Earth formed 4.5 billion years ago, and the oldest known fossils are about 3.5 billion years old, with possible fossil microbes dating to 3.7 billion years. Life must therefore have arisen within the first billion years, and scientists have proposed three main ways it could have happened.
In the 1920s Alexander Oparin and J. B. S. Haldane independently suggested that life formed in a “primordial soup” of organic chemicals. In 1952 Stanley Miller, working with Harold Urey, passed electric sparks through a mixture of simple gases and water and produced the amino acids glycine and alanine. Later, Leslie Orgel and Francis Crick suggested that the first life had no proteins and relied on RNA, which can both carry information and speed up chemical reactions. In 1986 Walter Gilbert named this idea the “RNA World”.
A third camp argues that metabolism came first, powered by hydrothermal vents on the sea floor, which were discovered in 1977 teeming with animals and bacteria. Geologist Mike Russell and biologist William Martin proposed that life began in cooler, alkaline vents whose porous rocks could have provided the natural proton gradients that cells still use to make energy. Others stress that life needs a cell, an enclosed compartment with a membrane, and all three ideas still have supporters.
via BBC Earth: The secret of how life on Earth began | Michael Marshall