Final answer:
The atmosphere 3.5 billion years ago did not contain ozone, a crucial component for protecting Earth from the Sun's ultraviolet radiation.
Step-by-step explanation:
The atmosphere 3.5 billion years ago was very different from what it is today. It did not contain ozone (O3). Ozone is a molecule made up of three oxygen atoms and is an important component of the Earth's atmosphere because it absorbs the majority of the Sun's harmful ultraviolet radiation.
During the Earth's early history, the atmosphere was primarily composed of carbon dioxide, nitrogen, and water vapor, along with other gases such as methane and ammonia. However, the high-energy ultraviolet radiation from the Sun would have broken down any molecular ozone that was formed, preventing a significant build-up of an ozone layer. It was not until photosynthetic organisms started producing oxygen around 2.4 billion years ago that enough ozone could accumulate to form a protective layer.