Final answer:
The gas missing from Earth's early atmosphere that is present today is oxygen. Initially, Earth's atmosphere was virtually devoid of free oxygen until the rise of photosynthetic organisms about 2 billion years ago, which began to substantially increase the oxygen levels.
Step-by-step explanation:
The gas that was missing from Earth's early atmosphere and is present today is oxygen. Initially, the planet's atmosphere had negligible amounts of oxygen, and it wasn't until about 2 billion years ago that it became abundant.
Explaining this change involves understanding the Earth's early conditions. Initially, Earth lacked an atmosphere and oceans. However, as the planet cooled, volcanic activity began releasing gases like ammonia, methane, water vapor, and carbon dioxide, forming the primordial atmosphere. Crucially, this atmosphere was anoxic, meaning it lacked free oxygen. The significant change came with the advent of photosynthetic organisms, which started converting carbon dioxide into oxygen.
Over time, the rising amounts of oxygen in the atmosphere could not be completely absorbed by chemical reactions with Earth's crust. This led to gradual accumulation of oxygen, a process aided by the increased sophistication and proliferation of plant life, as well as geological activities that buried organic carbon, preventing it from recombining with oxygen, which could have reduced its free form in the atmosphere.
As a result of these complex processes, Earth's atmosphere transitioned from a chemically reduced state to an oxidized state, leading to the presence of oxygen. It is largely thanks to the presence of life that Earth’s atmosphere is unique among its neighboring planets, with a significant portion of free oxygen.