Origin of life: from luca to prokaryote(Bacteria, Archaea)

From luca to chemotrophic prokaryotes

Luca(lasted universal common ancestors) left the warm alkaline hydrothermal vent and entered the vast ocean. These chemotrophic organisms utilized inorganic substances and proton concentration in seawater to produce organic matter. Over the next billions of years, Luca differentiated into archaea and bacteria. The emergence of cell walls prevented prokaryotes from absorbing too much water and bursting, and also provided resistance to physical damage. The energy generated from the oxidation of organic and inorganic substances was used to pump protons into the space between the cell wall and cell membrane. The energy for ATP synthesis came from the proton concentration gradient they created and it freed them from the constraint of external environment.

Prokaryotes adopted a strategy of rapid reproduction to maintain an advantage in competition. The cell membrane extended and folds inward to increase its area, and it was studded with many enzymes related to metabolism, which allowed chemical reactions to proceed more rapidly. Their DNA replication, transcription and translation occurred simultaneously. They also discarded some unused DNA segments to streamline their performance. Fossil records indicate that the earliest prokaryotes appeared around 3.5 billion years ago. Rocks dating back 3.8 billion years show traces of suspected prokaryotic existence.

Purple Earth hypothesis

Chemotrophic prokaryotes proliferated in the oceans and consumed numerous inorganic substances. Soon, bacteria and archaea encountered an energy crisis due to the insufficient raw material. Some attempted to move toward the surface of the ocean and discovered a new energy source there: sunlight. The sunlight alter the conformation of retinal to pump hydrogen ions into the gap between the cell membrane and cell wall. The concentration gradient made hydrogen ions flow through ATP synthase to generate ATP. These early photosynthesis were based on the retinal that absorped green light, rather than chlorophyll, so other colors in sunlight gave the archaea a purple color. They formed microbial mats on the water surface and occupied ecological niches rapidly. The oceans, lakes and rivers were dyed purple, leading to the hypothesis of a purple Earth.

Rise of Cyanobacteria or blue-green algae

Since the purple archaea occupied advantageous position, other organisms in the lower layers of water had to utilize more complex chlorophyll to absorb red and purple light. The earliest organisms to acquire this ability were blue green algae which emerged 3.2 billion years ago. They had many specialized thylakoids for photosynthesis. Chlorophyll absorbed light energy to pump protons into the thylakoids. As the low protons permeability of thylakoids membrane, all the protons flowed through ATP synthase into the cytoplasm. In contrast, the proton gradient created by other prokaryotes dissipated due to the permeability of their cell walls. This was one of the reasons why cyanobacteria could outcompete others.

Great Oxygenation Event and Snowball Earth

One byproduct of cyanobacterial photosynthesis was oxygen. The atmosphere of ancient Earth was primarily composed of a reducing atmosphere consisting of nitrogen, methane, sulfur dioxide, and hydrogen sulfide. Rocks and oceans contained significant amounts of ferrous iron, sulfur, and other reductants. Iron were the first to be oxidized in the oceans and deposited on the seafloor. The banded iron formations in sedimentary rock, rich in oxidized iron, bear witness to that period of history. Once the reductants were oxidized, oxygen accumulated in the atmosphere and oceans. They competed for electrons in anaerobic metabolism, making it difficult for anaerobic prokaryotes to survive.

Methane was 28 times stronger than carbon dioxide in greenhouse effect and was quickly oxidized into water and carbon dioxide. The weakened greenhouse effect resulted in rapid global cooling and glacier extending from the poles to the equator at 2.4 billion years ago. Earth became a white planet known as Snowball Earth. The cold and toxic oxygen removed most life on Earth, and even the blue-green algae nearly froze to death in the ice age.

A few organisms managed to survive near underwater volcanoes during the great glaciation. They either remained in the oxygen poor water bottom to continue their previous lifestyle, or evolved the ability to use oxygen. The end products of anaerobic respiration, including methane, acetic acid and hydrogen sulfide, can be further oxidized, so energy release from anaerobic respiration was relatively little. When oxygen is served as the final electron acceptor in the respiratory chain, organic compounds can be fully oxidized to release more energy. Additionally, the gradually accumulating oxygen in the atmosphere formed a barrier called the ozone layer, reducing the harmful ultraviolet radiation. The unprecedented energy from aerobic respiration greatly accelerated the evolution. Prokaryotes were ready to moved toward the more complex eukaryotes.

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