Did Darwinist evolution begin before life itself?

Proceedings of the National Academy of Sciences (2021). DOI: 10.1073 / pnas.2018830118 “width =” 770 “height =” 530 “/>

A study conducted by Ludwig-Maximilians-Universitaet (LMU) in Munich physicists shows that fundamental features of mopolymeric lecules, such as their subunit composition, are sufficient to induce selection processes in an acceptable prebiotic environment.

Before life on earth, many physicochemical processes on our planet were extremely chaotic. An excess of small compounds and polymers of different lengths, consisting of subunits (such as the bases found in DNA and RNA), were present in every conceivable combination. Before life-like chemical processes could occur, the chaos in these systems had to be reduced. In a new study, LMU physicists led by Dieter Braun show that basic properties of simple polymers, together with certain aspects of the prebiotic environment, can lead to selection processes that reduce disorder.

In previous publications, Braun’s research group investigated how spatial order could develop in narrow, water-filled chambers within porous volcanic rocks on the seabed. These studies showed that, in the presence of temperature differences and a convective phenomenon known as the Soret effect, RNA strands at different lengths could be dependent on different lengths. “The problem is that the base series of the longer molecules that one acquires are completely chaotic,” says Braun.

Developed riboenzymes (RNA-based enzymes) have a very specific base sequence that allows the molecules to fold into particular shapes, while the vast majority of oligomers formed on the Early Earth probably had random sequences. “The total number of possible base sequences, known as the ‘sequence space’, is unbelievably large,” said Patrick Kudella, first author of the new report. This makes it practically impossible to assemble the complex structures that are characteristic of functional ribozymes or similar. molecules by a purely random process. ‘This led the LMU team to suspect that the expansion of molecules into larger’ oligomers’ was subject to some sort of pre-selection mechanism.

At the time of the origin of life, there were only a few, very simple physical and chemical processes compared to the sophisticated replication mechanisms of cells, so the choice of sequences should be based on the environment and the properties of the oligomers. This is where the research of Braun’s group comes into play. For catalytic function and stability of oligomers, it is important that they form double strands such as the known helical structure of DNA. It is an elemental property of many polymers and makes complexes with double- and single-stranded parts possible. The single-stranded parts can be reconstructed by two processes. First by so-called polymerization, in which strands are completed with single bases to form complete double strands. The others are known by ligation. In this process, longer oligomers are joined together. Here, both double-stranded and single-stranded parts are formed, allowing the further growth of the oligomer.

“Our experiment begins with a large number of short strands of DNA, and in our model system for early oligomers we use only two complementary bases, adenine and thymine,” says Braun. “We assume that ligation of strings with random rows leads to the formation of longer strings, the base sequences of which are less chaotic.” Braun’s group analyzed the sequence mixtures produced in these experiments using a method that is also used to analyze the human genome. The test confirmed that the entropy of the sequence, i.e. the degree of disorder or randomness within the repeated series, 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 series obtained fell into two classes – with base compositions of 70% adenine and 30% thymine, or vice versa. “With a significantly larger portion of one of the two bases, the wire cannot fold on itself and remains as a reaction partner for the ligation,” Braun explains. Thus, almost no strands are formed with half of each of the two bases in the reaction. “We also see how small distortions in the composition of the short DNA pool leave clear position – dependent motif patterns, especially in long product strings,” says Braun. The result surprised the researchers because a string of just two different bases with a specific base ratio has limited ways to distinguish from each other. “Only special algorithms can detect such amazing details,” says Annalena Salditt, co-author of the study.

The experiments show that the simplest and most fundamental properties of oligomers and their environment can provide the basis for selective processes. Even in a simplified model system, different selection mechanisms may come into play, which have an impact on the growth of strings on different scales, and the results are 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 thus played an important role in the emergence of life from chaos.