ATMOSPHERE
A thin layer of air surrounds the Earth that we denominate the atmosphere as a whole, which protects us from the Sun’s UV radiation and from particles such as cosmic rays, and it also provides us oxygen to breathe. However, the current atmosphere has changed dramatically along time. Probably, the first layer of the atmosphere lasted a short time. It was mainly hydrogen and helium (yes, the same element which the Sun is made of), but being so light, and the Earth being a relatively small and low-mass planet, these elements “evaporated” and were released to space. Then, after the constant volcanic activity, the second atmosphere formed, mainly made up of methane, ammonia, water vapor, nitrogen, monoxide, and also carbon dioxide.
In that environment is where life begins. The first living organisms (bacteria) used solar light and these atmosphere elements to produce energy. They generated large quantities of free oxygen. Finally, as time goes by, these bacteria ended up changing the conditions of this layer, and today we have an atmosphere consisting of nitrogen (N2) and oxygen (O2), and other gases such as carbon dioxide, argon, neon, helium, and ozone.
It is interesting to note that though the atmosphere has changed over the last million years, life has remained as a constant, so the restrictions to its existence are not limited to an exact atmosphere like the one we have today.
