Final answer:
Photosynthesis by life forms contributed to the buildup of oxygen in the atmosphere, allowing colonizing of landmasses and evolution of oxygen-dependent life forms.
Step-by-step explanation:
The buildup of oxygen in the atmosphere was contributed by life through the process of photosynthesis. Around 2.4 billion years ago, the free oxygen produced by photosynthetic organisms began accumulating in the atmosphere. The interaction of sunlight with oxygen resulted in the formation of ozone, which acted as a protective layer against the Sun's harmful ultraviolet radiation, allowing life to colonize landmasses. This paved the way for the evolution of oxygen-dependent life forms.