Life Evolution (1):Origin of life and protocells in prebiotic soup

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Primitive ocean or Hadean ocean

Approximately 4.5 billion years ago, interstellar dust, ice, and rocks orbiting around the Sun gathered to form the Earth. At that time, the Earth temperature was extremely high as the gravitational potential energy of the dust and rocks converted into molecular kinetic energy during the formation process, resulting in a molten magma surface. As a result, liquid ocean has not yet formed and water exists in the air as vapor.

About 4 billion years ago, the heat energy was almost dissipated and the Earth's surface environment became moderated to form rocks and liquid oceans. But the Earth's core was still hot and active, so there were frequent earthquakes, volcanic eruptions and lightning.

Organic molecules and RNA world

During that time, the atmosphere of early earth was lacking in oxygen, and gases erupted from volcanoes enriched the atmosphere with methane, nitrogen, carbon dioxide, sulfur dioxide and other gases. Under the influence of lightning and high temperatures, these gases reacted with water to generate various amino acids, nitrogenous bases, sugars and lipids. They gathered in the primordial ocean to create a thick soup of organic matter. After a long evolution, these organics gradually generated large biological molecules such as peptides and nucleic acids under the high temperatures of submarine craters and catalysis of sand or clay.

The double helix DNA was more complex and it required enzymes to catalyze its replication. On the other hand, the single-stranded RNA was not only simpler, but it also acted as a catalyst. Therefore, it was unlikely that DNA was the primary genetic material in the early ocean, and RNA might be the best candidate. Some self-replicating RNAs with a specific three-dimensional structure appear in the primordial ocean that was full of diverse RNAs. They competed for nucleotide building blocks to replicate themselves, and create descendants with the same replication ability. These RNA molecules also catalyzed the synthesis of some simple proteins. This had been demonstrated to be feasible because the ribosome was a type of ribozyme whose active site was composed of RNA and the protein in ribosome maintained the conformation of RNA like clay. Due to the fact that proteins are more efficient in catalyzing chemical reactions compared to RNA, these proteins gradually replace RNA. Over time, these self-replicating RNA molecules outcompeted others and spread throughout the primitive ocean. This era was known as the RNA world.

Protocell

Lipids self-assembled into vesicles with bilayer membranes under the clay catalysis. Similar to cell membranes, these bilayers control the entry and exit of substances. Their thickness was closely related to the stability and permeability of vesicle. Thin membranes had poor stability, while thick ones had poor permeability. Vesicles with optimal thickness were more likely to survive. Eventually, vesicles composed of fatty acid chains with 14-24 carbons develop into protocell membranes. These vesicles can fuse with each other or divide into smaller ones, similar to cell endocytosis and exocytosis.

The selectively permeable membranes spontaneously absorbed some organic compound, and some inorganic substances such as gravels were blocked from the outside. These membrane separated the vesicle interior from their surroundings. It not only increased the inside organic compound concentration but also provided a stable environment for chemical reactions, facilitating the synthesis of more complex organic compound. Some of these vesicles swallowed self-replicable RNA, and this new combination revolutionized the situation. The vesicles whose division and RNA replication were perfectly coordinated would be the ultimate beneficiaries. Their descendants would inherit this ability and continue to expand their territories.


DNA world


The more stable double-helical DNA carried more genetic information and had fewer replication errors. If some vesicles containing RNA underwent reverse transcription to produce replicable DNA, they would have benefited again in competition. These protocells carrying DNA eventually evolved into the earliest prokaryotes. This era was known as the DNA world.

This process may have seemed long and incredible, but the materials and environments required for it were abundant and happened in the primordial oceans spontaneously. The earliest traces of life discovered by scientists dated back to approximately 3.8 billion years ago, indicating that the transition from inorganic matter to life may have taken only 200-300 million years. In contrast, the evolution from prokaryotes to eukaryotes took more than a billion years.

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