Did Darwinian Evolution Begin Before Life Itself?

A study by Ludwig-Maximilians-Universitaet (LMU) among physicists in Munich demonstrates that the fundamental characteristics of mopolymeric lecules, such as their subunit composition, are sufficient to trigger selection processes in a plausible prebiotic setting.

Before the emergence of life on Earth, many physico-chemical processes on our planet were highly chaotic. A plethora of small compounds and polymers of varying lengths, made up of subunits (such as the bases found in DNA and RNA), were present in every imaginable combination. Before life-like chemical processes could emerge, the level of chaos in these systems had to be reduced. In a new study, LMU physicists led by Dieter Braun show that basic characteristics of simple polymers, as well as aspects of the prebiotic environment, can give rise to selection processes that reduce disorder.

In previous publications, Braun’s research group explored how space order could have developed in narrow, water-filled chambers in porous volcanic rocks on the sea floor. These studies have shown that in presence Due to temperature differences and a convective phenomenon known as the Soret effect, RNA strands could be locally accumulated by several orders of magnitude depending on length. “The problem is that the base sequences of the longer molecules that you get are totally chaotic,” says Braun.

Evolved ribozymes (RNA-based enzymes) have a very specific base sequence that allows molecules to fold into particular shapes, while the vast majority of oligomers formed on early Earth most likely had random sequences. “The total number of possible base sequences, known as ‘sequence space’, is incredibly large,” says Patrick Kudella, first author of the new report. “This makes it virtually impossible to assemble the complex structures characteristic of functional or comparable ribozymes. molecules by a purely random process. This led the LMU team to suspect that the extension of molecules to form larger “oligomers” was subject to some sort of screening mechanism.

At the time of the origin of life, there were only a few very simple physical and chemical processes compared to the sophisticated mechanisms of cell replication, so sequence selection should be based on the environment and properties of the cells. oligomers. This is where the research of the Braun group comes in. For the catalytic function and stability of oligomers, it is important that they form double strands like the well-known helical structure of DNA. This is an elemental property of many polymers and allows complexes with both double and single stranded parts. Single-stranded parts can be reconstructed by two methods. First, by what is called polymerization, in which the strands are supplemented with single bases to form full double strands. The other is what is called the ligature. In this process, longer oligomers are brought together. Here, both double-stranded and single-stranded parts are formed, which allows further growth of the oligomer.

“Our experiment starts with a lot of short DNA strands, and in our model system for early oligomers we only use two complementary bases, adenine and thymine,” says Braun. “We assume that ligation of strands with random sequences leads to the formation of longer strands, whose basic sequences are less chaotic.” Braun’s group then analyzed the mixtures of sequences produced in these experiments using a method that is also used to analyze the human genome. The test confirmed that sequence entropy, that is, the degree of disorder or randomness within the recovered sequences, was in fact reduced in these experiments.

The researchers were also able to identify the causes of this “self-generated” order. They found that the majority of the sequences obtained belonged to two classes – with base compositions of 70% adenine and 30% thymine, or vice versa. “With a significantly greater proportion of one of the two bases, the strand cannot fold back on itself and remains as a reaction partner for ligation,” explains Braun. Thus, virtually no strand with half of each of the two bases is formed in the reaction. “We’re also seeing how small distortions in the composition of the short DNA pool leave distinct position-dependent motif patterns, especially in long strands of product,” says Braun. The result surprised the researchers, as a strand of only two different bases with a specific base ratio has limited means of differentiating from each other. “Only special algorithms can detect such amazing details,” says Annalena Salditt, co-author of the study.

Experiments show that the simplest and most fundamental characteristics of oligomers and their environment can serve as a basis for selective processes. Even in a simplified model system, various selection mechanisms can come into play, which impact strand growth at different length scales, and are the result of different combinations of factors. According to Braun, these selection mechanisms were a prerequisite for the formation of catalytically active complexes such as ribozymes, and therefore played an important role in the emergence of chaos life.