Great question. There are 2 main hypotheses:
1. Purely natural increasing complexity collection and assembly of fundamental elements into organic molecules with a wide range of options for development from external resources: carbon, nitrogen, hydrogen, sulfur, phosphorus. and other elements.
2. Transcendent extra-natural designer/creator caused the elements to combine and form complex biochemical assemblies that included all cellular functions, and ended with a single or group of living organisms.
Hypothesis 2 is not being investigated by conventional science.
Hypothesis 1 is made up of many theories and hypotheses that have not proven any aspect of the required processes and structures required for the origin of life. Nevertheless, what came first is a wide array of abiotic compounds, assemblies and structures that must form in the same location at virtually the same time. The simplest level of elemental reactions to produce polymers, sugars and amino acids are the logical first biochemical assemblies to form. But the process of chemical evolution to accomplish this does not exist at sufficient level to produce life. The critical amino acids are the 4 used in the genetic code, which all organisms contain. There are about 20 types needed throughout the cell and can only be made through the cellular machines that result from metabolic operations related to the genetic code. The amino acids must be 100% left-handed to function properly and there are no natural mechanisms that can accomplish this. Within a living cell it is common practice. The sugars must be right-handed 100% to attach to the amino acids and form the nucleotide chain that specifies how to build proteins, which are the fundamental building blocks of all organisms.
All the necessary assemblies and structures must be assembled at the same time for the next critical assembly of lipids and proteins, the cell membrane, to encapsulate them and prevent contamination while allowing key materials to enter and exit as needed to support the cell.
The result is that proteins, nucleotide chains of sugars and amino acids, polymers that provide structure, metabolic and reproductive elements, lipids, and a supply of essential nutrients must all come together to assemble all the elements necessary for a living entity, but what actually causes this assembly of materials to come to life is not known as it does not directly follow that combining all the necessary chemicals will produce a living organism.
