Early Modes of Evolution – Explanation

Key Concepts

• Evolution

• Theory of special creation

• Theory of panspermia

• Theory of spontaneous generation

• Theory of biogenesis

• Chemical evolution of life

• Lamarck’s hypothesis of acquired characteristics

• Darwinism

Introduction

Evolution is defined as change over time. New age scientists refer to evolution as genetic change in a population over time. Evolution states that various types of plants, animals, and other living organisms on Earth have their origin in the pre-existing life-forms. The distinguishable differences are due to modifications or adaptations in successive generations.  

The theory of evolution is one of the fundamental keystones of modern biological theory.

Explanation

Living organisms are constantly faced with challenges in their environment, such as severe weather, drought, famine, and competition for food and space. Not all living organisms may overcome these struggles. Animals that survive have the opportunity to reproduce and pass along the traits to their offspring that helped them survive.  

The diversity within species and the interactions of organisms with their environment help explain how populations can change over time and why some organisms survive while others become extinct.  

Millions of species on Earth today are only a small fraction of the species that have ever lived. In fact, it is estimated that 99 % of all species that have ever lived are now extinct. While some of the fossilized animals are ancestors of animals that are common today, others have long been extinct and are unlike anything in our modern oceans. 

Early ideas about evolution: 

Ideas revolving around natural selection and evolution began to be discussed in earnest in the early nineteenth century. Whenever the theory of evolution is mentioned, the name Charles Darwin often comes up. However, the work and ideas of many others has helped in shaping our current understanding of evolution.  

Also, as our technological and scientific techniques improve and our knowledge of the principles of evolution grows, our understanding of the processes of evolution will also improve. 

Charles Darwin was not the first person who thought about evolution. Several philosophers had put forward their theories of evolution before Darwin. 

In western culture, Plato and Aristotle were the most influential philosophers, but they did not support ideas that organisms could change. For example, Aristotle thought that all organisms that ever would exist were already created. He also believed that these organisms were permanent and perfect and would not change. 

Theory of special creation: 

It is the oldest theory and is based on religious belief. It is not backed by any scientific evidence. The theory stated that life was created by a supernatural power. It was proposed by Father Suarez as he was a strong believer in supernatural powers and believed in this theory. 

Cosmozoic theory/Theory of panspermia: 

This theory states that life did not arise on planet Earth. It was proposed by Helmholtz (1884). 

He said that micro-organisms or spores called cosmozoa/panspermia from space came on Earth along with meteorites and comets, and then evolved into higher organisms in the water. 

Theory of spontaneous generation (abiogenesis): 

This theory stated that life originated from non-living matter spontaneously. Aristotle proposed that life arose from non-living material if the material contained pneuma, i.e., vital heat.  

Many scientists and philosophers supported this theory. Later, the theory was disapproved by Louis Pasteur. 

Theory of biogenesis: 

This theory states that living organisms are always produced from pre-existing living forms by a process called reproduction. The Theory of biogenesis, however, could not explain the origin of the first life on earth but could explain only the continuity of life. 

Chemical evolution of life: 

This theory suggests that life originated on earth by combinations of various chemicals through constant chemical reactions over a long period. This theory is also called the self-assembly theory of the origin of life or the biochemical origin of life.  

This theory was first hypothesized by Haeckel but later developed by the Russian scientist Alexander I. Oparin (1924) and British biologist J. B. S. Haldane (1929). 

The process of chemical evolution can be divided into the following steps: 

1.  Origin of Earth and Primitive atmosphere:  

The Big-Bang theory of Georges Lemaitre explained the origin of the Universe. As per this theory, the Universe originated about 20 billion years ago by a single huge titanic explosion.  

As the Universe expanded, the temperature decreased, and various galaxies of solid objects were formed. Milky Way is one such galaxy, and Earth is one of the planets of the solar system, which originated about 4.6 billion years ago.  

When formed, it was a rotating cloud of hot gases and cosmic dust called Nebula. The condensation and cooling resulted in stratification, and the heavier elements like nickel and iron passed to the core, and lighter ones like helium, hydrogen, nitrogen, oxygen, carbon, etc., remained on the surface. They formed the atmosphere of the Earth. The primitive atmosphere of the Earth was quite different from the present one, and it was of a reducing type, devoid of free oxygen. 

2. Formation of ammonia, water, and methane: 

As the primitive atmosphere began to cool down, the lighter elements started reacting with each other. The early atmosphere was abundant in hydrogen, carbon, nitrogen, and sulfur, of which hydrogen reacted with other elements to form chemicals like CH₄, NH₃, H₂O, and H₂S.  

3. Formation of simple organic molecules: 

As the temperature of the earth decreased, the steam condensed into water and resulted in heavy rainfall, which gradually cooled the Earth. Rainwater got accumulated on the land to form rivers, streams, lakes, seas, and oceans.  

