How Life Came to Exist on Earth

The next time you step outside, take a second to look around. Most likely, you will be surrounded by lush green and the sounds of life. Birds chirp and squirrels make that weird squeaky noise. If not, just imagine it. The point is, we take most of it for granted and don't think of the long evolution process that spans super eons. The development of life is thought to be a two-stage process beginning somewhere along the past 4.6-4.7 billion years.
The very first phase of the development of life is chemical evolution. It was the process by which inorganic molecules combine and form into organic molecule in the early oceans. This is why the early oceans were known as "primordial soup." Chemical evolution took the better part of a billion years to complete. It started with the formation of the earth's early crust and atmosphere. After that was completed, small organic molecules form in the seas. Eventually, larger organic molecules began to form and are know as biopolymers. This is all occurring in the oceans. Finally, at the end of the process, the first protocells emerge.
After chemical evolution comes the familiar biological evolution that we all know and love. After protocells, the first single-cell prokaryotes form. They are organisms without a true nucleus or cell walls. Eventually, eukaryotes (organisms with true nucleus and cell walls) develop and go on to evolve into the variety of multi-cellular organisms that live on the land and in the seas.
Scientists were able to figure out the process of evolution through Dr. Miller's "primordial soup" experiment in which he demonstrated that amino acids could be formed by passing an electric current through a flask of methane. In addition, scientists study fossil records that are preserved in layers of rock to learn the order of evolution. They also study and use carbon dating and half-life to determine the exact age of some of the relics they uncover. This allows them to make an accurate time scale to track the progression of life through the early stages of the earth to relatively recent times.