The Origin of Life

When the natural scientists of past centuries set out to distinguish living beings from dead matter, they based this distinction on phenomenological characteristics: irritability, locomotion, metabolism, growth and reproduction are the core characteristics of life. When the microscope was invented, it was discovered that all living things are made up of cells that can change and divide. The characteristics of life can be traced back to the behavior of cells. Chemists also recognized that all living things are made up of complex molecules composed of the chemical elements carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulphur (S) and phosphorus (P). As chemists only knew these complex carbon compounds from living organisms, they were called organic substances. These elements are abundant in the earth's atmosphere (N, O, H, C) and in the earth's crust (P, S, O, C). However, in order to combine them in chemical reactions to form organic molecules, an energy flow is required - for example in the form of electrical discharges or a heat source. In addition, physiologically important molecules, including numerous enzymes or the chlorophyll required for photosynthesis, also contain metals such as iron, zinc or manganese.

If life on Earth arose through random processes, there must have been a variety of organic molecules in locally high concentrations over long periods of time. But where could the organic molecules have come from?

One possibility is that the organic molecules did not originate on Earth, but came from outer space. Indeed, in the early phase of our solar system, many meteorites crashed into the Earth and the other planets. Organic molecules have been detected in interstellar gases and asteroids using spectroscopic analysis. Nevertheless, it is rather implausible that the basic building blocks of life came to Earth from space. This is because organic molecules are very sensitive to heat and radiation. High-energy cosmic radiation and even UV light from the sun destroy most organic molecules in a short time. When meteorites enter the Earth's atmosphere, they heat up extremely strongly, so that any organic components could hardly reach the ground undamaged. Even if organic molecules from space reached the earth here and there, this cannot have led to permanently high local concentrations of organic molecules, because meteorite impacts were more frequent than today, but they were not a continuous rain.

A second possibility is that the organic molecules were formed in the atmosphere of the young Earth. In addition to nitrogen and water vapor, volcanic activity at that time produced large quantities of simple carbon and sulfur compounds (CO2, CO, H2S, SO2, SO3) in the Earth's gaseous envelope. Laboratory experiments have shown that electrical discharges in gas mixtures similar in composition to the early Earth's atmosphere lead to the formation of amino acids and other organic substances. Lightning was certainly not uncommon in Earth's early atmosphere, where there was probably more vapor and bubbling than today, and is a possible source of energy for the necessary reactions. However, lightning does not occur continuously, but only sporadically at different locations, making it unlikely that high concentrations of organic molecules would be present in the same place for long periods of time. In addition, phosphorus cannot have been bound by atmospheric processes - because phosphorus is mainly found in rocks, not in gaseous compounds in the Earth's atmosphere.

As a third option, another possible source is considered - the ocean floor. In the early stages of the Earth's formation, the crust had not cooled as much as it has today, so contact between water and hot rock or magma would have occurred over large areas. On Earth today, such conditions exist only in a few places, such as the Mid-Atlantic Ridge, where high concentrations of organic molecules have been measured near the so-called black smokers. The fact that phosphorus and metals such as iron, manganese, and zinc, which are essential components of many enzymes, are virtually absent from the Earth's atmosphere and are only found bound in rocks also supports the formation of organic molecules on the seafloor.

If we consider the seafloor as the primary source of organic matter, we have found a production site capable of producing organic matter in high local concentrations over a large area and continuously for many millions of years. However, the knowledge of geophysical correlations teaches us that the heat production by radioactive decay in the Earth's interior became weaker in the course of the Earth's history. As a result, the Earth gradually cooled, the Earth's crust became firmer and thicker, and the bubbling and steaming on the ocean floor subsided. As a result, the sources of organic molecules gradually dried up. What had once been abundant became an increasingly scarce resource. The only way for early life forms to survive was to find a way to make the materials they needed. Necessity is the mother of invention.

Since all known forms of life consist of cells, it is reasonable to assume that the original forms of life were small vesicles surrounded by a lipid layer and containing various organic molecules that could be transported in and out through the lipid layer, similar to today's cells. Lipids consist of a hydrophilic head and a long hydrophobic tail. As a result, they tend to spontaneously form thin films at water-gas interfaces, such as the oil films on puddles. When the thin films collapse due to water movement, vesicles are formed - think of the familiar formation of soap bubbles. Since there must have been many bubbling black smokers on the seafloor in the early days of the Earth, it is conceivable that thin films of lipids and other organic molecules formed around the small gas bubbles. As the gases gradually escaped through the semipermeable lipid films and dissolved in the surrounding seawater, the vesicles filled with salty seawater. Even today, all cells contain an aqueous salt solution.

As the energy source for the synthesis of organic molecules, the heat flow from the Earth's interior, gradually dried up, the original forms of life had to tap into a new source of energy - sunlight. Organic molecules such as chlorophyll can use sunlight to split water molecules into atomic hydrogen and atomic oxygen, which are then available for further reactions.

Other early molecular life forms were able to synthesize fats and sugars from other molecules by using proteins as catalysts. However, they needed a suitable energy source for this. Interestingly, all living organisms on Earth today use the same universal carrier of operating energy - namely adenosine triphosphate (ATP). The fact that this is a phosphate compound strengthens our hypothesis that the origin of life is to be found on the seabed, because only there could phosphates from rocks pass into aqueous solution and combine with organic molecules such as adenosine. Presumably, ATP was initially formed in abundance on the seabed. As the amount of energy released during the conversion to adenosine diphosphate by splitting off a phosphate residue is sufficient for many organic syntheses, it was able to establish itself as an energy source. There may have been other molecules that were used as energy sources at the time. But when the supply became scarce, it was the ATP / ADP system for which nature invented a recycling system and which therefore prevailed in the course of prebiotic evolution. All modern life forms recycle ADP into ATP by oxidizing energy-rich substances such as carbohydrates or fats.

At that time, the ocean must have been teeming with macromolecular machines capable of compensating for individual deficiencies. As not only individual components became scarce, but also the general supply of organic molecules diminished, the only survival strategy left was for various primordial life forms to combine their abilities and join together as endosymbionts. Thus, all modern cells contain various organelles surrounded by a double lipid layer, suggesting that the ancestors of these organelles were once independent. The mitochondria are the power plants of the cell, producing ATP; the blueprint of the cell is stored in the nucleus in the genetic molecules RNA or DNA; and the chloroplasts, found only in plant cells, can use sunlight to split water.

However, the vast majority of the original forms of life have perished without leaving any traces that we could use to study them today. In addition to the organelles that we find in all cells today, only one other distant echo of the original forms of life remains: Viruses. Viruses contain only the genetic material required to produce their protein envelope. In order to reproduce, however, they are dependent on other life forms that can translate their hereditary molecules into proteins.