Introduction:
The study of the origin of life and the evolution of different forms of life on Earth is known as evolutionary biology or bioevolution, as named by Mayer in 1970. The term “evolution” refers to the process of unfolding or revealing hidden potentialities, resulting in an orderly change from one condition to another. This process of evolution is slow, continuous, and ongoing, never coming to a complete stop.
The origin of life is the scientific inquiry into how living organisms arose from non-living matter. This process is believed to have occurred on Earth between 3.8 and 4.2 billion years ago. The first signs of life appeared during this time period.
Scientists have proposed various theories to explain the origin of life, including the Big Bang theory, which suggests that the rapid expansion of the universe played a role in the formation of life; the theory of special creation, which proposes that a supernatural entity created life; the theory of eternity, which suggests that life has always existed; and the cosmozoic theory, which proposes that life may have arrived on Earth through interstellar travel.
Despite the numerous theories proposed, the precise mechanism by which life originated on Earth remains a mystery. Scientists continue to investigate the origin of life through various experiments and research, such as the Miller-Urey experiment, which attempted to recreate the conditions of early Earth to study the formation of organic molecules.
Big Bang theory
Abbe Lemaitre proposed the Big Bang theory, which suggests that the universe originated approximately 15 billion years ago due to a thermonuclear explosion of a dense entity. This explosion, known as the Big Bang, resulted in the formation of gaseous clouds that collapsed and converted into a flat disc-like structure made up of atoms and small particles due to their own gravitational pull. This flat-disc like structure is called the Solar Nebula.
About 4.6 billion years ago, the origin of the solar system took place through the condensation of atoms and dust particles moving around the sun. The very hot central part of the solar nebula became still hotter and converted into the sun. Formation of other planets, including Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, also took place during this time.
The solid part of Earth is known as the lithosphere, while the gaseous part is referred to as the atmosphere. As the Earth’s surface cooled down, water formed on it when the temperature decreased to 100°C.
Theories for origin of life:
The theory of special creation proposes that life and everything in the universe were created by a supernatural entity. Father Suarez was one of the greatest supporters of this theory.
According to the Bible, God created life and everything in the universe in six days. Similarly, Hindu mythology proposes that God Brahma created the world. According to this theory, life has remained unchanged since its origin.
However, the theory of special creation lacks scientific evidence and is not accepted by the scientific community as a valid explanation for the origin of life. Instead, scientists rely on empirical evidence and the scientific method to understand the natural world and its origins.
Cosmic panspermia theory –
The cosmic panspermia theory proposes that life on Earth originated from outer space. Some scientists believe that spores or other units of life were transferred to different planets, including Earth, through comets, asteroids, or other interstellar objects.
The idea of panspermia has been around since ancient times. Early Greek thinkers believed that spores, or the seeds of life, could be transported to different planets in the cosmos.
Today, the concept of panspermia remains a favorite idea among some astronomers and astrobiologists. However, it is still a matter of debate and lacks definitive scientific evidence to support it. While the discovery of organic molecules on comets and other celestial bodies supports the idea of panspermia, it does not necessarily prove that life originated elsewhere and was transported to Earth. Nevertheless, the study of panspermia and other theories on the origins of life continue to be a fascinating topic for scientists and the general public alike.
The theory of spontaneous generation
The theory of spontaneous generation, also known as abiogenesis or autogenesis, was a popular hypothesis supported by many ancient Greek philosophers, including Thales, Anaximander, Xenophanes, Plato, Empedocles, and Aristotle.
According to this theory, life could originate from non-living matter spontaneously. For example, some believed that mud from the Nile River could give rise to frogs, snakes, and crocodiles. Similarly, Von Helmont claimed that mice could form in 21 days if a sweat-soaked dirty shirt was kept in a wheat barn.
However, the theory of spontaneous generation was eventually disproven by a series of experiments conducted by scientists such as Francesco Redi (1668), Abbe Lazzaro Spallanzani (1767), and Louis Pasteur (1867). These experiments demonstrated that life could only arise from preexisting life, and not from non-living matter.
Redi’s experiment involved placing meat in jars and observing that maggots only appeared on the meat in open jars, while sealed jars remained free of maggots. Spallanzani showed that boiling broth in a sealed flask prevented the growth of microorganisms, while Pasteur’s experiment involved boiling broth in a specially designed flask with a curved neck that allowed air to enter but prevented microorganisms from contaminating the broth. The broth remained free of microorganisms, demonstrating that life does not arise spontaneously from non-living matter.
The modern theory of origin of life, also known as primary abiogenesis, was formulated by Russian scientist A.I. Oparin and J.B.S. Haldane. This theory suggests that life originated from the composition of chemicals, and is based on artificial synthesis. Four basic requirements for life to arise are a primitive atmosphere, the right chemicals (including water, various inorganic ions and organic molecules), an energy source, and infinite time.
The energy needed for chemical evolution on primitive earth mainly came from solar radiation and cosmic rays, electric discharges, volcanic eruptions, heat, etc. The lightest atoms of nitrogen, hydrogen, oxygen, etc., formed the primitive atmosphere, which was reducing because hydrogen atoms combined with all available oxygen atoms to form water and leaving no free oxygen.
In an experiment to prove that organic compounds were the basis of life, Miller took a flask and filled it with methane, ammonia and hydrogen in the proportion of 2:1:2 respectively at 0°C. This proportion of gases probably existed in the environment at the time of the origin of life. This flask was connected with a smaller flask that was filled with water, with the help of glass tubes. In the bigger flask, two electrodes of tungsten were fitted. Then a current of 60,000 volts was passed through gases containing the bigger flask for seven days. At the end of seven days, when the vapours condensed, a red substance was found in the U-tube. When this red substance was analyzed, it was found to contain amino acids, glycine, and nitrogenous bases which are found in the nucleus of a cell.
Macromolecules which were synthesized abiotically in primitive oceans later came together and formed large colloidal drop-like structures called protobionts (later called coacervates by Oparin, Fox called them microspheres, and Deamer called them vesicles). Each protobiont was a cluster of macromolecules containing proteins, nucleic acids, lipids, polysaccharides, etc. They grew by absorbing molecules from their environment, could divide by budding like bacteria, and many chemical reactions including the decomposition of glucose took place inside the protobionts. The sun provides energy for chemical reactions. According to Oparin, coacervates were the first sole living molecules which gave rise to cells.
Evolution from coacervates to simple cell structures is known as biological evolution. The first living form named protocell originated in the primitive oceans. The protocell was a cluster of nucleoproteins formed by the composition of nucleic acids and enzymatic proteins. Nucleoproteins had the property of self-duplication. Nucleoproteins were the first sign of life. The protocell represented the beginning of life. From protocells or eobionts, few cores of nucleoproteins got separated free in oceans and became inactive, but when they entered another eobiont they became active, so virus-like structures were formed.
Evidences of Evolution
To be COntinued