Earth's initial atmosphere probably contained higher levels of water vapor than today's atmosphere. Most of the water vapor left the atmosphere due to the a) cooling of the earth.
Explanation:
To understand this answer we need to understand the characteristics of cold and hot air. First of all, hot air or in this case water vapor due to physics tends to go up, while cold air or vapor tends to go down. This can be observed in the process of raining. Vapor always goes up and comes down to earth in the form of water, mist or snow. However, in that time the vapor didn't come down and kept going up because the atmosphere was not developed and the earth's cooling was responsible for that. Because it didn't have the pressure and gravity it has today.