At that time, the atmosphere did not contain an ozone layer, and thus ultra-violet radiations reached the surface of the earth directly. Under the influence of available energy sources such as UV rays, radiations, lightning, and volcanic activities, the early molecules of hydrocarbons, ammonia, methane, and water underwent several reactions like condensation, polymerization, oxidation, and reduction. These reactions resulted in the formation of simple molecules like monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol, etc. and accumulated at the bottom of water bodies. Haldane described it as the ‘hot dilute soup’ or ‘primitive broth.’ It did not show any degradation due to the absence of free oxygen and enzymes. 

4. Formation of complex organic molecules: 

The primitive broth was neutral and oxygen-free. Simple organic molecules aggregated to form new complex organic molecules like polysaccharides, fats, proteins, nucleosides, and nucleotides due to polymerization. Polymerization of amino acids formed protoproteins which later formed proteins. The formation of protein molecules is considered a landmark in the origin of life. Proteins (enzymes) accelerated the rate of other chemical reactions. 

5. Formation of Nucleic acids:  

Nucleotides may have formed due to the reaction between phosphoric acid, sugar, and nitrogenous bases. Many nucleotides join together to form nucleic acids (RNA, DNA). Nucleic acids acquired self-replicating ability, a fundamental property of the living form. 

6. Formation of Protobionts or Procells: 

Protobionts are the first form of life. They are prebiotic chemical aggregates having some properties of the living system. They have formed due to coacervation, i.e., aggregation of organic molecules. Oparin (1924) called them coacervates, and Sidney Fox called proteinoids or microspheres. 

7. Formation of first cell:  

When RNA or DNA systems evolved within protocells, they resembled bacteria or viruses. They controlled various metabolic activities. The first cell was anaerobic, heterotrophic, and obtained energy by chemoheterotrophic processes.  

Experimental evidence of chemical evolution: 

Stanley Miller and his teacher Harold Urey provided the first experimental evidence in support of chemical evolution theory of Oparin in 1953.  

They designed a glass apparatus which was called spark-discharge apparatus. The apparatus was first sterilized and then evacuated. Gases such as methane, ammonia and hydrogen were pumped in the proportion of 1:2:2 into the glass chamber. A tube carrying water vapor was also connected to the chamber. The lightning effect was stimulated by electric discharge carbon arc spark in the chamber.  

Evaporation and precipitation processes were also stimulated by using the heating mantle and condenser, respectively. The mixture of CH₄, NH₃, H₂ was continuously exposed to electric discharge for several days causing the gases to interact and then condense. The liquid that was collected in the U-tube turned brown. When this liquid was analyzed, it reported the presence of simple organic compounds. (Urea, amino acids, lactic acid, etc.). This experiment provides evidence that the simple molecules present in the earth’s early atmosphere combined to form the organic building blocks of life. 

Organic evolution: 

Organic evolution is described as gradual, continuous, and irreversible changes through which the present-day complex forms of life evolved from their simple pre-existing forms. Approximately 50 years before Darwin published his ideas on evolution, a French naturalist Lamarck gave his theory of inheritance of acquired characters. 

Hypothesis of acquired characteristics: 

In 1809, Jean Baptiste de Lamarck proposed a hypothesis to explain how species change over time. Lamarck believed that microscopic organisms appear spontaneously from inanimate materials and evolve gradually into more complex forms. He also thought that creatures could develop new organs or change the structure and function of old ones due to their use or disuse. 

He suggested that characteristics, or traits, developed during a parent organism’s lifetime are inherited by its offspring. His hypothesis is called the inheritance of acquired characteristics. 

For example, Lamarck believed that giraffes evolved their long necks by stretching farther each generation to reach the leaves in the trees and that this change in body shape was then inherited. 

Objections to Lamarck’s theory: 

Scientists collected data on traits that are passed from parents to offspring.  

The data showed that traits that are developed during a parent’s lifetime, such as large muscles built by hard work or exercise, are not passed on to offspring. The evidence did not support Lamarck’s hypothesis. 

Darwinism: 

Charles Darwin was a naturalist who travelled extensively. Based on his collections of living and fossil material he published a book ‘‘The origin of species by Natural Selection’’ in 1859.  

Darwinism is based on 5 main postulates: 

1. Overproduction 

2. Struggle for existence 

3. Variations 

4. Natural selection 

5. Origin of new species 

Summary

• Evolution is defined as change over time. New age scientists refer to evolution as genetic change in a population over time.

• The theory of special creation states that life was created by a supernatural power.

• Theory of panspermia states that life did not arise on planet Earth. Micro-organisms or spores called cosmozoa/panspermia from space came on Earth along with meteorites and comets, and then evolved into higher organisms in the water.

• The theory of spontaneous generation states that life originated from non-living matter spontaneously.

• The theory of biogenesis states that living organisms are always produced from pre-existing living forms by a process called reproduction.

• Chemical evolution theory suggests that life originated on earth by combinations of various chemicals through constant chemical reactions over a long period.

• In 1953, Stanley Miller and Harold Urey gave the first experimental evidence in support of Oparin’s chemical evolution theory called the “Urey-miller experiment.”

• Organic evolution is described as gradual, continuous, and irreversible changes through which the present-day complex forms of life evolved from their simple pre-existing forms.

• Lamarckism suggested that characteristics, or traits, developed during a parent organism’s lifetime are inherited by its offspring.