Life is the result of evolution.
We generally think that only living beings can evolve but, in fact, remember that, like we have defined, evolution is a process of increasing complexity. In this sense, even inanimate matter can evolve. For example, when the methane becomes ethane as a result of a chemical process, that is an evolution.
Keeping this in mind, it is easy for life to emerge anywhere in the Universe where a liquid environment is available. The liquid medium facilitates the chemical reactions by dissolving the reactants. Usually we think only about water as a liquid, but it is not necessary. Any liquid can do this, like methane or any other.
Over time, simple substances, such as carbon dioxide and hydrogen, must evolve to produce proteins and they will evolve to produce more complex molecules, such as RNA and DNA.
On the other hand, the components of cells, such as mitochondria, may be unique to the cells that have emerged on our planet. Of course, extraterrestrial cells must have another structure to replace mitochondria with the same function. The same can occur with other components of our cells.
Planets without liquid environment probably will have no life unless life was implanted there by a meteor coming from a planet with life or by an intelligent life with space travel capability.
It is clear that complex lives that came from different planets may be completely different as to the shape and function of their internal organs, although externally they are similar.
But why does all this occurs?
The answer is simple but controversial.
Evolution is the response of the Universe to compensate for the increase in entropy that results from any physical process to maintain the universe’s entropy unchanged.
Keep in mind that in a closed system the entropy must remain unchanged because any increment in one part of this system needs to be compensated with a reduction elsewhere. If our Universe had its entropy increased, it would entail energy gain.
Imagine a box with air inside. If we move 3/4 of this air to one part of this box the rest of the box should have the other 1/4. In the first part the entropy was increased, but in the second the entropy was reduced, but in sum the entropy of the whole system is exactly the same.
When simple substances react to make more complex substances the entropy is reduced, being entropy the measure the disorder of a physical system. The sum of the decrease in entropy caused by these reactions should compensate for the increase in entropy of the physical process related to the event.
The same occurs in the process of fusion within a star. When two hydrogen atoms merge to create a helium atom, the entropy inside the star is reduced, but the released energy compensates for that reduction and maintains the entropy unchanged.
In this sense, chemical evolution plus biological evolution should reduce entropy at the same rate as the physical process increases the entropy to have a null result, while the physical process that reduces the entropy, like fusion, releases energy that will keep the entropy unchanged.
That is why life emerges.
Life is the final result of the process of chemical evolution.
That is how life emerges.