Metabolism First: It’s not a new diet plan. It’s how some scientists think life originated. Over the course of the last few years insights into this class of evolutionary models indicate there is little weight of evidence in their favor. New work by scientists from Hungary and Spain makes this evidence even more scarce.1
Evolutionary origin-of-life models require the generation of self-replication and metabolism, two of life’s defining biochemical features. Consequently, two fundamentally different approaches to the evolutionary explanation for life’s beginning exist: replicator-first and metabolism-first scenarios.
Metabolism defines the entire set of chemical pathways in the cell. Foremost are the routes 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, and cell membrane and cell wall components. Life’s metabolic pathways often share many molecules. The shared molecules serve as connection points, causing the cell’s metabolic routes to interconnect and form complex, reticulated webs of chemical pathways..
Since origin-of-life researchers recognized many intractable problems with replicator-first models, metabolism-first scenarios for life’s emergence have gained popularity. For a detailed discussion of the problems with replicator-first explanations, see an article I wrote for the Today’s New Reason to Believe feature.
Most metabolism-first scenarios suggest that, once prebiotic materials formed, these relatively small molecules self-organized to form cycles and networks of chemical reactions that over time gave rise to life’s metabolic systems. Once encapsulated or sequestered within a membrane, these complex systems of reactions became the first prebionts.
According to this view, molecular self-replicators emerged later, along with the enzymes that catalyze each step in the chemical cycles and networks.
Still, before information-rich, self-replicators emerged, these ensembles of cycles and networks would have had to possess some self-replicating capability. Otherwise, they could not have evolved to become a living entity. Some origin-of-life scientists have suggested that proto-metabolic systems may have functioned initially as ensemble replicators. These systems could have “grown” as more and more compounds were added to their cycles and networks. Occasionally, these systems would have become unstable and fissured, producing two daughter systems. If the chemical composition and structural makeup of the cycles and networks in the “daughters” proved the same as the original system, then, in effect, they would have replicated.
For ensemble replicators to evolve toward a living entity, the replication process must occur with a high degree of fidelity. If not, then those ensembles that possess greater fitness won’t be maintained by selection; they won’t be preserved.
The researchers from Hungary and Spain assessed the replication fidelity and evolvability of these replicators. Based on a theoretical analysis, they concluded that ensemble replicators reproduce so inaccurately that selection cannot maintain fitter ensembles. They noted that the absence of this critical evolutionary constraint “cautions against metabolism-first theories of the origin of life.”2
Just like dieters who can’t lose weight no matter what they try, origin-of-life researchers experience failure every time they try to account for life’s beginnings without the involvement of a Creator.
- Vera Vasas, Eörs Szathmáry, and Mauro Santos, “Lack of Evolvability in Self-Sustaining Autocatalytic Networks Constraints Metabolism-First Scenarios for the Origin of Life,” Proceedings of the National Academy of Sciences, USA, advanced on-line, doi: 10.1073/pnas.0912628107.