This is an interesting question with unclear answers. There are many theories about how life began and how it evolved.

One theory is that lightning in Earth's early, hostile atmosphere provided sparks that allowed the chemicals found in early oceans and atmosphere to react and form the building blocks of life (amino acids for proteins, sugars and nucleic acids for RNA, DNA, etc.). The idea here is that these compounds amalgamated into units that would undergo replication, cross-over (sharing information), and mutation, and eventually constituted single-celled organisms that took in sunlight, used chemicals from the surroundings, and released by-products that slowly changed the atmosphere of the Earth.

These ideas are consistent with "metabolism-first" models -- the idea that life began with small molecules that underwent cycles of reactions that became encapsulated in cell-like units over time. An opposing view is the idea that the first forms of life began as RNA ("RNA world hypothesis") and then eventually proteins and DNA developed from RNA.

There are other theories that suggest life may have formed in the deep sea vents of the oceans, which are rich with chemical and thermal energy, or that organic molecules may have been arranged in patterns in clay. There are also theories that life began in space on asteroids and that through successive collisions with the early Earth, it was introduced to the planet.

For the origin of life, we really don't know. There are several possible ways it could have happened, but all involve complex chemistry in an environment without oxygen using bubbles of soap-like hydrocarbons containing other chemicals like RNA. There may have been crystals of certain types of clay involved, too.

Life can evolve for several reasons. There are three main reasons: the first is mutation, where errors or changes happen in the DNA code that life uses to store information. This is the source of all differences between living things, and is required in order for the other types of evolution to happen. The second is natural selection, where some existing mutations are better at surviving or reproducing than others. Third, there is genetic drift, where some existing mutations survive due purely to chance.

Generally, natural selection is responsible for the creation of new species with complex adaptations: unlike genetic drift and mutation, natural selection exists because of the laws of physics, which are nonrandom, and so can act in a single direction. Drift and mutation are random, and so have a harder time making anything complicated and useful.

When the earth was formed, it is thought that there were only non-living chemicals present, and no life. However, the gas atmosphere surrounding the planet and the presence of water on the surface provided the conditions necessary for the first life to begin. There are many theories for how the first life appeared, and scientists do not agree on any one explanation. One possibility is that life originated at volcanic vents under water, where the high temperatures could provide the energy needed for chemical reactions that could form the first primitive life. Scientists have replicated these high energy conditions in lab to show that such reactions could occur.