The concept of chemical evolution is based on the possible origin of life by combination of chemicals under suitable environmental conditions. This concept suggests that the first forms of life on Earth emerged through a series of chemical reactions that occurred in a primordial soup-like environment.

Chemical Reactions:
- The early Earth was rich in various chemicals such as ammonia, methane, water, and carbon dioxide. These chemicals were constantly exposed to various energy sources such as lightning, volcanic activity, and UV radiation from the Sun.
- Under such conditions, it is believed that these chemicals could have undergone chemical reactions, leading to the formation of more complex organic molecules. These reactions include polymerization, condensation, and hydrolysis reactions.

Formation of Organic Molecules:
- The combination of these chemical reactions and the availability of various chemicals led to the formation of organic molecules such as amino acids, nucleotides, sugars, and lipids.
- Amino acids are the building blocks of proteins, nucleotides are the building blocks of nucleic acids (DNA and RNA), sugars are the building blocks of carbohydrates, and lipids are the building blocks of cell membranes.

Formation of Protocells:
- The next step in chemical evolution is the formation of protocells, which are simple, membrane-bound structures that can carry out some basic functions of life.
- These protocells could have formed when lipids spontaneously assembled into bilayer structures, enclosing other organic molecules inside. The lipid bilayer acts as a selective barrier, allowing certain molecules to enter or leave the protocell.

Emergence of Self-Replicating Molecules:
- One of the key aspects of life is the ability to replicate. The concept of chemical evolution suggests that self-replicating molecules, such as RNA, could have emerged within protocells.
- RNA can both store genetic information and catalyze chemical reactions, making it a potential candidate for the first self-replicating molecule.

Evolution of Complexity:
- Over time, these simple protocells and self-replicating molecules could have undergone further chemical reactions and natural selection, leading to the emergence of more complex organisms.
- Through a process of gradual change and adaptation, these organisms eventually evolved into the diverse forms of life we see today.

In conclusion, the concept of chemical evolution is based on the idea that the combination of chemicals under suitable environmental conditions can lead to the formation of more complex organic molecules, protocells, and eventually, the emergence of life. This concept provides a plausible explanation for the origin of life on Earth and is supported by experimental evidence and observations of early Earth conditions.