Origin and Evolution of Life

The term "evolution" refers to the process of unfolding or revealing hidden potentialities. It signifies an orderly transition from one state to another. 

This concept is best illustrated by Darwin's idea of "Descent with modification."

1.0What is Life the Origin of Life?

To study the history of life forms, including the changes in flora and fauna over millions of years on Earth, we must also understand the context of life's origin. This involves exploring the evolution of Earth, stars, and the universe itself.

2.0The Beginning of Life and Evolution- Big Bang Theory- Proposed by Abbe Lemaitre

• This  Theory is the leading cosmological model for explaining the origin and evolution of the universe. 
• This theory posits that the universe started as an incredibly hot and dense singularity around 13.8 billion years ago and has been expanding ever since. It is supported by several lines of evidence, including the redshift observed in distant galaxies, the cosmic microwave background radiation, and the abundance of light elements. 
• The Big Bang Theory offers a comprehensive framework for understanding the universe's large-scale structure and its development over time.

3.0Theories of Origin and Evolution of Life -Theory of Special Creation

• The greatest supporter of this theory was Father Suarez. 
• This is a mythology-based theory. 
• This theory has three connotations: All living organisms (Species or types) we see today were created as such. 
• Diversity has always been the same since creation and will be the same in future. 
• The Earth is about 4000 years old. 
• All these ideas were strongly challenged during the nineteenth century based on the observations of Charles Darwin, Wallace, etc. They believed that life forms varied over time.

Cosmic Panspermia Theory 

• Some scientists propose that life originated from outer space. Ancient Greek philosophers speculated that life forms, referred to as spores, were transported between planets, including Earth.
• This concept, known as 'panspermia,' remains a popular hypothesis among some astronomers.

Theory of Spontaneous Generation (Abiogenesis/Autogenesis) 

• For a long time, it was also believed that life spontaneously emerged from decaying and rotting matter like straw, mud, etc. 
• Ancient Greek philosophers supported this hypothesis. They believed that the mud of the River Nile could give rise to fish, frogs, crocodiles, etc. when warmed by light rays. 
• By careful experimentation, Louis Pasteur demonstrated that life comes only from preexisting life and not from dead matter.

Theory of Biogenesis 

• Harvey and Huxley proposed the concept of "Omnis vivum ex ovo or vivo," which means "New life can originate from preexisting life." 
• Louis Pasteur's experiment supported the theory of biogenesis and disproved abiogenesis. 
• However, this theory did not answer how the first life-form came on Earth.

Evidence in Favour of Chemical Evolution

• Harold Urey & Stanley Miller Experiment
• In 1953, S.L. Miller, an American scientist, created similar conditions at a laboratory scale, which were thought to be primitive on Earth. 
• He took CH4, NH3, H2 and water vapour at 800°C in a large flask (spark flask). 
• He created electric discharge by using two tungsten electrodes to raise the temperature inside the spark flask to 800° C.
• He observed the formation of simple amino acids like glycine, alanine, and aspartic acid.
• In similar experiments, other scientists observed the formation of sugars, nitrogen bases, pigments, and fats. 

Evidences for Evolution

• Ernst Haeckel proposed embryological support for evolution, observing that certain features appearing during the embryonic stage are common to all vertebrates but absent in adults. 
• For instance, embryos of all vertebrates, including humans, develop a row of vestigial gill slits just behind the head, which functions as an organ only in fish and is not found in any other adult vertebrates. 
• However, Karl Ernst von Baer later disapproved of this proposal, noting that embryos do not pass through the adult stages of other animals.

Palaeontological Evidences

• Palaeontological evidence refers to the scientific study of fossils to understand the history of life on Earth.
• Fossils, which are preserved remains or traces of organisms from the past, provide crucial information about the morphology, behaviour, and evolution of extinct species. 
• By analyzing fossil records, scientists can reconstruct organisms' evolutionary lineages, identify transitional forms, and study changes in biodiversity and environmental conditions over geological time. 
• Palaeontological evidence thus plays a vital role in supporting the theory of evolution and understanding the biological and ecological history of our planet.

Evidences From Connecting Links- Archaeopteryx

• There needs to be a link between reptiles and birds. 
• The connecting links that are not found at present are called missing links. 
• It was found in the rocks of the Jurassic period. 

Homology

• Organs with a common origin, embryonic development, and the same fundamental structure but perform similar or different functions are called Homologous organs, and this phenomenon is called Homology. 

Adaptive Radiation/Adaptive Divergence

• Adaptive radiation is the evolutionary process by which a single ancestor species rapidly diversifies into a wide variety of forms, each adapted to different environments and ecological niches. This usually occurs when species colonize new habitats or face significant environmental changes.This phenomenon is an example of homology based on divergent evolution.
• One famous example of adaptive radiation is Darwin's finches. During his voyage to the Galapagos Islands, Charles Darwin observed a remarkable variety of species. Among these, he was particularly fascinated by a group of small black birds, which were later named Darwin's finches. These birds showcased a wide range of adaptations, demonstrating the concept of adaptive radiation in action.

