No Good Options for the Origin of Life
The hall-of-fame catcher Yogi Berra is reputed to have said, “When you come to a fork in the road, take it.” Origin-of-life researchers have been following his advice for years as they search for an evolutionary explanation for life’s origin.
There are two fundamental approaches to explain life’s beginning from an evolutionary standpoint: (1) replicator-first; and (2) metabolism-first scenarios.
Chemist Robert Shapiro argues in the cover article of the June 2007 issue of Scientific American that the replicator-first approach to the origin-of-life is a failed paradigm. From his vantage point, metabolism-first scenarios offer the best hope to explain the origin of life.
This week I would like to explain how Shapiro reaches this conclusion. Next week I will describe Shapiro’s proposal for life’s origin and point out some of the chemical difficulties with metabolism-first models.
Evolutionary origin-of-life models require pathways that ultimately generate two of life’s defining biochemical features: self-replication and metabolism. From a molecular standpoint, self-replication describes the capacity of a complex molecule to guide its own reproduction, typically by serving as a template that directs the assembly of chemical constituents into molecules identical to it.
DNA is a self-replicating molecule. DNA not only orchestrates its own reproduction, but also houses the information needed to carry out the cell’s operation. Prior to cell division, the cell’s biochemical machinery generates two identical DNA molecules from the “parent” DNA. These two molecules become partitioned into the "daughter" cells during the cell division process. In this way, the information needed to operate the cell is passed on to the next generation.
Metabolism defines the entire set of chemical pathways in the cell. Foremost are the ones that chemically transform relatively small molecules. Metabolic pathways (1) generate chemical energy through the controlled breakdown of fuel molecules like sugars and fats; and (2) produce, in a stepwise fashion, the building blocks needed to assemble proteins, DNA, RNA, cell membrane, and cell wall components. Life’s metabolic pathways often share many molecules. This sharing causes the cell’s metabolic routes to interconnect to form complex, reticulated webs of chemical pathways.
Most origin-of-life researchers maintain that the first step toward a living entity took place when a self-replicating molecule emerged. Only later did this naked self-replicator become encapsulated within a primitive membrane. According to this view, after encapsulation, metabolism emerged as a means to support the production of the self-replicator by providing the necessary building block molecules to sustain its activity.
Origin-of-life researchers have proposed a number of possible candidates for the original self-replicator. But Shapiro has pointed out that the identity of the first self-replicator doesn’t matter. Why? All replicator-first scenarios suffer from a fatal flaw known as the homopolymer problem. Let's observe.
Candidates for the first self-replicating molecule possess common chemical features. All potential self-replicators are relatively complex molecules made up of smaller chemical subunits that link up to form chain-like molecules. The side groups that extend from the self-replicator’s backbone must be chemically and physically varied to provide the physicochemical information necessary to carry out the self-replication process. However, the self-replicator’s backbone must mundanely consist of a repetitious structure.
To function as a self-replicator, a molecule must serve as a template to direct the assembly of subunit molecules into an identical copy of itself. Self-templating, and hence, self-replication is possible only if the backbone’s structure repeats with little, if any, interruption. This means that the subunit molecules that form the self-replicator must consist of the same chemical class.
Chemists call chain-like molecules with structurally repetitive backbones homopolymers. (Homo = same; poly = many; mer = units). DNA, RNA, proteins, and the proposed pre-RNA world self-replicators, such as peptide-nucleic acids, are all homopolymers and satisfy the chemical requirements necessary to function as self-replicators.
Shapiro has pointed out that while undirected chemical processes can produce homopolymers under carefully controlled, pristine laboratory conditions, they cannot generate these types of molecules under early Earth’s conditions. The chemical compounds found in the complex chemical mixture that origin-of-life researchers think existed on early Earth would interfere with homopolymer formation. Instead, polymers with highly heterogeneous backbone structures would be produced. And these molecular entities could not function as self-replicators. The likely chemical components of any prebiotic soup would not only interrupt the structural regularity of the self-replicator’s backbone, but they would also prematurely terminate its formation or introduce branch sites.
The homopolymer problem is fundamentally intractable, devastating all replicator-first models. The only remaining option is to explain life’s origin via a metabolism-first scenario. And this approach has troubles of its own as I will discuss next week.
For a detailed discussion of problems with evolutionary models for the origin of life, see the book I wrote with Hugh Ross, Origins of Life: Biblical and Evolutionary Models Face Off.