Origin of life: submarine alkaline hydrothermal vents, organic matter, protocells

Underwater alkaline hydrothermal vents, also known as white smokers or white chimney, were discovered in the 1970s. The abundance of life in these environments caught the attention of scientists and led to the proposal of new theories regarding the origin of life. One of the most plausible theories was that life originated at alkaline hydrothermal vents or white smokers on the ocean floor.

Submarine white chimneys and simple organic matter

Unlike underwater volcanic vents, alkaline hydrothermal vents did not come into contact with the superheated magma directly. They were located several kilometers away from the seafloor spreading regions. Seawater infiltrated into the cracks of the rocks and got heated by the scorching olivine at depths of several kilometers. The heated seawater released alkaline fluids containing gases from the cracks, with temperatures around 100°C. When the fluids cooled down, elements like calcium, magnesium, iron, nickel, and sulfur precipitated above the hydrothermal vents to form white mineral structures known as white chimneys or white smokers. The sponge-like interconnected cavities in these chimneys ranged in size from hundreds of micrometers to millimeters. They acted as barriers to separate acidic seawater from the alkaline fluids, much like cell membranes.

As warm seawater continuously rose from the cracks, hydrogen, methane, ammonia and seawater seeped from bottom to the top of chimney through the porous structures. Inside these tiny cavities, the fluid was warm and alkaline (pH around 9), while the exterior was acidic seawater (pH around 6, due to high carbon dioxide levels in the hadean oceans). The proton concentration gradient increased the electrochemical potential, and the semiconductive iron sulfide compounds on the cavity walls readily transfered the electrons of the reactants, making chemical reactions faster and easier. Through compartmentalization, high-concentration reactants, warm seawater, proton gradient and iron-sulfur catalysts, nature took its first step in chemical evolution within the hydrothermal chimneys.

Simple organic compounds, such as amino acids, nucleotides, fatty acids and thioesters could be synthesized within the cavities at the top of the rocks. Thioesters and phosphates produced acetyl phosphate to store energy from the proton gradient. They served as precursors to ATP and drove the initial biochemical reactions. Due to the thermophoresis, these organic compounds tended to aggregate within the cavities at the top of chimney. When their concentration became high enough, they easily formed peptides, short-chain RNA, and vesicles with bilayer membranes. The vesicles continuously absorbed organic materials, grew and then budded off smaller vesicles.

The earliest metabolic pathway and genetic material

Sometimes peptides encapsulated iron sulfide compound to form a special structure that could catalyze biochemical reactions and embed themselves within the lipid vesicles. The biochemical reactions from the rocks were transferred into the vesicles, leading to the emergence of increasingly complex organic compounds and metabolism. By chance, several iron-sulfur proteins aligned together to allow the sequential transfer of electrons until they were neutralized by hydrogen ions. The alkalinity inside the vesicles increased and the hydrogen ions outside tended to enter the interior. If there were ion channels in certain membrane proteins, they could utilize the concentration gradient to perform additional functions, such as producing the simplest high-energy compound, acetyl phosphate.

RNA was considered the original genetic material because of its simple structure, ability to record genetic information and catalyze protein synthesis. RNA did not easily form double strands and it tended to fold. Inside the vesicles, some RNA folded into a specific shape to catalyze protein synthesis, while others had structures that enabled self-replication. If they could perfectly coordinate with vesicle division, they gained an advantage in competition and gradually eliminated other vesicles. Protocells were formed. The double helix structure of DNA provided stability to genetic information. Some vesicles used DNA to produce RNA for protein synthesis, allowing them to gain an advantage in competition and eventually become dominant.

These early cells evolved proton pumps and flagella, enabling them to leave the hydrothermal vents. All of these processes took place within the cavities of rocks and they were waiting for an opportunity to leave.

Last universal common ancestors or LUCA

Countless submarine hydrothermal vents existed in the hadean ocean and each with endless tiny cavities. Every cavity was constantly performing such experiments. One of these cavities, like winning the lottery, produced cells capable of DNA controled metabolism. They were known as the Last Universal Common Ancestors or LUCA. Earthquakes destroyed the chimneys and LUCA were released. They used inorganic substances and proton concentration to manufacture organic matter. Over hundreds of millions of years, LUCA colonized the oceans and differentiated into bacteria and archaea.

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