Final answer:
The disappearance of hydrogen and helium from Earth's early atmosphere is attributed to gravitational escape. These light gases have largely escaped into space due to their high velocities. Volcanic activity contributed to the original formation of Earth's atmosphere, along with comet and asteroid impacts.
Step-by-step explanation:
The reason for the disappearance of hydrogen and helium from the early atmosphere is d) Gravitational escape. These elements are rare in our atmosphere because gas molecules with higher speeds can escape from the atmosphere into space, and hydrogen and helium move at higher speeds due to their lower mass. Over billions of years, more hydrogen and helium molecules have escaped relative to heavier molecules such as nitrogen and oxygen.
The early atmosphere lacked oxygen because a) Oxygen is a byproduct of photosynthesis, and there was scant oxygen until photosynthetic organisms evolved. The atmosphere of early Earth had a very different composition and was subject to extreme radiation. Moreover, volcanic eruptions did not shape the early atmosphere; instead, the gravitational escape of lighter gases predominantly influenced its composition over time.
Source of Earth's Original Atmosphere
The original source of Earth's atmosphere may have come from (1) the accumulation of debris from the formation of the Sun, (2) release from the interior through volcanic activity, or (3) impacts by comets and asteroids. Current evidence supports a combination of the second and third sources.