In 1952, Stanley Miller and Harold Urey made sparks fly in a gas-filled flask meant to reflect the composition of Earth's atmosphere around 3.8 billion years ago. Their results suggested that lightning could have led to prebiotic molecules necessary for the evolution of life, such as amino acids. At the time, scientists thought the early atmosphere would have been primarily methane and ammonia, but by the 1990s, experts argued for an atmosphere filled with carbon dioxide and molecular nitrogen.