Convergent evolution or Adaptive convergence

• When multiple adaptive radiations occur within an isolated geographical region, each corresponding to different habitats, this phenomenon can be termed convergent evolution.

Analogy 

• Similar environments have led to the evolution of similar adaptive traits in different groups of organisms, resulting in analogous structures. 
• These structures, known as analogous organs, originate from different ancestral lineages and fundamental structures but perform similar functions. This phenomenon is referred to as analogy.

Convergent evolution or Adaptive convergence

• When multiple instances of adaptive radiation occur in a distinct geographical region, each targeting different habitats, this phenomenon can be described as convergent evolution. 
• In Australia, placental mammals have undergone adaptive radiation, evolving into diverse forms that mirror their marsupial counterparts. For example, the placental wolf and the Tasmanian wolf (a marsupial) exhibit remarkable similarities.

Lamarackism/ Theory of Inheritance of Acquired Characters

• Internal vital forces: Due to some internal vital forces, all organisms tend to increase in size of their organs or entire body.  
• Impact of Environment and Emerging Needs: The environment affects all organisms. Changes in the environment create new needs, which in turn lead to the development of new structures (based on the doctrine of desire/appetency) and alterations in an organism's behavior.
• Usage and Disuse of Organs: Organs that are consistently used over many generations become more developed, while those that are not used may degenerate into vestigial organs.
• Inheritance of Acquired Traits: New traits can develop in an organism due to internal vital forces, environmental influences, new needs, or the use and disuse of organs. These acquired traits can be inherited by subsequent generations.
• Accumulation and Speciation: Over multiple generations, the accumulation of these variations can lead to the emergence of new species.

Darwinism/Theory of Natural Selection

• Darwin traveled by H.M.S. Beagle ship through S. America, S. Africa, Australia & Galapagos Islands. 
• Two books influenced Darwin- 
• (i) "Principles of population" of Malthus 
• (ii) "Principles of Geology" by Charles Lyell 
• Alfred Wallace, a naturalist who worked in the Malay Archipelago, had also reached similar conclusions around the same time. He sent his findings to Darwin in the form of a chart. 
• Darwin later explained this theory in his book, 'On the Origin of Species by Means of Natural Selection'.

Genetic Drift (Sewall Wright effect) 

• Genetic drift is a random change of gene/allelic frequencies in a population merely by chance. 
• It operates rapidly in a small population.
• The founder effect and bottleneck effect are two forms of genetic drift.

 Hardy- Weinberg Principle 

• One can find the frequency of alleles of a gene or a locus in a given population. This frequency should remain fixed and even the same through generations. 
• This principle says that allele frequencies in a large, randomly mating & non-evolving population are stable and remain constant from generation to generation. 
• The gene pool (total genes and their alleles in a population) remains constant. 
• This is called genetic equilibrium. The total of all the allelic frequencies is 1.

Reproductive Isolation

• It prevents interbreeding between the populations of two different or closely related species. 
• It preserves the traits of the species but has the potential to give rise to new species.

4.0Origin and Evolution of Man

Human is a member of order Primata of class Mammalia. 

First real primate ancestors were tree shrews. 

5.0Origin And Evolution NEET Questions

Q.1 Which of the following is a vestigial organ in humans?

(1) Appendix
(2) Liver
(3) Heart
(4) Lungs

Answer: (1) Appendix

Q.2 Which one of the following was the first human-like hominid?

1) Australopithecus

2) Homo habilis

3) Neanderthal man

4) Homo erectus

Answer: 2) Homo habilis

Q.3 Which of the following evidence supports the theory of evolution?

(1) Fossil records

(2) Comparative anatomy

(3) Molecular biology

(4) All of the above

Answer: (4) All of the above

Q.4 The concept of "survival of the fittest" was proposed by:

(1) Charles Darwin

(2) Alfred Russel Wallace

(3) Jean-Baptiste Lamarck

(4) Gregor Mendel

Answer: (1) Charles Darwin

Q.5 Which era is known as the "Age of Reptiles"?

(1) Cenozoic

(2) Mesozoic

(3) Paleozoic

(4) Proterozoic

Answer:( 2) Mesozoic

Q.6 Which of the following is an example of convergent evolution?

(1) Wings of butterflies and birds

(2) Flippers of dolphins and fins of fish

(3) Eyes of octopus and mammals

(4) All of the above

Answer: (4) All of the above

Q.7 The homologous organs are evidence of:

(1) Convergent evolution

(2) Divergent evolution

(3) Adaptive radiation

(4) Artificial selection

Answer: (2) Divergent evolution

Q.8 Which of the following is considered the oldest known fossil?

(1) Cyanobacteria

(2) Trilobites

(3) Archaeopteryx

(4) Stromatolites

Answer: (4) Stromatolites