Evolution of life on Earth

Evolution of life on earth occurred billion of years ago with simple organic compounds bonding together to form complex molecules that could grow and reproduce. Speculations have been made by scientists for all these years about the origin and evolution of life on Earth. Newer theories have been put forward revaluating the old ones and discarding certain ideas. With the advancement of science, new facts are being discovered and new approaches such as that of molecular biology are being taken up to explain the origin and evolution of life.

Read More: Darwin’s Contribution

Key Terms: Evolution, Panspermia, Abiogenesis, Spontaneous generation, Reducing atmosphere, Prebiotic soup.

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What is Evolution?

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The term evolution may be explained as the development of newer varieties/ types of organisms from pre-existing ones due to certain genetic changes that take place over time. This does not happen overnight but takes a huge length of time to occur.

Read More: Organisms and Population

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Formation of the Earth

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Formation of the Earth was estimated to be about 4.5 billion years ago in the form of a hot mass of molten rock by the accretion of the solar nebula. As the earth cooled, the water vapour present in the atmosphere condensed into a liquid form that accumulated on earth as chemically rich oceans. 

The Big Bang Theory

Big Bang theory is the most consistent with the observation of the past and present universe. According to this theory, all the materials of this universe have been created by a huge explosion and then rapid cooling of the superheated ejected bodies.

Early Earth Conditions

Early earth conditions were as follows: 

1. Immense heat due to volcanoes and accretion
2. There was a bombardment of asteroids and the formation of the Moon.
3. Gradual cooling caused the condensation of vapour to rain. From volcanoes, H₂, N₂ and CO₂ gases were released. These comprised the early Earth’s atmosphere which is devoid of free molecular O₂.

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Origin of Life

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There is various theories regarding the origin of life. Some of them are as follows: 

Theory of Panspermia

Svante Arrhenius, a Swedish scientist, put forward this theory. According to this theory, life has originated in space in the form of certain biochemical compounds or microbes and had been brought to earth by comets and meteorites. However, this theory never became popular due to a lack of strong evidence such as the absence of life forms on nearby planets and lethal radiations that are present in outer space.

Theory of Abiogenesis

Abiogenesis refers to the generation of life from non-living matters. Abiogenesis is also known as the theory of Spontaneous Generation. According to this theory, living organism originates from non-living, decaying organic matter e.g. maggots are seen to appear in uncovered meat. Greek philosopher Aristotle believed in this theory.

However, scientists like Francesco Redi, Lazzaro Spallanzani, and Louis Pasteur, disproved the theory.

Read More: Test For Protein

Redi’s Experiment: 

Francesco Redi, in 1668, performed an experiment to check if the maggots came from decaying meat. He placed pieces of meat in a number of jars, covering some and keeping a few uncovered. Only the uncovered jars showed the development of maggots in meat. From this, it was concluded that maggots were not generated from meat, but from eggs that fly had laid on the pieces of uncovered meat. Since flies could not land on the covered pieces of meat, hence, no maggots were seen. Therefore, he concluded that decaying meat did not generate maggots.

Redi’s experiment to disprove the theory of spontaneous generation 

Spallanzani Experiment: 

An Italian naturalist, Lazzaro Spallanzani, performed an experiment, similar to the above, but with broth. He put the broth into two flasks and boiled them in order to sterilize them. Then one of the flasks was sealed and the other was left open to the air. Growth of micro-organisms was seen in the unsealed flask and this made him conclude that micro-organisms did not come from the broth but were already present in the air that entered the flask. 

Spallanzani experiment to disprove the theory of the spontaneous generation

Read More: Air Composition 

Pasteur Experiment: 

A French chemist, Louis Pasteur, performed the same experiment using a straight-necked flask and an S-shaped-necked flask. He allowed air to enter the sterilized broth in both flasks. However, only air could enter the S-shaped neck while the straight-neck flask permitted the entry of both air and micro-organisms. Contamination was seen in the broth of the straight-necked flask. Louis Pasteur concluded that even though the air had entered the S-necked flask, the broth was not contaminated with the growth of microbes.

This, finally convinced the other scientists, that any living matter does not originate from non-living things.

Pasteur experiment to disprove the theory of spontaneous generation

Theory of Biogenesis

Theory of Biogenesis was put forward by Louis Pasteur. This theory states that living things only come from living things, i.e. life originates only from pre-existing life forms.

Theory of Chemical Evolution

Theory of chemical evolution was proposed by A.I. Oparin and J.B.S. Haldane which led to the current understanding of the prebiotic earth and the gradual chemical evolution towards life. This theory suggested that early Earth had a strong reducing atmosphere (free molecular oxygen was absent), under this condition, inorganic molecules would form small and simple organic molecules like simple sugars and amino acids. This, as they referred to as the primordial soup.

The 4 steps of chemical evolution theory are:

• The first formed molecules were small carbon compounds like formaldehyde (HCHO) and hydrogen cyanide (HCN).
• These smaller molecules react to form simple sugars, amino acids, different nitrogenous bases and the prebiotic soup.
• Small molecules of this prebiotic soup link together to form proteins and nucleic acids (DNA/RNA).
• Among these, a single molecule attained the ability to self-replicate, becoming the first living organism thereby ending the chemical evolution and starting biological evolution.

Oparin suggested that these organic compounds could have undergone several reactions to form more complex molecules. And these complex molecules in turn formed colloid aggregates or coacervates in an aqueous environment. The coacervates are considered to have taken part in the evolutionary process to form the first living beings.

Read More: Structure of DNA

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Miller- Urey experiment

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S. Miller, a student of H.C. Urey had set up an apparatus for stimulating early earth conditions at the time of the evolution of organic substances.

Miller - Urey experiment’s design consisted of the atmospheric conditions of the primitive atmosphere of Earth in a closed chamber. To simulate lightning discharges of the primitive earth, a spark chamber was constructed with two electrodes of 75000V of energy.

A mixture of methane, ammonia, hydrogen and water vapour was introduced. A flask for boiling at a temperature of 800˚C and a condenser (for simulating rain) was also constructed in the apparatus. 

After 18 days, it was observed that biomolecules like amino acids, hydroxyl acids, urea, etc. had accumulated in the flask.

This experiment, thus, demonstrated how biological molecules have arisen abiotically under the conditions of the earth’s primitive atmosphere.

Miller - Urey Experiment

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Things to remember

• Evolution is the development of newer varieties/ types of organisms from pre-existing life forms over the course of time.
• It is estimated that the Earth was formed 4.5 billion years ago. The Big Bang theory suggests the formation of the earth.
• The Theory of Spontaneous generation was discarded by Francesco Redi, Lazzaro Spallanzani and Louis Pasteur.
• Oparin and Haldane suggested the homogeny theory of the origin of life.
• The early Earth had a strong reducing atmosphere devoid of free molecular oxygen.
• The early Earth also did not have the ozone layer for protection from UV radiations; the ozone layer is believed to have formed nearly 600 million years ago.
• Miller and Urey conducted an experiment to support Oparin and Haldane’s theory.