulation of the fish heart to the double circulation of the 3 or 4 chambered heart requires a completely new pathway for fluid flow, including valves, veins-to-arteries connections, coordinated heart muscle contraction, new pressure differentials and means to control them (read nerves and centers in the brain0, and the list goes on and on with both major and minor requirements to "change" from 2 chambers to 3. It is very tempting to look at the hearts of different animals and conclude that the more complex "evolved" from the less complex. In fact, even the 2 chambered heart of the fish is remarkably complex. Trying to go from 2 chambers (living in water) to 3 chambers (living out of the water) involves more than one nonfunctional intermediate form (read: lethal to the organism), so it is incorrect to say that the amphibian heart evolved from the fish heart. Even given eons of time, this could not have happened in a Darwinian fashion! Here's a link that explains it in more detail if you're interested: http://www.ideacenter.org/contentmgr/showdetails.php/id/1113 Bottom line answer: fish have 2 chambers because they use gills to perform the CO2-oxygen exchange, and all creatures with lungs have 3 or 4 chambers to perform the same exchange. Or, stated differently, they are that way because that's the way they are - almost as if it were designed that way, eh? And please remember that this is a very oversimplified answer. I encourage you to go beyond this site and seek more detailed answers. Anatomy and physiology are phenomenally complex studies, far more so than we think about day to day!