The oldest fossil record of prokaryotic organisms was obtained from the store of 3.5 billion years of age. From this, it can be presumed that the first life appeared on earth many years before this. However, the most prevailing idea is that the first life appeared within 1 billion years from the origin of the earth.
Atmosphere on Earth when the First Life Arose:
The primitive atmosphere of the earth which promoted the origin of life was absolutely different from the present-day condition. At that time, there was no free O2 on the earth. The environment was principally full of nitrogen, CO2, and water vapor. Besides there were H2S, NH3 and CH4, etc. The temperature of the earth in this period was also very high and used to favor various chemical reactions.
Frequently the occurrence of volcanic eruptions would increase the temperature of the earth. Besides this in the sky of the primitive earth, frequent electric sparking was a common event. High energy radiation also came from the sun and other celestial bodies. It is predicted that all these conditions of the primitive earth were favorable to the origin of the earth.