Why are hearts in humans and other higher animals not able to regenerate like salamander hearts? Answering that question would be a step on the road to recreating that ability when needed: "A new study has shed light on why adult human cardiac cells lose their ability to proliferate, perhaps explaining why our heart have little regenerative capacity. The study, done in cell lines and mice, may lead to methods of reprogramming a patient's own cardiac myocytes, or muscle cells, within the heart itself to create new muscle to repair damage ... Recent research suggests that mammals do have the ability to regenerate the heart for a very brief period, about the first week of life. ... During human development, cardiac myocytes are made by progenitor stem cells and proliferate to form the heart. Once the heart is formed, the myocytes transform from immature cells into mature cells that cannot proliferate. That's not so for newts and salamanders, whose cardiac myocytes can go back and forth between immature, or primitive, states to proliferate and repair damage and then revert back into mature cells once the damage is repaired. [Researchers believe] the reason adult human cardiac myocytes can't do this is quite simple - when the myocytes are in a more primitive state, they are not as good at contracting, which is vital for proper heart function. Because humans are much larger than newts and salamanders, we needed more heart contraction to maintain optimum blood pressure and circulation."