Summarize the modern theory of evolution my natural selection

Natural selection refers to "survival of the fittest", in which organisms mate with those who have a certain superior quality, and over time, this leads to an evolutionary change as the quality becomes more present.

The atmosphere wasn’t strong enough to contain more oxygen when compared earths atmosphere today, the exosphere was full of rips and couldn’t contain enough oxygen to provide for land creatures, although aquatic earthlings could continue i exist in this environment