Spontaneous generation is the idea that organisms could form miraculously from non-living material.
But Pasteur's results posed a new riddle for evolutionary biologists: If life could only arise from life, how did living organisms initially appear on the planet?
The answer came in the 1920's when A.I. Oparin of Russia and J.B.S. Haldane of England independently presented compelling arguments that the origin of life could be explained, not as the result of rapid spontaneous generation of whole organisms in a few weeks, but from a long and gradual process of chemical evolution.
Much progress has been made since the 1920's, but, as with most complex scientific questions, many uncertainties remain and many new avenues of inquiry have been uncovered.
Today, research into the origin of life is interdisciplinary with workers trying to answer four main questions:
1. What was the Earth's physical environment like when life first evolved?
2.What sorts of chemical reactions could produce the building blocks of life and could these occur naturally in the early Earth's environment?
3. How could the complex organic molecules be compartmentalized into a contained unit?
4. How did the genetic code evolve?