Did Protein, DNA Or RNA Come First?

Substantial evidence indicates that all life on Earth today developed from a shared common ancestor. The process by which that common ancestor formed from nonliving matter is called abiogenesis. How this process took place is not yet fully understood and is still a subject of research. Among scientists interested in the origin of life, whether proteins, RNA or some other molecule came first is a hotly debated topic.

In the famous Urey-Miller experiment, scientists mixed methane, water, ammonia and hydrogen in an effort to simulate the atmosphere of the early Earth. Next they fired electric sparks through this mixture to simulate lightning. This process yielded amino acids and other organic compounds, demonstrating that conditions like those on the early Earth could create amino acids, the building blocks of proteins.

But getting from a mixture of amino acids in solution to an intact, functioning protein presents many problems. For example, over time, proteins in water tend to break apart rather than assemble into longer molecular chains. Also, asking whether proteins or DNA appeared first presents a familiar chicken-or-egg problem. Proteins can catalyze chemical reactions, and DNA can store genetic information. However, neither of these molecules alone is sufficient for life; DNA and proteins must be present.

One possible solution is the so-called RNA World approach, in which RNA came before either proteins or DNA. This solution is attractive because RNA combines some of the features of proteins and DNA. RNA can catalyze chemical reactions just like proteins, and it can store genetic information just like DNA. And, the cellular machinery that uses RNA to synthesize protein is made partly of RNA and relies on RNA to do its job. This suggests that RNA might have played a crucial role in the early history of life.

One problem with the RNA World hypothesis, however, is the nature of RNA itself. RNA is a polymer or chain of nucleotides. It's not entirely clear how these nucleotides formed or how they would have come together to form polymers under early-Earth conditions.

In 2009, British scientist John Sutherland proposed a workable solution by announcing his lab had found a process that could build nucleotides from building blocks that were probably present on the early Earth. It's possible that this process could have given rise to nucleotides, which were then linked by reactions taking place along the surface of microscopic layers of clay.

Although the RNA-First scenario is very popular among origin-of-life scientists, there is another explanation, which proposes that metabolism came before RNA, DNA or protein. This metabolism-first scenario suggests that life arose near high-pressure, high-temperature environments such as deep-sea, hot-water vents. These conditions drove reactions catalyzed by minerals and gave rise to a rich mixture of organic compounds. These compounds in turn became the building blocks for polymers such as proteins and RNA. At the time of publication, however, there isn't enough evidence to explain conclusively whether the metabolism-first or RNA World approach is correct.