Well, originally Earth's life was extremely simple; there were only single-celled organisms. As time went on, they adapted to different environments. After that, some organisms turned into macrophages, or cells that eat things, including other cells.
Soon, some cells began to bisexually reproduce, creating a multicellular organism. These organisms were still very simple and not as complex as today's living things, but definitely more complicated than the earlier generations of life before them.
They started to adapt and evolve, as well. Another important thing to mention was Earth's atmosphere at the time. Earth didn't have it's protective little blanket of ozone that it does today, so nothing could live on the surface.
As time went on, Earth's ozone layer began to develop, preventing harmful ultraviolet light from piercing the atmosphere. Organisms slowly began to appear on land, while many others stayed in the ocean.
Evolution and adaptation occured over and over to a countless number of species, creating a huge diversity of fauna and flora. Then, most of them died. It is commonly believed today that a large asteroid struck the Earth, killing off most of its organisms.
Then, the cycle repeated. This time, however, things were different. Many organisms weren't wiped out, both on land and in water, allowing for new species to develop.
Some organisms ate other organisms. This made the prey evolve to have a higher chance of survival against the predators. Some evolved, other's didn't, causing a split. Many species did this, so even though many of them were related, they are considered different organisms all together.
This cycle happened over and over, prey vs. predator, evolving and adapting, eating and being eaten. They all grew according to each others' traits.
For example, if a predator can only see certain colors, some prey will evolve to have those colors and/or camoflauge with the ecosystem. This can be seen in the case of a cheetah and a zebra.
We can clearly see the zebra against a field, because the zebra is black and white and the field is orange, but a cheetah doesn't see the orange. It only sees some stripes (the zebra) against more stripes (the field.
Hopefully this helps. In short, everything grew according to everything else's characteristics.
Fun fact: About 99% of all species that lived on Earth are extinct. Crazy, huh?
Two Pairs
The two oxygen atoms share two pairs of electrons, so two covalent bonds hold the oxygen molecule together.
