Protobionts are organisms that are controversially considered to have possibly been the precursors to prokaryotic cells.
A protobiont is an aggregate of abiotically produced organic molecules surrounded by a membrane or a membrane-like structure. It has been proven that protobionts could have spontaneously formed early in the earth's development, according to the laws of physics and chemistry. Protobionts exhibit some of the properties associated with life, including simple reproduction and metabolism, as well as the maintenance of an internal chemical environment different from that of their surroundings. It has been suggested that they are a key step in the origin of life on earth. Experiments by Sidney W. Fox and Aleksandr Oparin have demonstrated that they may be formed spontaneously, in conditions much like what the early Earth is thought to have been like. These experiments formed liposomes and microspheres, which have membrane structure similar to the phospholipid bilayer found in cells.