How did the first cell in the world come about?

Viewed 44

On April 28, 2012, the US Discovery News website revealed a new research result from scientists at the University of Oslo in Norway: they discovered a single-celled organism from a billion years ago.

This oldest known organism was found in the mud of Lake As, 30 kilometers south of Oslo. It was 30-50 micrometers long and moved its body with four tail-like flagella. This was the only single-celled organism in the world at that time, because it was neither an animal, nor a plant, nor a fungus or algae.

However, according to the research on the origin of life, this organism is not the earliest cell in the world, because according to measurements and calculations, the earth has a history of 4.6 billion years, and cells have appeared on the earth for at least 3.6 billion years.

How did the first cell in the world come about? This is an old problem. Today, the best explanation for this question is the chemical evolution hypothesis.

Scientists infer that during the formation of the earth, a large amount of gas produced by violent changes in the earth’s interior broke through the crust with frequent volcanic activities and sprayed into space, forming an atmosphere above the crust.

In the newly formed atmosphere, there were methane, ammonia, water vapor, carbon monoxide, carbon dioxide, hydrogen sulfide, etc., but no oxygen or nitrogen. Under the combined action of cosmic rays, radioactive materials on the earth, solar ultraviolet rays, lightning flashes, etc., inorganic molecules in the atmosphere gradually formed organic small molecules such as amino acids, purines, pyrimidines, ribose, deoxyribose, porphyrins, etc.

This is the first stage of chemical evolution. Organic small molecules synthesized and accumulated over billions of years became more and more abundant. In this process, the surface temperature of the earth gradually decreased, water vapor gradually condensed into water, and organic small molecules in the atmosphere gathered into the primitive ocean with rainwater. The organic small molecules in the primitive ocean became more and more abundant.

At that time, because life had not appeared yet, there were no microorganisms that could decompose organic matter, so organic molecules would not rot or deteriorate. The whole ocean was like a pot of nutritious and warm and clean "primordial soup". In the "primordial soup", amino acids, nucleotides and other organic molecules interacted with each other for a long time. Through dehydration and combination reactions such as "condensation", small molecules became organic macromolecules such as proteins and nucleic acids. This is the second stage of chemical evolution.

When the concentration of organic macromolecules in the primitive ocean increased continuously, under certain external conditions, these organic macromolecules separated from the ocean and interacted with each other to form droplets.

At this time, organic macromolecules can avoid being disintegrated due to seawater constantly "squeezing" into the molecular interior. The droplets formed by organic macromolecules are surrounded by a membrane that separates them from the ocean environment. This membrane is a combination of lipids and proteins. It has a dual function of "sentry" and "pump".

With this membrane, harmful substances outside cannot enter the droplet interior, while nutrients are just the opposite. Even if they are very low in concentration, they will be "pumped" into the droplet interior. Such an independent droplet is a multi-molecular system. This kind of independent multi-molecular system can exchange some substances with the external environment. This is the third stage of chemical evolution.

The evolution of the multi-molecular system into the initial cell is the fourth stage of chemical evolution, which is also the most complex and decisive stage. In this stage, the macromolecules such as nucleic acids and proteins in the multi-molecular system "take their positions", with nucleic acids located in the center of the multi-molecular system and proteins around the multi-molecular system.

The central part is called the nuclear region, and the surrounding part is called the cytoplasmic region. The multi-molecular system after nuclear-cytoplasmic separation is the "prokaryotic cell".

There are two types of cells on earth today: prokaryotic and eukaryotic. Prokaryotic cells only have nuclear material, and there is no membrane around the cytoplasmic material. There are also no organelles such as mitochondria and chloroplasts in the cytoplasm. Eukaryotic cells have a nuclear membrane surrounding the nuclear material.

In the 1960s, a scientist named Margulis had a brainstorm and proposed a bold hypothesis. He believed that after anaerobic cells, aerobic cells and autotrophic cells (i.e., cells that can perform photosynthesis) existed on the earth at the same time, anaerobic cells engulfed aerobic cells into their cells, and aerobic cells coexisted and prospered with anaerobic cells. Aerobic cells eventually became an organelle in anaerobic cells, which is mitochondria.

If anaerobic cells engulfed autotrophic cells again, when autotrophic cells entered prokaryotic cells that already had mitochondria, they could also coexist and prosper. Then autotrophic cells also became an organelle, which is chloroplast. Anaerobic cells with engulfing ability not only engulfed other cells, but also engulfed their own cell membrane. When the engulfed cell membrane surrounded the nuclear material, eukaryotic cells appeared.

Margulis’s hypothesis of continuous engulfment is called the "endosymbiosis" hypothesis. However, if a hypothesis is not confirmed by experiments, it is not science. While scientists were simulating volcanic eruption environments to explore the origin of cells, research on artificial synthesis of life substances was also in full swing.

At the end of the 20th century, artificial synthesis of insulin and yeast alanine transfer RNA was successful. At the beginning of the 21st century, artificial synthesis of life macromolecules such as proteins and nucleic acids was successful. People believe that one day cells will also be artificially "manufactured", and by then there will be a clear answer to how the first cell on earth came about.

0 Answers