05 Origin and Evolution of life - part 11- Evidences of organic evolution

05 Origin and Evolution of life - part 11- Evidences of organic evolution

Evidences of organic evolution : 

• The theory of organic evolution states that the present day complex organisms have originated from earlier simpler forms of life. 
• The process of evolution is supported by evidences provided by various branches of biology such as : 

1. Palaeontology
2. comparative anatomy
3. embryology and 
4. molecular biology. 

A. Palaeontology : 

• The study of ancient life with help of fossils is called palaeontology. 
• Fossils are the dead remains of plants and animals that lived in past in various geological layers. 
• The study of fossils provides the most convincing and direct evidence of evolution. 
• Fossils are formed in sedimentary rocks, amber (yellowish fossils resin), ice, peat bogs etc. 
• During fossilization, the primitive forms of organisms occupy the older, lower layers and the advanced forms occupy the upper, more recent layers of the earth. 

Types of fossils : 
1. Actual remains : 

• These are most common type of fossils. 
• The plants, animals and human bodies got embedded in permafrost of arctic or alpine snow remain preserved in the actual state.
• e.g. Wooly Mammoth in Siberia. 
• Amber or hardened resin contains preserved bodies of many insects and arthropods.

2. Moulds : 

• These are the hardened encasements formed in the outer parts of organic remains which later decayed leaving cavities. 
• Body parts of plants or animals later decays but the impression still remains and becomes permanent. 
• For example Foot ­ prints are formed in this manner. 

3. Cast : 

• They are hardened pieces of mineral matterdeposited in the cavities of moulds. 

4. Compressions : 

• Internal structure is absent but a thin carbon film indicates the outline of external features. 

Significance of Palaeontology : 

1. It is useful in reconstruction of phylogeny. 
2. It helps in studying various forms and structures of extinct animals. 
3. It provides record of missing link between two groups of organsims. 
4. It helps in the study of habits of extinct organisms. 
5. Palaeontology provides the following types of evidences

Connecting link (missing link) : 

• These are fossil forms transitional or intermediate between two groups of organisms. 
• It shows some characters to both the groups. 
• Thus it indicate the evolutionary line Seymouria (between amphibians and reptiles). 
• e.g. Archaeopteryx (between reptiles and birds). 

Archaeopteryx lithographica : 

• It is fossilized crow size toothed bird found from jurassic rocks in Germany. 
• It is known as missing link between reptiles and birdsbecause it shows characters of both. 

Reptilian characters : 

• Presence of long tail, claws and scales on the body. 
• Single headed ribs. 
• Abdominal ribs are present which look like ribs of crocodile. 
• Jaws with homodont teeth.
• Sternum without keel. 
• Bones are solid (nonpneumatic). 
• Hind limbs had four clawed digits.

Avian characters : 

1. Feathery exoskeleton. 
2. Forelimbs are modified into wings. 
3. Jaws are modified into beak. 
4. Skull bone is completely fused. 
5. Large rounded cranium. 
6. Cranium with large orbits and a single condyle. 
7. Limb bones are bird like. 
8. Hind limbs with four toes first toe is opposible. 

• Thus from the above study it is very clear that birds evolved from reptiles. Huxley justified this by calling birds as glorified reptiles.

Do you know ? 

1. First fossil of Archaeopteryx was found in Jurassic rocks of Bavaria. 
2. It was discovered in 1861 and preserved in British museum by Andreas Wanger. 
3. The second specimen found in 1877, known asArchaeornis, is kept in the Berlin museum. 

B. Morphology : 

• Morphology deals with study of external structures while, anatomy deals with study of internal structures. 
• From comparative study of morphology and anatomy we can understand the evolutionary aspects in the form of -  

1. Homologous organs
   
2. Analogous organs and 
3. Vestigeal organs. 
4. Molecular Evidences

1.Homologous organs : 

• Homologous organs are those organs, which are structurally similar but perform different functions.
•  For example : 

1. Forelimbs of vertebrates such as lizzard, bird, bat, horse, whale and man,all of them have humerus, radius-ulna, carpals, metacarpals and phalanges in their forelimbs. Forelimbs of these vertebrates are structurally similar but perform different functions. 
2. Vertebrate heart and brain. 
3. In plants, thorns of Bougainvillea and tendrils of cucurbita represent homology. 

• The structural similarities between the homologous organs indicates that they have a common ancestory. 
• Differences in homologous organs are examples of divergent evolution or adaptive radiation. 

2. Analogous organs : 

• Analogous organs are those which are structurally dissimilar but functionally similar. 
• These organs haveexternal superficial similarity due to similar functions but they are different anatomically. 
• For e.g. wings of butterfly (insects) and of birds look superficially alike but they are no anatomically similar structures though they perform similar functions. 
• Other examples of analogous organs. 

1. Eye of the octopus (mollusca) and of mammals.They differ in their retinal position, structure of lens and origin of different eye parts. 
2. The flippers of penguins (birds) and dolphins (mammals). 

• Sweet potato (root modification) and potato (stem modification) store food in form of starch. 
• Analogous organs leads to convergent evolutioni.e. different organisms shows same superficial structural similarities due to similar functions or habitat. 
• These organs do not help to trace the common ancestry. 
• Thus analogous organs do not have significant role in evolution.

3. Vestigeal organs (Rudimentary organs) : 

• Vestigeal organs are imperfectly developed and non-functional, degenerate structures which were functional in some related and other animals or in ancestors. 
• The vestigeal organs are no longer required by the organism but indicate the relationship with those organisms were these organs are fully developed.
•  Examples : Human beings show some vestigeal organs like 

1. Presence of vestigeal nictititating membranes. 
2. Presence of wisdom teeth (third molars). 
3. Coccyx (tail bone) : It is greatly reduced in man since the tail is of no use due to erect posture.
4. Vermiform appendix and the caecum. It is functional in herbivorous mammals for digestion of cellulose. In man due to eating of cooked food it has lost its function. 

• Presence of these vestigeal organs provide evidence that man has (evolved) descended from simple primates.

4. Molecular Evidences : 

1. Cell is the basic structural and functional unit of life in all organisms. 
2. Similarities in proteins and genetic materialperforming a similar function among diverse organisms gives evidence of a common ancestry. 
3. Basic metabolic activities also occur in a similar manner in all organisms. 
4. ATP is the energy source in all living organisms.