Here is one brief explanation:
Conditions for synthesis of organic molecules on the early Earth
Essential to the spontaneous origin of life was the availability of organic molecules as building blocks. The famous "prebiotic soup" experiment by Stanley Miller (Miller 1953, Miller-Urey experiment) had shown that amino acids, the building blocks of proteins, arose among other small organic molecules spontaneously in the lab by sparking a mixture of methane, hydrogen, ammonia and water. These conditions were assumed to simulate those on the primitive earth. Already in 1922 Oparin had proposed that the early Earth had such a reducing atmosphere (in his classic "The Origin of Life" from 1936 he expanded on these ideas). Observations of Jupiter and Saturn had shown that they contained ammonia and methane, and large amounts of hydrogen were inferred to be present there as well (it is now known that hydrogen is the main atmospheric component of these planets). These reducing atmospheres of the giant planets were regarded as captured remnants of the solar nebula and the atmosphere of the early Earth was assumed by analogy to have been similar.
Only in a reducing atmosphere like this, synthesis of organic molecules – also sugars and organic bases, building blocks of nucleotides – would have been possible in large amounts (Chyba, Sagan 1992).
Later research had cast doubt on the existence of a reducing atmosphere, and suggested a neutral atmosphere instead – see also Chyba 2005, the accompanying article to Tian F et al. (see below).
However, new calculations indicate that hydrogen escaped from the early atmosphere at a much slower rate than previously thought, yielding an atmosphere where hydrogen was a major component (about 30%) and which was therefore highly reducing (Tian F. et al. 2005; see also press release). The authors measured the production of organic molecules through UV photolysis under those conditions, and conclude that at 1010 kg/year it "would have been orders of magnitude greater than the rate of either the synthesis of organic compounds in hydrothermal systems or the exogenous delivery of organic compounds to early Earth".
Another new study supports an early reducing atmosphere as well. Chondrites are primitive material from the solar nebula and are generally believed to be the building blocks of the Earth and other rocky planets, asteroids and satellites. During and after planet formation, gases escape from the chondritic material due to high temperature and pressure. Systematic, detailed calculations on what these gases must have been show that they are mainly the highly reducing hydrogen, methane and ammonia – the same gases as in the Miller-Urey-type experiments (Schaefer, Fegley 2006; see also press release). The composition of the gases varied with temperature only to a moderate extent, and was found to be largely independent of the actual pressure under which outgassing may have occurred, which appears to support robustness of the conclusions.
The authors mention that it had been found that a reducing atmosphere of methane and ammonia is extremely vulnerable to destruction by UV sunlight (Kuhn, Atreya 1979, Kasting et al. 1983). They also point out, however, that recent developments suggest that a reducing atmosphere is more stable than previously believed:
1. It was found that hydrogen escape from the Earth’s atmosphere was less efficient than previously thought (referring to the study above).
2. Observations of the atmosphere of Titan, Saturn’s moon, which is composed primarily of methane and nitrogen, show that photochemically produced hydrocarbon aerosols form a haze layer in the upper atmosphere that protects the lower atmosphere from photochemical destruction. Such a haze layer could also have been produced on the early Earth from outgassed methane and ammonia (Zahnle 1986, Sagan and Chyba 1997, Pavlov et al. 2000).
Of course, if life arose in deep-sea hydrothermal vents (see below), the composition of Earth’s early atmosphere would become largely irrelevant. To a certain extent, this also holds true for organic building blocks delivered to the earth by interplanetary dust particles and on carbonaceous meteorites.
Although some estimates assume a relatively concentrated prebiotic soup of organic molecules in the earth’s ocean or other waters (e.g. De Duve, Miller 1991 and references therein), others have argued that the prebiotic soup would have been too dilute. However, locally it might have been concentrated by such simple processes as, for example, evaporation in puddles or shallow lakes, possibly with long-term wet/dry cycles. It should be kept in mind for evaluating all chemical scenarios that, due to its nature, the origin of life must have been a very local event; this is also important for the issue of the origin of homochirality of amino acids and sugars, see below.