Organic compounds are those upon which life depends, and they all contain carbon. In fact, the definition of an organic compound is one that contains carbon. It's the sixth most abundant element in the universe, and carbon also occupies the sixth position on the periodic table. It has two electrons in its inner shell and four in the outer one, and it's this arrangement that makes carbon such a versatile element. Because it can combine in so many different ways, and because the bonds carbon forms are strong enough to remain intact in water – the other requirement for life – carbon is indispensable for life as we know it. In fact, an argument can be made that carbon is necessary for life to exist elsewhere in the universe as well as on Earth.
Carbon is the only element that can form so many different compounds because each carbon atom can form four chemical bonds to other atoms, and because the carbon atom is just the right, small size to fit in comfortably as parts of very large molecules.
