The Birth of the Evolution

The first primitive cells are presumed to have the form of self-assembled vesicles. Basically, because the universe moves in the direction of increasing entropy according to the second law of thermodynamics, it is necessary to make a structure that distinguishes cells from the outside environment. Interactions with outer environments took place through the cell membrane, and the cell membrane had to consist of a very stable material. So, cross-linked fatty acids and phospholipids that make up most of the cell membrane components today were chosen. Under the protection of cell membranes, metabolism could occur within the cell that would be well suited to the primitive earth environment, and exchange of small molecules would have been possible.

However, there was no evolution of these primitive cells. They may have been able to maintain their form and divide and maintain an inner world that is different from the outside world, but they did not have a way to learn through interaction with the outside environment, record it, and transmit it to future generations.

So, when did evolution come to life? This required some material that is the collection of important information. Today, we all know that the DNA of almost every organism plays a role, but is DNA really the first evolutionary material? To date, it seems that the protagonist was RNA.

The "RNA World" hypothesis that RNA has begun to play an evolutionary role in the early Earth is now accepted by the greatest number of people. There are some discrepancies as to the origin of the RNA, but RNA is the most prolific of the materials at that time. The RNA world hypothesis was first proposed by Alexander Rich in 1962, and the term was used by Walter Gilbert since 1986.

In order for RNA to be the starting point for evolution, it must be able to produce very short peptide cofactors and enable RNA duplication. To date, the ancestral ribosome has been composed entirely of RNA. Of course, the roles were taken over by proteins.

Molecular structure of the ribosome 30S subunit from Thermus thermophilus from Wikipedia

In primitive earth, the reaction between organic materials gave rise to nucleotides, and these nucleotides were linked and RNA was generated. It is estimated to be about 4 billion years ago. Among them, a number of RNAs self-replicated, so-called 'RNA world' began in 3.8 billion years ago. Some of these RNAs continue to connect with the nucleotides and acquire other functions. For example, the ability to synthesize proteins has been added.

For early life, RNA was important because it stored information and acted as a catalyst for chemical reactions (as a ribozyme). In fact, in early-Earth-like environments, relatively short RNA molecules have been artificially produced in labs, which are capable of replication. For example, replicase RNA functions as both a code and a catalyst. Jack W. Szostak argues that certain catalytic RNAs can self-replicate after binding smaller RNAs. If this is true, it is likely that cells with this kind of gene have flourished by Darwin's natural selection. In 2009, G. F. Joyce demonstrated the evolution of a new system capable of more efficient replication through evolutionary competition experiments. This was the first demonstration of evolutionary adaptation to a molecular genetic system.

As RNA appeared, Darwin's natural selection could proceed and the heritability of the traits, the birth of diversity, and the differential reproductive output, resulting from interaction with the external environment, could be seen, so the birth of life can be defined after the appearance of RNA. Because RNA has inherently unstable structure, short life span was disadvantage. So the RNA world does not last that long, and it is thought that the world of DNA and protein has begun with the emergence of DNA with a more stable structure, similar to RNA about 3.6 billion years ago.


The Origins of Cellular Life
Evolution in an RNA World


Popular Posts