The final, most important events leading to the origin of life are perhaps the least understood chapters of the story. Life began during the first billion years of an Earth history which is 4.5 billion years old. The illustration depicts an early Earth in which volcanoes, a gray, lifeless ocean, and a turbulent atmosphere dominated the landscape. Vigorous chemical activity is represented by the heavy clouds, which were fed by volcanoes and penetrated both by lightning discharges and solar radiation. The ocean received organic matter from the land and the atmosphere, as well as from infalling meteorites and comets. Here, substances such as water, carbon dioxide, methane, and hydrogen cyanide formed key molecules such as sugars, amino acids, and nucleotides. Such molecules are the building blocks of proteins and nucleic acids, compounds ubiquitous to all living organisms. A critical early triumph was the development of RNA and DNA molecules, which directed biological processes and preserved life's "operation instructions" for future generations. RNA and DNA are depicted in the illustration, first as fragmets and then as fully assembled helices. These helices formed some of the living threads, as shown in the illustration, however, other threads derived from planetary processes such as ocean chemistry and volcanic activity. This evolving bundle of threads thus arose from a variety of sources, illustrating that the origin of life was triggered not only by special molecules such as RNA or DNA, but also by the chemical and physical properties of the Earth's primitive environments.