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The term "chemical evolution" can be used in many different ways depending on the context of the words. If you are speaking to an astronomer, then it could be a discussion about how new elements are formed during supernovas. Chemists may believe chemical evolution pertains to how oxygen or hydrogen gases "evolve" out of some types of chemical reactions. In evolutionary biology, on the other hand, the term "chemical evolution" most often is used to describe the hypothesis that organic building blocks of life were created when inorganic molecules came together. Sometimes called abiogenesis, chemical evolution could be how life started on Earth.
The Earth's environment when it was first formed was very different than it is now. The Earth was somewhat hostile to life and so the creation of life on Earth did not come for billions of years after the Earth was first formed. Because of its ideal distance from the sun, the Earth is the only planet in our solar system that is capable of having liquid water in the orbits the planets are in now. This was the first step in chemical evolution to create life on Earth.
The early Earth also did not have an atmosphere surrounding it to block ultraviolet rays which can be deadly to the cells that make up all life. Eventually, scientists believe a primitive atmosphere full of greenhouse gases like carbon dioxide and perhaps some methane and ammonia, but no oxygen. This became important later in the evolution of life on Earth as photosynthetic and chemosynthetic organisms used these substances to create energy.
So just how did abiogenesis or chemical evolution happen? No one is completely certain, but there are many hypotheses. It is true that the only way new atoms of non-synthetic elements can be made are through the supernovas of extremely large stars. All other atoms of elements are recycled through various biogeochemical cycles. So either the elements were already on Earth when it was formed (presumably from the collection of space dust around an iron core), or they came to Earth via the continuous meteor strikes that were common before the protective atmosphere was formed.
Once the inorganic elements were on Earth, most hypotheses agree that the chemical evolution of the organic building blocks of life began in the oceans. The majority of Earth is covered by the oceans. It is not a stretch to think that the inorganic molecules that would undergo chemical evolution would be floating around in the oceans. The question remains just how these chemicals evolved to become organic building blocks of life.
This is where the different hypotheses branch off from each other. One of the more popular hypotheses says that the organic molecules were created by chance as the inorganic elements collided and bonded in the oceans. However, this is always met with resistance because statistically the chance of this happening is very small. Others have tried to recreate the conditions of early Earth and make organic molecules. One such experiment, commonly called the Primordial Soup experiment, was successful in creating the organic molecules out of inorganic elements in a lab setting. However, as we learn more about the ancient Earth, we have found out that not all of the molecules they used were actually around during that time.
The search continues to learn more about chemical evolution and how it could have begun life on Earth. New discoveries are made on a regular basis that help scientists understand what was available and how things may have happened in this process. Hopefully one day scientists will be able to pinpoint how chemical evolution happened and a clearer picture of how life began on Earth will emerge.