Which major gas in today's atmosphere was introduced by living organisms about 2.7 billion years ago?

Answer 1

Oxygen.

Answer 2

How does the atmosphere protect us from harmful ultraviolet radiation?

Answer 3

The atmosphere protects us from harmful ultraviolet (UV) radiation by absorbing most of it before it can reach the Earth’s surface. The ozone layer, which is a layer of ozone gas in the Earth’s stratosphere, plays a crucial role in absorbing the most damaging UV radiation, called UV-C, before it reaches the surface. The remaining UV radiation that reaches the Earth's surface is mostly UV-A and UV-B. However, the ozone layer is also depleted by human-made chemicals such as chlorofluorocarbons (CFCs), which can lead to increased levels of UV radiation reaching the surface and increasing the risk of skin cancer and other health problems in humans and other organisms.