MCAT Review - Biological Sciences: The Origin of Life

The MCAT is the exam taken by those seeking entry into medical school. I earned a score of 41 out of 45 when I took the exam two years ago. I wanted to share some information with those of you who will be taking this grueling exam. The following information is what I felt I needed to know to answer questions regarding the origin of life in the Biological Sciences section. In writing this, I assume you have knowledge of certain terms, gained from your biology classes.

Earth is about 4.5 billion years old. The oldest fossils are estimated to be approximately 3.5 billion years old. The oldest fossils were outlines of primitive bacterial cell walls. These ancient cell walls were found in stromatolites, which are layered mats of sedimentary structure made up of carbonate. This structure is produced over a long time by the trapping of sediment by groups of microorganisms. All life evolved from bacteria.

The atmosphere of early Earth was full of electron donors. Thus, it was a reducing environment. The early atmosphere of Earth consisted of water, carbon monoxide, carbon dioxide, and nitrogen gas. It is important to note that oxygen gas was at first absent. Without oxygen present to break bonds, simple organic molecules could spontaneously form. Lab recreations have concluded that the energy for this organic synthesis was provided by lightning, radioactive decay, volcanic activity, and the Sun's radiation. In experiments that recreated the early environment, there resulted spontaneous formation of amino acids, ribonucleotides, carbohydrates, lipids, and other organic compounds. There were no enzymes when polymerization occurred for the first time in nature. It is theorized that metal ions on the surface of clay and rocks played the role of catalysts. This theory of Earth's first execution of organic synthesis is referred to as abiotic synthesis.

Polypeptides made via abiotic synthesis were known as proteinoids. When protenoids are in water, they spontaneously form groups called microspheres. Liposomes form when lipids are added to the solution. The lipids form a layer on the surface of the proteins. A coacervate, a compound that is comprised of nucleic acids, polysaccharides, and polypeptides, can catalyze reactions if they are made with preexisting enzymes.

Protobionts is the term that collectively describes microspheres, liposomes, coacervates. Protobionts bear a resemblance to cells in that they contain a protected inner environment. Protobionts can also undergo chemical reactions. Furthermore, protobionts can "reproduce" when they grow too large. When a protobiont grows too large, they split in half.

Eventually single-stranded RNA evolved as the first mechanism for heredity. In the appropriate environment, RNA chains can self-replicate. While replicating, the RNA chain can use a nonspecific catalyst. A metal ion, for example, would increase the efficiency RNA replication. Eventually, the earliest cells switched to DNA as a genetic material. This switch was favorable because DNA is more stable due to its 2' deoxy structure. Also, DNA spontaneously forms a compact, double-stranded helix.