Question

Why did the earth have a reducing atmosphere earlier?

Answer 1

Hint: An atmosphere is a layer or series of layers of gases that surround a planet or other material body and are held in place by gravity. If the gravity on which an atmosphere is subjected is high and the temperature of the atmosphere is low, the atmosphere is more likely to be retained.

Complete answer:
A reducing atmosphere is one in which oxidation is prevented by the absence of oxygen and other oxidizing gases or vapors, and which may contain actively reducing gases like hydrogen and carbon monoxide, as well as gases like hydrogen sulfide that would be oxidized by any present oxygen.
Scientists believe that the early Earth's atmosphere was severely depleted, implying that oxygen was scarce. Due to the lack of oxygen, the atmosphere would have been filled with noxious methane, carbon monoxide, hydrogen sulfide, and ammonia. To this day, there are many theories and studies about how life on Earth might have evolved from this deadly atmosphere cocktail.
As the earth began to cool, stable bonds between atoms formed, forming molecules (this was the beginning of the chemical history of the earth which has finally yielded the present-day life and atmosphere). Because hydrogen is active and abundant, it can easily combine with other atoms to form simple compounds like ammonia, methane, molecular hydrogen, hydrogen cyanide, water vapors, and carbon dioxide.
All of the oxygen in the atmosphere was found in combination with other elements. In the early atmosphere of the earth, there was no free molecular oxygen, so the atmosphere was reducing.

Note:
Planets follow the same principle. Many scientists believe that the early Earth, like Mars, Venus, and Titan, had a reducing atmosphere. This would have been an ideal environment for cyanobacteria to evolve the first photosynthetic metabolic pathways, gradually increasing the oxygen content of the atmosphere and transforming it into an oxidizing environment.