Why is the center of the earth so hot?

It used to be thought that the earth was just still cooling. But more recently its been discovered that much of the heat at the center of the earth is caused by nuclear reactions (fission or fusion, I'm not sure). This led to a re-evaluation of the age of the earth, just as finding that the sun works not by hydrogen 'burning' and combining with oxygen but hydrogen -fusing- to become helium led to a re-evaluation of the age of the solar system.