Biological Evolution – Genetics, Paleontology with Evidence of Evolution

Immunology is one of the Biology Topics focused on understanding the immune system and its response to pathogens and diseases.

Evidence of Evolution – Fossils, Comparative Anatomy, and Embryo Development Pattern

Biological evolution denotes the origin of new and gradually complex organisms from the pre-existing simpler form of life. Along path leads from the origins of primitive life which existed at least 3.5 billion years ago, to the profusion and diversity of life that exists today. This path is best understood as a product of evolution. The word ‘evolution’ first appeared in the English language as a non-biological connection in 1647 and it became widely used for all sorts of progressions from the beginning. Biological evolution was ‘descent with modification’ referred to by ‘Darwin’. On the basis of ‘Natural selection’, Darwin proposed that evolution could be explained by the differential survival of organisms. The present-day biodiversity is the result of this biological evolution.

Origin of life deals with the mechanism describing the appearance of the first life form on the earth and biological evolution deals with the modification of the initial life form to be evolved into some other descended life form usually complex in organization. There is ample evidence in favour of this biological evolution. From the very remote past after the origin of first life, it started evolving in nature. On the earth, many life forms evolved and also went extinct and relics of these extinct biological forms could be preserved in the form of fossils in the earth’s crust. Scientists could be able to explore this history of evolution and therefore, we have no way to reject the facts of descent with modification.

Besides, evidence of evolution was presented by the researchers from different angles which are worthy of their description with the necessary explanation. However, evolution occurred in two phases. The first phase is chemical evolution which resulted in the development of life on Earth and the next phase is biological evolution which deals with descent with modification. Before placing the evidence in favour of biological evolution on earth, the history of this biological evolution mentioning different life forms appearing at different time periods should be indicated.

Distribution of Life Forms in Time and Space

From the discussion of the history of the origin of life, it appears that the first evolved life was very simple in organization and probably similar to the microsphere. Subsequently in the environment of Earth with the appearance of O2, the evolution of life occurs in various ways. In the history of more than a thousand million years of earth along with the physical changes, appearance and extinction of various life forms in the earth’s crust printed the distribution of life forms in time and space. The fossils of the extinct life available in the earth strata of the different periods appear as relics of the past history. From the primitive period as the diversity appears at different time periods, their distribution may be presented in the following table. However, to understand this life distribution along different time periods starting from the initial stage of origin of life, a discussion on the geological time scale and its division should be essential.

Age of Earth and Its Division:
According to the calculation of geologists the age of Earth appears to be about 4.6 billion years, that is, earth originated about 4.6 billion years ago. Starting from the period, the period up to the recent time may be divided into two eons, namely Precambrian and Phanerozoic. The period before 545 million years is included in Precambrian and the period after that is known as phenerozoic. Each eon is divided into several eras. Precambrian eon has been divided into the Hadean, Archean, and Proterozoic eras. On the other hand, the phanerozoic eon has been divided into the Palaeozoic, Mesozoic, and Cenozoic eras. Among these Palaeozoic is the oldest. Each era is again divided into more than one period and a period again is divided into epochs.

Geological Time Scale with duration of Years:

From the analysis of the fossils at the different petrified layers of the earth, a history of the life distribution may be realized. With the division of time period, the distribution of life may be noted as given in the table.

Geological Time Scale of Earth (To be read from below upward. Duration of each epoch and period is in million years):

Evidence in Favour of Biological Evolution

There are different pieces of evidence in favour of organic evolution and among them are palaeontological evidence, evidence from comparative anatomy, embryological evidence, and evidence that needs a special explanation.

A. Palaeontological Evidence

Paleontology is the branch of science that deals with the study of life in the geological past based on the analysis of plant and animal fossil records. The animal and plant body or body part trapped within the stratified rocks may also be petrified to produce fossils. Therefore, fossils are petrified relics or remains of bodies of living organisms of the past. Such relics of old existing organisms were discovered by the scientists and they could successfully determine the age of fossils within the earth’s strata. The study of the fossilized remains of the organisms may give light on the history of the organisms and these appeared as the best evidence in favour of the biological evolution on the earth.

Leonardo da Vinci (1452-1519) is regarded as the father of Paleontology. However, George Cuvier (1769-1832) is the father of modern Paleontology. The subject Palaeontology has two divisions as paleobotany which deals with the plant fossil, and palaeozoology which deals with animal fossils. However, in discussing paleontology evidence on biological evolution some ideas on fossils, fossilization and dating of rocks should be developed.

