How did life start on Earth?

How did life Start on Earth?
This is an excellent question that has challenged scientists for many years. The answer to this question is still in the realm of theoretical speculation however, this does not mean that there are not strong theories about how life first appeared on Earth. The earliest evidence of life on Earth can be found in fossilized matts of cyanobacteria in Greenland that are about 3.7 billion years old. Whereas this is the first concrete evidence of the beginning of life, there is other evidence dating back to around 4.1 billion years which indicates a form of carbon that is believed to be connected to life.
There are many competing theories about the beginnings of life on Earth. One of them is known as panspermia which stipulates that life actually exists throughout the universe and is carried to knew planets through the impact of comets and meteorites on lifeless planets. Another theory postulates that life started off with amino acids and polypeptides and through a combination of chemical processes and electric sparks via lightning life started.
Today's life is dominated by DNA which drives is code of life. However, this is believed to not always to have been the case. It is believed by many scientists that RNA was once the driving molecule of life. RNA today still has a role to play in all cells as a integral part to play in protein synthesis and also can be found in mitochondria. Mitochondria are the engines of the cell and many scientists believe that they were primitive life that later joined with other cell for mutual symbiotic advantage. It could be that there have been many genesis of life on earth given that over 99 percent of life that has ever existed has gone extinct.
References:
https://www.livescience.com/1804-greatest-mysteries-life-arise-earth.html
http://www.nature.com/scientificamerican/journal/v197/n1/pdf/scientificamerican0757-131.pdf

There is evidence that early Earth was different than Earth today. The atmosphere had no free oxygen—instead it consisted of methane, ammonia, water vapor, and hydrogen gas. It has been proven by experiments like one done by Stanley Miller in the 1950s that organic compounds can be formed spontaneously in the absence of living organisms, given the proper conditions. There was volcanic activity and lightning that provided an energy source along with an abundance of elements in the early Earth that reacted together and formed organic compounds.
Eventually, they would have accumulated and would have been stable for a long period of time—like a hot, rich organic soup. As molecules collided, larger, more complex molecules formed. These are polymers. Their formation is known as spontaneous polymerization.
It is thought that eventually, these chemicals became able to self-replicate and produce proteins capable of catalyzing chemical reactions that would be a key step to life evolving on Earth. Self-replicating polynucleotides—the building blocks of proteins—were established on Earth 3.5 billion years ago. It is believed that this brought about RNA, which can be used as a catalyst and can act as a template for replication. RNA is a single-stranded nucleic acid and was probably the first molecule on early Earth to carry the genetic information necessary to make proteins.
It is next hypothesized that the first cell arose on ancient Earth when a self-replicating RNA, along with its protein products, was surrounded by a phospholipid "bubble," which acted like a cell membrane, enclosing the RNA and its proteins. This would then be a primitive cell since RNA was capable of self-replicating.
Later on, DNA arose, which is a more stable molecule consisting of a double helix. In modern cells, DNA in the nucleus directs the production of proteins indirectly because it is transcribed into RNA first, and then during translation of the RNA by a ribosome, proteins are produced. This is the central dogma—DNA to RNA to protein—and describes the flow of information within a cell. Due to the endless combinations of nucleic acids, there is a large variety of life on Earth.
On ancient Earth, the first cells depended on the rich organic materials in the sea broth for food, and these early organisms were heterotrophic: dependent on pre-formed organic molecules to feed on. These organisms eventually multiplied, and as the organic substances began to be consumed, any organism with the ability to manufacture organic nutrients from simple molecules would have an advantage, as it could produce its own food. Therefore, autotrophs arose next on early Earth. These are producers.
As autotrophs like early blue-green algae accumulated, oxygen would accumulate in the atmosphere as a by-product of their metabolism. The atmosphere changed from an anaerobic to an aerobic one, and evidence shows oxygen accumulated on ancient Earth around 2 billion years ago. The oxygen would be toxic to the early bacteria accustomed to an anaerobic environment. Those that couldn't adapt would only be able to live in anaerobic parts of the biosphere. Other aerobic bacteria evolved. The accumulation of oxygen allowed eukaryotic one-celled organisms to arise around 1 billion years ago. And from then on, the wealth of multicellular organisms on Earth evolved into every part of the biosphere.
https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment