Flexi Says: All living things consist of organic molecules, centered around the element carbon. Therefore, it is likely that organic molecules evolved before cells, perhaps as long as 4 billion years ago. How did these building blocks of life first form? Scientists think that lightning sparked chemical reactions in Earth’s early atmosphere. The early atmosphere contained gases such as ammonia, methane, water vapor, and carbon dioxide. Scientists hypothesize that this created a “soup” of organic molecules from inorganic chemicals. In 1953, scientists Stanley Miller and Harold Urey used their imaginations to test this hypothesis. They created a simulation experiment to see if organic molecules could arise in this way (see Figure below). They used a mixture of gases to represent Earth’s early atmosphere. Then, they passed sparks through the gases to represent lightning. Within a week, several simple organic molecules had formed. Some scientists speculate that RNA may have been the first organic molecule to evolve. In fact, they think that early life was based solely on RNA and that DNA and proteins evolved later. This is called the RNA world hypothesis. Why RNA? It can encode genetic instructions (like DNA), and some RNAs can carry out chemical reactions (like proteins).