Evolution of Horse
The present-day horse (Equus equus) is fast running and stands on four long legs containing a single hoof. The Paleontologists could trace the evolution of this horse (known also as an up-horse) from about 54 million years old animal called Eohippus or Hyracotherium in the Eocene epoch. Within the rocks of the Eocene epoch, the fossils of this form of animal were recovered. Examination of the fossils of Eohippus revealed that they were of the size of jackals having a height of only 30-50 cm. On their legs, there were four effective digits when the first digit of the forelimbs was slightly smaller than any of the other three digits. Whereas in the hind limbs, there were three effective digits and the 1st and 5th digits were shorter. Eohippus thus was a small ungulate and it was named as Dawn horse.

Besides, they had a complete series of teeth with smooth molars and premolars having low crowns. As a result, they wanted to browse soft vegetables. In comparison to this dawn horse, the present-day horse is quite large in height being about 165 cm. The teeth on the jaws are also not complete. Due to the loss of canines, a gap called diastema is developed on the jaws and the incisors and molars are of equal size and have thick crowns. Each limb is having single effective digit forming the hoof. Because of this feature, the horses are swift runners and they are efficient in browsing over grass in the field. The present-day horse evolved about 2-2.5 million years ago during the Pleistocene of the Quaternary period.

The evolution of the modern horse from the dawn horse took about 51-52 million years and in between several forerunners of the horse also took birth. Several of those were Mesohippus of Oligocene, Merychippus of Miocene, and Pliohippus of Pliocene. Besides two other forms namely Orohippus and Mesohippus evolved between Eohippus and Merychippus. The sequence of origin of the horse from Eohippus may be shown by the given diagram.

In the course of the evolution of horses, gradual changes in some features may be noticed. These are

1. Increase in size.
2. Modification relating to the structure of the limbs
   • Lengthening of limbs and feet
   • Reduction of lateral digits
   • Increase in length and thickness of the third digit.
3. Development of a hoof to move on the rigid surface of the ground.
4. Development of thick crown on the molars and premolars.
5. Formation of stout muscles in the body

The comparative discussion on the above two features is shown below-
1. Increase in Size:
The evolution of modern horses began about 60 million years ago in the Miocene epoch and the first fossil named Eohippus was found in North America. The horse was about the size of a fox, only 30 cm high at the shoulders. It had a short head and neck.

• Mesohippus, the intermediate horse of the Oligocene epoch, was of the size of modern sheep, about 60 cm high at the shoulders.
• Merychippus, the ruminating horse of the Miocene epoch, was the size of a small pony, about 100 cm high at the shoulders. It was with a longer neck.
• Pliohippus, the Pliocene horse of the Pliocene epoch, was of modern pony, about 120 cm high at the shoulders.
• Equus, the modern horse of the Pleistocene epoch, is about 150 cm high at the shoulders. It has a long head and a long neck.

Increasing in size:

Forms of Horses	Size (cm) in Height	Appearance
1. Eohippus	30 cm	Like a fox (short head and neck)
2. Mesohippus	60 cm	Like modern sheep
3. Merychippus	100 cm	Like a small pony (longer neck)
4. Pliohippus	120 cm	Like modern pony
5. Equus	150 cm	Modern horse (long head and long neck)

2. Modifications Relating to the Structure of the Limbs:
Lengthening of limbs and feet, Reduction of lateral digits, and Increase in length and thickness of the third digit.

• In Eohippus, the forefeet were with four complete fingers (2, 3, 4, 5) and one splint of the first finger and the hind feet with three functional toes (2, 3, 4) and two splints of first and fifth toes. Splints are reduced and non-functional side fingers and toes of horses.
• In Mesohippus, four feet had three fingers and one splint of the fifth finger, and hind feet were three-toed but & the middle one was longer than others and supported most of the body weight.
• In Merychippus, the fore and hind feet had three fingers and three toes, the middle finger and toe being longer than others and supporting entire body weight. There was no splint.
• In Pliohippus, each fore and hind foot had one complete finger and one complete toe, and two splints hidden beneath the skin. It is the first one-toed horse.
• In the Equus horse, each fore and hind foot of the modern horse has one finger and one toe, and two splints.

