The earth's first atmosphere (approximately 4.6 billion years ago) was most likely composed of

4.6 billion years ago - most likely hydrogen (H2) and helium (He)

probably rich in hydrogen compounds like methane (CH4), ammonia (NH3), hydrogen cyanide (HCN), water (H2O), and formaldehyde (HCHO). Also some N2, CO2 but LITTLE or NO free oxygen.

Molecules of H2 and He move really fast, especially when warm, that they eventually all escaped Earth's gravity and drifted off into space.

The answer to this question is explained below:

Tha earth's first atmosphere (approximately 4.6 billion or several billion years ago) was most likely composed of the following;
First, "water vapor"
Second, "Nitrogen"
Third, "Carbon Dioxide" 

Due photosynthesis, the oxygen increase.

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