Most objections have come from religious groups that support special creation, or the theory they term creationism, or creation science. Individuals and organizations that support this concept defend the belief that all beings were created by God. Fundamentalist Christians and others feel that the premise that species are continually changing conflicts with literal interpretations of the Bible. The first serious attack on the idea of spontaneous generation of life was made in 1668 by Francesco Redi, an Italian physician, who proved that maggots did not arise spontaneously in decaying matter, as commonly believed, but from eggs deposited there by flies. Many theories have been developed to explain how life first originated on Earth.
Some people theorize that microorganisms reached Earth from another planet.
Most scientists discount this idea, called panspermia, because the radiation in space would kill cells or spores before they reached Earth. Instead, they believe that terrestrial life evolved from nonliving matter on the primitive Earth. The way in which life first formed may always remain a mystery, but the following explanation is likely. Since Earth is thought to be about 4.5 billion years old, the first living things probably evolved within a billion years of Earth's formation. In the 1920s Aleksandr Oparin, a Soviet biochemist, pointed out that the atmosphere of the primitive Earth was probably very different from today's.