earths early atmosphere formed during the archean era. Which gas was generally absent from the atmosphere at this time
http://en.wikipedia.org/wiki/Earth's_atmosphere#Evolution_of_the_Earth.27s_atmosphere
During the Archean Era, which lasted from about 4 billion to 2.5 billion years ago, Earth's early atmosphere was very different from its present-day composition. The atmosphere during this time was primarily composed of gases such as methane (CH4), ammonia (NH3), water vapor (H2O), and carbon dioxide (CO2).
One gas that was generally absent from the atmosphere during the Archean Era was oxygen (O2). Oxygen was not a significant component of the atmosphere until much later in Earth's history, during the Proterozoic Eon. The buildup of oxygen in the atmosphere was a result of photosynthetic organisms, such as cyanobacteria, which began releasing oxygen through the process of photosynthesis.
To confirm this information, you can refer to the link you provided, which is the Wikipedia page on Earth's atmosphere. The section titled "Evolution of the Earth's atmosphere" provides a detailed explanation of the composition of Earth's early atmosphere during different geological eras.