Evidences of Organic Evolution

Last Updated on Friday, 10 February 2012 11:26 Written by Sandesh Monday, 23 January 2012 04:17

EVIDENCES IN FAVOUR OF ORGANIC EVOLUTION:

1. EVIDENCES FROM MORPHOLOGY AND COMPARATIVE ANATOMY

2. EVIDENCES FROM PALEONTOLOGY

3. EVIDENCES FROM BIOCHEMISTRY AND COMPARATIVE SEROLOGY

4. EVIDENCES FROM EMBRYOLOGY

5. EVIDENCES FROM GENETICS

6. EVIDENCES FROM GEOGRAPHICAL DISTRIBUTION OF ANIMALS

1. EVIDENCES FROM MORPHOLOGY AND COMPARATIVE ANATOMY:

The study of morphology and anatomy of plants and animals suggests that their each group possess their own pattern of organization but they possess a common set of characteristics.

1. Homologous organs: Organs which have similar structure and embryonic origin but are different in function are known as homologous organs and the phenomenon is called homology. This is due to their common ancestry. Homology is exhibited by every organ system of vertebrates from lowest to highest including man.

The limbs of all vertebrates have a common structural plan. For example forelimbs of variety of vertebrates like whale (flippers), bat (patagia), birds (wings), horse (legs), man (hands). The forelimb of man, used for handling things is similar in structure to those of frog which is used for hopping, similar to those of rabbit which is used for leaping, flippers of whale or seal which are used for swimming, wings of bat and bird used for flying. Though forelimbs in these vertebrates are modified for a variety of uses they are constructed on same fundamental plan, the pentadactlye plan. The skeleton of forelimb of all the vertebrates contains humerus, radio-ulna, carpals, metacarpals and phalanges. The existence of homologous anatomical organs structures implies a common evolutionary origin of amphibians, reptiles, birds and mammals.

1. Analogous organs: Certain organs which perform similar functions but are different in development and origin are called analogous organs and the phenomenon is called analogy. For example the wing of an insect, the wing of a bat and the wing of bird serve the same purpose of flying but their basic structure is totally different. The wings of bat and birds are in fact modified forms of forelegs and internally have the bones, nerves and blood vessels. While the wings of insects are membranous, made up of thin flap of chitin and supported by hollow tubes called veins.

Analogous organs show the convergent evolution. This means that they evolved from different ancestry but developed similar characteristics when put in similar habitats.

1. Vestigial organs: Vestigeal organs are those organs which are rudimentary and non-functional in one group of animals while same organs are well developed and functional in another group of animals. Presence of vestigial organ is a vital evidence of organic evolution. From the evolutionary point of view, vestigial organs are structures that were well- developed, functional and necessary in ancestors but are now in process of disappearance.

Human beings are considered as the walking museum of vestigial organs because they have more than 100 such vestigial organs. A few of them are as follows:

• Vermiform appendix- the vermiform appendix found at the junction of small intestine and the large intestine of man is a vestigial organ. It is slender about 2.5 inches long serving no useful function but often is the site of infection requiring surgical removal. But in other mammals like rabbit it is well developed functional part of digestive system for cellulose digestion.

• Plica semilunaris- In the inner angle of the eye of man there is a whitish semilunar membrane called plica semilunaris. It corresponds to the nectitaing membrane but is completely unstretchable. It is well-developed and protects the eye in frogs but has no apparent function in man.

• Coccyx: In human tail is absent but he still carry 3 to 5 fused tailbone vertebrae.
• Ear muscles: In human ear muscles are rudimentary and immovable. While in other mammals it is used to move external ear.
• Wisdom teeth: This is third molar teeth supposed to be vestigial organ.
• Body hair: In human body hair is vestigial organ which has no real use like in other mammals.

2. EVIDENCES FROM PALAEONTOLOGY

Palaeontology deals with the study of fossils. Leonardo da Vinci (1452-1519) is regarded as the father of Palaeontology. The branch of Palaeontology which deals with plant fossil is known as Palaeobotany where as the branch dealing with animal fossil is known as Palaeozoology. Palaeontology links biology with geology.

What are fossils?

Usually plants and animals after death are decomposed by bacteria. But sometimes they are preserved as fossils.

Fossils are any form of preserved remains or traces, thought to be derived from living organisms. They provide concrete proof to the fact that variety of animals and plants had lived in various geological ages of earth.

How are they formed?

Process of formation of fossil is known as fossilization. All the organisms which die do not become fossils because most of them are rapidly decomposed. The best place for fossilization is the ocean because salt water checks the decay of organisms. In most cases, soft parts are not preserved and only hard parts such as bones, teeth, shells, woody parts of plants are preserved.

