Life needs a source of energy for chemical reactions. On the early Earth, the atmosphere did not have oxygen. Oxidation using the Krebs cycle, which is common today, was not possible. The Krebs cycle may have acted backwards, doing reduction instead of oxidation, and the cycle may have made larger molecules. To make life, molecules needed to make copies of themselves. DNA and RNA make copies of themselves, but only if there is a catalyst — a compound which speeds up the chemical reaction. One guess is that RNA itself served as a catalyst. At some time, the molecules were surrounded by membranes, which made cells. 