Scientists do not know how life began on Earth, but they do know that the early Earth’s atmosphere was very different from the atmosphere now.
In 1952, Stanley Miller was working with Harold C. Urey and they designed an experiment to see how complex organic molecules might have formed under the conditions of early Earth. They believed the early Earth atmosphere would have been composed of methane, ammonia, hydrogen and water vapor. They sealed these gases in an airtight container, and then exposed the gases to sparks of electricity to simulate lightning. They continued the lightning for a week, and by the end, a reddish-brown substance had coated the walls of the container. This substance contained 11 of the 20 amino acids used by life on earth. Since Miller and Urey performed this experiment, its results have been confirmed many times by other scientists. Many scientists now believe that the early Earth’s atmosphere was composed of carbon dioxide, nitrogen and water vapor.
Modern experiments with this mixture of gases produce similar results suggesting that early conditions on Earth produced complex organic molecules that probably became the basis for the development of more complex organisms. However, scientists have not been able to replicate the formation of even simple organisms, or anything that can really replicate itself. There are several theories as to how the amino acids might have made the leap into the complex, self-replicating life we see today.
This animated video from Youtube channel Stated Clearly describes the Miller-Urey Experiment.
Metabolism - First
Some scientists believe that metabolism, in other words - the ability to break down carbon dioxide in the presence of a catalyst into small organic molecules - was how the first life developed. These reactions might have evolved to become more complex, and then genetic molecules somehow formed and joined in later. There are many different theories as to exactly what types of molecules and catalysts would have been involved.
Genes - First
Other scientists believe that the first living organisms were genes. These genes were single molecules that had developed in such a way as to be able to catalyze their own replication. This theory seems more likely, since even simple systems such as crystals, have been demonstrated to evolve with modifications that breed true. Some scientists have suggested that certain compositions of clay create the right environment for these reactions to propagate.
RNA is a complex molecule found in all living things that seems to be able to catalyze its own reproduction. Many scientists believe that simple RNA molecules developed and eventually became more complex and developed into the organisms we see today.
This animated video from Youtube channel Stated Clearly explains the idea of the RNA world hypothesis which states that before DNA evolved, life on Earth was made up of self-replicating RNA molecules.
Astrobiologists and biochemists want to understand something they call LUCA (the Last Universal Common Ancestor). The idea is that all life on Earth has a common ancestor, kind of like a great-great-great-....-great grandmother. They search for traits that are common across all life forms and assume that any traits that are common to all life forms today must have been inherited from LUCA, who had them all as well.
Biochemists know quite a bit about LUCA and her biochemistry. She stored her genetic information in DNA, she had several hundred proteins performing a variety of functions, and she used the same 20 amino acids we use in our proteins. She used RNA and had some kind of double-layer lipid membrane. She was probably the ancestor of the three kingdoms of life: Archaea, Eukaryotes and Bacteria.
LUCA lived at least 2 billion years ago, before there was much oxygen in the atmosphere. She used enzymes containing iron in her metabolic pathways the way much life on early Earth did. Studying how life arose on Earth is useful to astrobiologists, but they keep in mind that the way life formed on Earth is not the only way life could have formed. It is simply one way that it did.