Fossils are found mostly in sedimentary rocks. The sedimentary rocks are formed due to slow settling down of silt, mud or volcanic ash in rivers, lakes and sea. As the particle settle layer after layer each layer is called stratum. The lowest layer which was deposited first contains the fossil of primitive organisms where as those lying at top layers deposited recently contains the fossils of much complex and advanced plants and animals. Thus fossil records suggest that modern day plants and animals have evolved from much simpler forms of past.

Types of fossils:

1. Dead and preserved bodies: In this type of fossil, entire organism gets preserved including original tissue. The preserving material can be ice, amber, tar, oil seeps etc. Eg. Insects trapped in tree resin (amber), Baby mammoth found in Siberia (10,000 years old) etc.
2. Petrified fossils: In this type of fossil the preserved parts of animals become hard as they get replaced by mineral substances such as iron, calcium carbonate, silica etc. As a result the preserved material turns into hard stone. This process of mineralization is known as petrifaction. Eg. Bones, skull, shells or tree turnks (of Petrified Forest national park of Arizona, USA) etc.

1. Moulds and casts: A mould is an impression of a complete organism or a part of it in the rock that surrounds it. Animals or plants are surrounded in mud or clay to leave impressions. The mud hardens and the organic matter disintegrates. This leaves the impression which is exact replica of the organism. For example, fossils of coelenterates and mollusks shells, Saligram in kaligandaki etc.
2. When moulds get filled with minerals they are known as casts. They form replicas of buried organisms. Eg mollusks.
3. Trails and footprints: Fossils in form of foot-prints, tracks, tunnels of different organisms in mud. When animal walk in soft sediment like mud, their feet or tail leaves impressions which may be harden and preserved. For eg. Foot prints of dinosaurs.
4. Compression and carbonization: Oils in the plant’s cells are leached out and the remaining matter is reduced to a carbon film. Plants have an inner structure of rigid organic walls that may be preserved in this manner, revealing the framework of the original cells. Plants are mostly fossilized through carbonization. For example:  Coal.
5. Coprolites: fossilized fecal matter

Pseudo fossils: Sometimes rocks present structures resembling plant remains. Such structures present in rocks are normally products of minerals in rocks crevices. In fact mineral substances crystallize and develop patterns that resemble the outline of leaves. Such structures are called pseudofossils or false fossils.

Examples of fossils:

Dinosaurs: They existed on earth some 200 million years ago  in the Jurassic and Triassic periods. The dinosaurs known as Tyrannosaurs were the largest living carnivore ever. It stood 20ft tall and was about 50 ft in length. It had dagger like teeth measuring about six inches.

Archaeopteryx: Commonly known as reptile-bird, was a primitive bird-like form. Its fossil was found in Germany in the rocks of Jurassic period, of about 150 million years ago. It was about the size of a crow and represented both the characters of birds and reptiles.

• Like birds it had wings with feathers but, wings had clawed digits like in reptiles.
• It also had beak like in birds. But, beaks had conical teeth like in reptiles.
• It had feathery tail like in birds. But, Tail was long like lizards and had tail vertebrae.
• Body was covered with feathers like in birds but also had scales like in reptiles.
• It skeletal framework resembled more with reptiles than the birds.

It is probable that it used its forelimb for flying as well as climbing. Thus Archaeopteryx provides a connecting link between reptile and birds.

Evolution of horse: The history of evolution dates back some 60 m.y.a. in Eocene epoch and involves about 20 genera. The phylogeny of horse starts with Eohippus, a small fox like creature with longer head. It had shorter legs with four toes on each front foot and three on each hind foot. It was a forest dweller, feeding upon soft vegetations. It passed through evolutionary line to give rise to modern day horse.

Major changes seen in modern horse are:

• Enlargement and elongation of third digit
• Loss of other digits
• Elongation of fore part of skull
• Lengthening of limbs
• Increase in the length and mobility of neck
• Development of premolars and molars into high crowned
• Increase in body size

Geological time scale:

Method of determining the age of fossil:

The age of the fossil can be determined by determining the age of the rock in which the fossil was found. The age of a rock in years is called its absolute age. Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock. When rocks are formed, small amounts of radioactive elements usually get included. Radioactive elements are unstable and are gradually changing into stable compounds in a fixed rate. As time passes, the “parent” radioactive elements change at a regular rate into non-radioactive “daughter” elements. For example, Uranium, thorium, Potassium, Carbon etc are radioactive substances.

