Final answer:
Oxygen was absent in early Earth's atmosphere. Its absence was important because it is a byproduct of photosynthesis and is critical for the survival of aerobic organisms.
Step-by-step explanation:
The atmospheric component that was absent in early Earth's atmosphere and is critical to our survival is oxygen. The early atmosphere lacked oxygen because it is a byproduct of photosynthesis, which had not yet evolved. Oxygen plays a vital role in supporting aerobic organisms, allowing them to carry out respiration and obtain energy.