Final answer:
The troposphere is the most important layer of Earth's atmosphere for sustaining life, containing most of the air and water vapor necessary for life's processes. The ozone layer within the stratosphere also plays a vital role in protecting life from harmful UV-B radiation, but the troposphere contains the breathable air and the climate conditions that allow life to exist.
Step-by-step explanation:
The layer of the atmosphere most important to life on Earth is the troposphere. This is the lowest layer of the atmosphere, extending up to about 6 miles or 10 kilometers above Earth's surface. The troposphere contains roughly 80% of the Earth's atmosphere by mass and is the region where most weather events occur. Additionally, it holds virtually all of the water vapor in the atmosphere, making it the layer where almost all water-based clouds are formed.
The presence of the ozone layer in the stratosphere also plays a critical role in protecting life by absorbing the bulk of the Sun's harmful ultraviolet B rays. This protection is vital as UV-B radiation can cause skin cancers and genetic damage. Although the stratosphere is also significant for life on Earth due to the ozone layer, the troposphere is where we find the conditions needed for life to thrive, including the right mix of breathable air and a stable climate.
Furthermore, the composition of the atmosphere in the troposphere, which consists predominantly of nitrogen (78.1%) and oxygen (21.0%), is essential for life processes. These gases, along with minor components like carbon dioxide, argon, helium, and water vapor, form a life-sustaining environment. Life itself has shaped the atmosphere's composition over time, with processes such as photosynthesis and respiration influencing the levels of carbon dioxide and oxygen.