Which gas was absent in the atmosphere at the time of the origin of life?

The atmosphere at the time of the origin of life on Earth was vastly different from what we observe today. One of the significant characteristics of the primordial atmosphere is that it lacked oxygen. In the early stages, the atmosphere was primarily composed of methane, ammonia, water vapor, and other gases, but contained very little to no free oxygen.

The absence of oxygen is crucial when considering the origin of life, as many of the current forms of life on Earth, particularly aerobic organisms, rely on oxygen for respiration. The presence of oxygen in significant amounts is largely the result of photosynthetic organisms, which evolved later and contributed to the gradual oxygenation of the atmosphere, a process known as the Great Oxygenation Event.

This lack of oxygen in the early environment likely facilitated the formation of organic compounds from simpler molecules, contributing to the development of early life forms. Therefore, identifying oxygen as absent in the atmosphere at the time life originated highlights the evolutionary transition that occurred with the advent of photosynthetic organisms.