Process of determining age of rock is called as radioactive disintegration method. For example, 74,00 million grams of Uranium produce 1 gm of Lead per year. Therefore, the age of the rock containing uranium and lead can be calculated as:

Age of the rock = Wt. of lead/ Wt. of Uranium X 74, 00 million years

3. EVIDENCE FROM BIOCHEMISTRY AND SEROLOGY:

Occurrence of similar molecules in a complete range of organisms suggests the existence of biochemistry homology.

i. Chemical composition: C,H,O,and N are the basic elements which form organic compound. In plants and animals, these combine to form carbohydrate, fats and protein. Protoplasm in all living organism is similar as they have same composition.

ii. Protoplasm Chemistry: biochemical study of protoplasm suggests that it has the similar chemical composition in various animals and plants. They contain substances like protein, carbohydrate, fats etc. This supports the evolutionary development has occurred in all organisms.

iii. Chromosome chemistry: another remarkable similarity in all living organisms are found in chromosome chemistry. All chroomosomes are made up of DNA and RNA. This suggest the common origin of all living organisms.

iv. Enzymes: a large number of enzymes are same in all animals. For example. Trypsin, a protein digesting enzyme is present from protozoan to mammals. So this enzyme is called as ancient enzyme. Similarly, amylase is found from poriferons to mammals.

v. Hormones: Various hormones like thyroxine from thyroid gland is common in all vertebrates. For the treatment of thyroid deficiency in human beef thyroid has been successfully used.

vi. Comparative serology (Precipitation test): Precipitation tests are used to confirm and determine the natural relationship due to similarity of blood protein. The degree of similarity between plasma proteins of various animals can be tested by antigen-antibody technique.

Protein molecules present in serum act as antigens. When it is injected into the blood of another animal that lack these proteins, this causes the animal to produce antibodies against them.  This results in antigen-antibody reaction. In this experiment by Dr. George Nuttal, human serum was injected into rabbit; antibodies against human are produced in the rabbit’s body. After few days blood from rabbit was drained. This serum is called as anti-human serum. If it is mixed with human serum, a white precipitate is formed. It was also mixed with blood of dog, cats, primates (apes, monkeys, gorilla etc.). With the blood of dogs and cats precipitation was not formed. But, less precipitation was formed with blood of primates. The relative degree of precipitation of serum gives an idea about closeness between two different organisms. This test shows the relationship between man and other primates and not with the dogs and cats.

4. EVIDENCE FROM EMBRYOLOGY:

Embryology is the science that deals with the study of the development of embryo. A comparative study of embryology of different groups of animals reveals certain features which provide evidence for organic evolution. This feature has been described by Earnest Haeckel as Recaptulation theory. This theory states that ontogeny recapitulates phylogeny. Ontogeny is the individual history or the development of an individual. However, phylogeny is the ancestral history, which is the series of stages during development of a species. It means that during the embryological development (ontogeny) of an individual, the characters of the race (Phylogeny) are repeated.

All multicellular organisms exhibit common pattern of development. Comparative study of embryological development of different vertebrate like fish, frog, lizard, chick, rabbit, human etc. shows similarity in early stages. For example:

• All vertebrates begin their life history as a single celled zygote.
• The zygote in all organisms undergoes series of division to produce hollow ball of cells called blastula.
• Blastula further develops into gastrula.
• Gastrula in all the vertebrates has the same three primary germ layers, namely ectoderm, mesoderm and endoderm.
• The young ones of fishes resemble very closely to the tadpole larvae of amphibians. This shows the origin of amphibians from fish-like ancestors.
• The embryos of all the vertebrates at one stage or the other possess pharyngeal gill slits, two chambered heart, tail etc. Even human embryo at eight weeks develops the gill slits and tail. But as the development proceeds, gill clefts persist only in fishes and get closed in other vertebrates.
• The heart remains two chambered as in adult fish, but it becomes three chambered in amphibians and most reptiles and four chambered in crocodiles, birds and mammals.
  
  

5. EVIDENCE FROM GENETICS:

Genetics is the branch of biology which deals with the study of genes and hereditary characters. Hereditary change in animals are due to mutation and recombination of genes. Mutation is sudden change in gene which create new genetic material. This is the raw material for evolution. While recombination is the formation of new combination of genes by hybridization. This also causes evolution. For example mule is the artificial hybrid of horse and ass, which is sterile. Natural hybrids are butterflies, moth, fishes, ducks etc.

Patterns of Organic Evolution

1. Divergent evolution or adaptive radiation:

It was first observed by H.F. Osborn in 1898. The evolution of a single ancestral species to a variety of forms which occupy different habitats is called adaptive radiation or divergent evolution.

1. Convergent Evolution:

Distantly related animals inhabitating similar habitats often develop independently similar morphological features that make them look similar. This phenomenon is known as convergent evolution which is opposite of divergent evolution. For example all aquatic vertebrates possess similar morphological features like streamline body, paddle-like limbs, webbed digits  etc. inspite of belonging to different classes. This is due to living in same environment.