The origin of life is easy to explain if it is the result of intelligent design, because intelligent agents know how to engineer the building blocks, and how to assemble them into the components of a living system: proteins and nucleic acids. But how can you do it when there are no intelligences allows in your worldview? There is a new article in Nature about that.
The authors write:
To understand how life might have begun, researchers must stop cherry-picking the most beautiful bits of data or the most apparently convincing isolated steps, and explore the implications of these deep differences in context. Depending on the starting point, each hypothesis has different testable predictions. For example, if life started in a warm pond on land, the succession of steps leading from prebiotic chemistry to cells with genes is surprisingly different from those that must be posited if the first cells emerged in deep-sea hydrothermal vents.
Building coherent frameworks — in which all the steps in the continuum fit together — is essential to making real progress. To see why, here we highlight two of the most prominent frameworks, which propose radically distinct environments for the origin of life.
Let’s see what the authors say about the prebiotic soup:
This framework posits that nucleotides are concentrated in a small pond. To form RNA, the simplest and most versatile genetic material, nucleotides must polymerize. That is most easily achieved by drying them out (polymerization is a type of dehydration reaction). Proponents imagine a succession of wet–dry cycles, in which the pond dries out to form polymers of RNA, then fills again with water containing more nucleotides and so on, cycle after cycle, making more and more RNA3.
This is the “RNA World” hypothesis, which I’ve blogged about before. The authors don’t like it:
But this concept raises some difficult questions. It places the onus on an ‘RNA world’, in which RNA acts both as a catalyst (in a similar way to enzymes) and as a genetic template that can be copied. The problems are that there is little evidence that RNA can catalyse many of the reactions attributed to it (such as those required for metabolism); and copying ‘naked’ RNA (that is not enclosed in compartments such as cells) favours the RNA strands that replicate the fastest. Far from building complexity, these tend to get smaller and simpler over time. Worse, by regularly drying everything out, wet–dry cycles keep forming random groupings of RNA (in effect, randomized genomes). The best combinations, which happen to encode multiple useful catalysts, are immediately lost again by re-randomization in the next generation, precluding the ‘vertical inheritance’ that is needed for evolution to build novelty.
If selection on RNA in drying ponds could somehow be made to generate greater complexity, what must it achieve? To make cells that grow and reproduce, RNA must encode metabolism: the network of hundreds of reactions that keeps all cells alive. Modern-day metabolic reactions bear no resemblance to the cyanide chemistry that makes nucleotides in this model. Evolution would therefore need to replace each and every step in metabolism, and there is no evidence that such a wholesale replacement is possible.
The authors are saying that they need to build up complexity from simple to more complex, in order to get the bare minimum they need for simple life. Life basically requires four different capabilities: membrane, energy capture, metabolism, and information storage. And they all have to be there and working right from the start. It’s a frightful amount of complexity to get right – unless you appeal to intelligence.
The authors also talk about life forming from non-living materials in hydrothermal vents:
Our own favoured scenario is that the chemistry of life reflects the conditions under which life began, in deep-sea hydrothermal systems on the early Earth4. In broad brush strokes, this means that gases such as carbon dioxide (the near-universal source of carbon in cells today) and hydrogen feed a network of reactions with a topology resembling metabolism. Genes and proteins arise within this spontaneous protometabolism and promote the flux of materials through the network, leading to cell growth and reproduction. There are plenty of problems here, too, but they differ from those in the prebiotic soup framework.
The first problem is that they need enzymes in order go from simple gases to nucleotides, and they don’t have them:
The first problem is that H2 and CO2 are not particularly reactive — indeed, their chemistry was largely ignored for decades, although rising interest in green chemistry is changing that. But deep-sea vents are labyrinths of interconnected pores, which have a topology resembling cells — acidic outside and alkaline inside. The flow of protons from the outside to the inside of these pores can drive work in much the same way that the inward flow of protons can drive CO2 fixation in cells today5.
[…]But many chemists are troubled by the idea that, in the absence of enzymes to serve as catalysts, hydrothermal flow could drive scores of reactions through a network that prefigures metabolism, from CO2 right up to nucleotides. The chemist Leslie Orgel once dismissed this scenario as an “appeal to magic”. Certainly, further data are required, supporting or otherwise.
They have problems sequencing nucleotides into functioning components:
Polymerization is another stumbling block. Nucleotides have been polymerized in water on mineral surfaces9, but this raises similar questions to those noted for wet–dry cycles about how selection could act. If the problem is solved by polymerizing nucleotides inside growing protocells, mineral surfaces would not have been available. Polymerization would then have needed to happen in cell-like (aqueous gel) conditions, but without enzymes. If serious attempts to synthesize RNA under those conditions fail, the overall framework would need to be modified.
I’m not convinced that mineral surfaces can help with the nucleotide sequencing problem, but they don’t even have those available in the hydrothermal vents.
I studied computer science in university, so biochemistry is all new to me. I am trying to learn it, but I also have to write code all day for work. But thankfully, there are experts who can sort this out for us.
Here is a podcast from the fellows over at the Discovery Institute, and they talked about this article. Podcasts are a great way to try to understand these complex problems.
Be effective and influential:
WINTERY KNIGHT 