Did life start with a replicating molecule or a metabolism?

Early biologists may have been at a loss when they wondered how life on Earth could ever have possibly arisen. Modern biochemists face the opposite quandary. They know of many possible prebiotic chemistries that could have produced cells and therefore life; the challenge lies in figuring out which one actually occurred.

The idea of a primordial "RNA world" was proposed 50 years ago, and it has since been demonstrated that biological compounds—i.e. DNA, proteins, and membranes that can enclose vesicles—can be generated with prebiotic means. That's a lot of progress. But these prebiotic means require minerals like boron and molybdenum that are only present in the requisite quantities on Mars. And there is still a big gap between the synthesis of such compounds and their organization into Life As We Know It: understanding how those biological compounds, once synthesized, became capable of replication.

According to Science's latest perspective on the origin of life, "biochemistry occurred on geological time scales, in which millions of years of a poor replicator (a blink on the geological time scale) might well have been necessary to craft a feedback cycle that led to a slightly better replicator."

Once an efficient replicator arose, it still remains unknown how it might have started using pure materials, like ribose, that would have been found in a complex mixture of prebiotic chemicals. But "once an early replicator established itself ... the feedback cycle leading to the evolution of additional catalysts [enzymes to perform metabolic functions other than replication] would have been difficult to derail."

Alternatively, some argue that metabolism may have evolved before replication. The necessary ingredients for primordial carbon fixation pathways were present near hydrothermal vents—in this model, the earliest life forms don't have to rely on materials found on Mars. Once important metabolites become available, "the chance of 'kick-starting' self-polymerizing ribozymes is an increasingly realistic option."

Ribozymes—pieces of RNA with catalytic activities, like enzymes—are obviously essential for envisioning an RNA world. But we need not even begin with pure ribose; RNA molecules with different backbones can exist. In fact, they may have actually helped prebiotic chemistry, since a mixture can lower the temperature required to separate and rearrange the double helix.

So far, ribozymes have been found that are capable of not only linking two RNA molecules together, but directly copying an RNA molecule. Once this copying ability became self-replicating, all the molecule needed was a compartment to protect them and enclose the substrates they needed, and to divide in half periodically. This results in—voila—a protocell.

Science, 2014.  DOI: 10.1126/science.1246704  (About DOIs).