About a billion years ago, all of the continents came together forming a super continent called Pangea. An ice age caused the whole land mass to ice over, destroying all terrestrial life. After the ice melted, terrestrial life re-evolved from sea creatures. It is not known why the earth's core is hot, but changing thickness of the crust indicates a constant cooling, which indicates that the heat was created at the beginning of the earth's formation. And that, kids, is why we have Christmas.