The modern era of abiogenesis research can be considered to start from
Charles Darwin's speculations about life emerging ?in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present.? He also speculated that no environments for abiogenesis would exist on the present-day Earth, because such environments would be quickly consumed by various organisms.
But there was no further progress since the 1920's, when
Aleksandr Ivanovich Oparin and
John Burdon Sanderson Haldane independently worked out scenarios of "chemical evolution". They concluded from various chemical grounds that the Earth had originally had hydrogen, ammonia, carbon dioxide, methane, and other simple compounds in its atmosphere -- but no oxygen molecules. And larger and larger molecules formed by various chemical processes until some of them succeeded in making copies of themselves, forming the first living things.
Much of the Oparin-Haldane scenario was little better than hand-waving, but the first concrete support was provided by the famous
Urey-Miller experiments, conducted in 1953 by
Stanley Miller and
Harold Clayton Urey in 1953. These involved a simulated early Earth atmosphere and ocean, complete with simulated lightning: an electric spark. After about a week, the simulated ocean accumulated some brown tar -- and a variety of dissolved organic molecules, including several biological amino acids.
Urey-Miller experiments have been repeated with a variety of energy sources and other conditions, and as long as the source mixture is chemically reducing, organic molecules readily form. Contrary to creationists' claims, Urey-Miller experiments are not rigged to produce the "right" results; no rigging seems to be necessary.
But the Earth's early atmosphere is nowadays thought to be neutral, consisting mostly of nitrogen and carbon dioxide, instead of hydrogen, ammonia, and methane (reducing), as had been suggested from cosmochemical grounds. Urey-Miller experiments performed with a neutral mixture are much less successful than those with a reducing mixture; however, the early Earth could easily have had reducing microenvironments, like hot springs and hydrothermal vents.
