Final answer:
Ozone (O3) in the stratosphere forms the protective ozone layer that absorbs damaging UV radiation from the Sun, shielding life on Earth. It is composed of three oxygen atoms and originated from atmospheric oxygen accumulated via photosynthesis.
Step-by-step explanation:
The gas in today's atmosphere that shields us from ultraviolet radiation is ozone. Ozone, which is a molecule composed of three oxygen atoms (O3), forms a layer in the stratosphere known as the ozone layer. This layer is crucial for life on Earth as it absorbs most of the Sun's harmful UV radiation, which can damage living organisms. Without the ozone layer, more UV radiation would reach the Earth's surface, posing a serious threat to all forms of life.
Ozone is formed when oxygen molecules (O2) are split by ultraviolet light from the Sun into single atoms, which then bond with other oxygen molecules to form ozone (O3). Hence, even though oxygen began accumulating in our atmosphere about 2.4 billion years ago thanks to photosynthesis, it is the special form of oxygen, ozone, that provides crucial protection against UV radiation.