It is theorized that life began as the result of organic molecules being synthesized in the early atmosphere and rained down into the oceans. These nucleotides would have been similar to the RNA and DNA, which are the building blocks of life.
Based on this theory, life on early Earth would have consisted of little more than self-replicating organic molecules. Much later, these molecules would have combined with simple membranes, such as those formed by amino acids in suspension. These simple cells would have done no more than replicate themselves through division.
Eventually, a variant would have emerged with an advantageous difference, allowing it to become the dominant form of life on Earth, out-competing other forms of early life. This would continue until a stable form of reproduction emerged and instead of self-replication, sexual reproduction would evolve. This major leap forward allowed for a wider range of biodiversity due to the many different potential combinations of DNA sequences made possible.
The greater number of combinations allowed for more frequent genetic mutation and eventually the process of evolution would take many leaps forward in a short period of time, known as the Cambrian Explosion. Every living thing on Earth began its evolutionary journey in the atmosphere as simple organic molecules.