Final answer:
The spontaneous formation of organic molecules like formaldehyde and hydrogen cyanide is less likely today due to an oxygen-rich, non-reducing atmosphere, compared to the prebiotic Earth that had a hypothesized reducing atmosphere facilitating complex chemical reactions for life's origins.
Step-by-step explanation:
The spontaneous formation of formaldehyde and hydrogen cyanide is less likely on Earth today compared to the prebiotic Earth due to the change in the planet's atmosphere over time. Initially, Earth's atmosphere lacked oxygen and was more reducing, which facilitated various complex chemical reactions and allowed the formation of organic molecules necessary for life. In contrast, present-day Earth's atmosphere is oxygen-rich and not reducing, making it less conducive to abiotic synthesis of such compounds.
During the prebiotic era, Haldane and Oparin hypothesized that Earth had a reducing atmosphere, rich in inorganic molecules like H₂, NH3, CH4, and H₂S, which provided the free energy required for the origin of organic molecules. However, new evidence suggests that the early Earth may not have had a reducing atmosphere but instead one dominated by N₂ and CO2. This has led to alternative hypotheses for the origin of life, such as the role of hydrothermal vents in deep oceans providing the necessary conditions and materials for the synthesis of organic compounds.