Evolutionary biology is the study of the history of life forms on Earth. There are several theories for how life originated, including that life came from outer space or spontaneously generated from non-living matter. The currently dominant theory is that the universe began in a massive explosion around 13 billion years ago known as the Big Bang. Over time, gravity caused gas and dust to condense, forming the sun and planets. Earth formed around 4.5 billion years ago without an atmosphere. Early theories of organic evolution include those proposed by Lamarck and Darwin, with Darwin's theory of descent with modification through natural selection now the prevailing explanation of how evolution has occurred. Evidence for evolution comes from morphology, embryology, physiology and paleontology.