Modification relating to the structure of limbs:

Form of Horse	Modification of Limbs and Feet		Length and thickness of the third digit
	Forefeet	Hind Feet
1. Eohippus	4 toe, one splint of 1st toe	3 toe, two splints of 1st and 5th toe	Very slender
2. Mesohippus	3 toe, one splint of 5th toe	3 toe, middle one longer than others	Little thick
3. Merychippus	3 toe, no splint, middle toe being longer	3 toe, no splint, middle toe being longer	Moderately thick
4. Pliohippus	1 toe, two splints hidden under the skin	1 toe, two splints hidden under the skin	Stout
5. Equus	1 toe, two splints	1 toe, two splints	Very stout

Along with all these modifications in the different forms of horses, the size of the skull also increased in a progressive order with the teeth organization. The fossilized stages of horse evolution show these gradual developments as evidence in favour of biological evolution. Therefore, the evolution of horses supports the theory of descent with modification and this comes from the study of fossils.

The Missing Link as Evidence in Favour of Biological Evolution from Palaeontology
Missing link represents the organism that no longer exists on the earth but contained the features of two groups of organisms having different statuses in the living world. They are also regarded as transitional fossils. Among the animals Archaeopteryx and among plants Runcaria are recognized as missing links. As the organisms belonging to the missing link contain the features of two distinct and different groups of organisms, they may be taken as proof of evolution.

Archaeopteryx: Archaeopteryx is a fossilized animal identical in many respects to birds. The first fossil of Archaeopteryx was discovered in Bavaria of Germany in 1861 by Andreas Wagner. Subsequently, 10 other fossils of this animal could be discovered. This animal could belong to the Therapsid group of Dinosaur and the animal has been named Archaeopteryx lithographica. Archaeopteryx (about 1.6 feet long) was much identical to a bird, but it contained many features as found in reptiles and therefore, Archaeopteryx may be taken as a link between birds and reptiles. Hence, Archaeopteryx has been a befitting proof in favour of the concept that birds may have originated from reptiles.

Archaeopteryx lived in the late Jurassic around 150 million years ago. It had more in common with other small Mesozoic dinosaurs than with modern birds. According to Palenological Archaeopteryx is a transitional fossil between dinosaurs and modem birds. Archeopterise has since become central to the understanding of evolution. In this regard, a consideration on reptilian and avian features of Archaeopteryx appears to be worthy of discussion.

Reptilian Features of Archaeopteryx	Avian Features of Archaeopteryx
1. Presence of homodont teeth on the jaws.	1. Presence of feathers over the body.
2. Presence of claws on the digits.	2. Ovai skull with firmly attached skull bones.
3. Presence of long tail supported internally by free caudal vertebrae.	3. Conversion of forelimbs into wings.
4. Presence of rigid sternum.	4. Presence of three digits on the fore limb.
	5. Presence of four digits on the hind limb.
	6. Presence of furcula or wishbone.

In consideration of the above-mentioned characters ‘the origin of birds from the reptiles cannot be considered as illogical. Based on this probably Julian Huxley advocated that ‘Birds are glorified reptiles’.

Connecting Link
Connecting links may be defined as living organisms that possess the characteristics of two different groups.
Examples:
Peripatus – Connecting link between arthropods and annelids.

Duck-Billed Platypus (Egg laying mammals) – Connecting link between reptiles and mammals.

Difference between Connecting link and Missing link:

Connecting link	Missing link
They are living in today’s world.	They are not living in today’s world, only available as fossil records.
e.g., Platypus	e.g., Archaeopteryx

Fossil
The word Fossil originated from the Latin word ‘Fodere’ – which means dig up.
Fossils may be defined as the remnant, impression, or trace of plants and animals of prehistoric life, excavated from the earth’s crust.

Formation of Fossil:
Body of any organism, its body parts, or any relic of its existence when is engraved within the earth’s crust, which may be petrified in the course of time. However, the hard parts of the body are generally petrified and the soft parts get decomposed. When the body of an organism is entrapped within the strata of the earth, different component molecules of the body are replaced by minerals. As a result, different parts of the body of the organisms get fossilized. Therefore, fossilization occurs in two steps. In the first phase, the body of the organism is entrapped between the earth’s strata, and in the second phase, petrification occurs through mineralization. It is noteworthy that the intact body of an organism is never petrified, rather the petrified form of the body gets distorted to some extent. However, the fossil may give a general idea about the organism.

Determination of the Age of Rock or Dating of Rocks:
In the study of fossils, the determination of the age of a fossil is a very important aspect. There are two methods in the determination of the age of the rock namely relative dating and absolute dating. In relative dating, the age of a fossil is determined based on the relative position of the fossil in a rock and depending on the decay rate of the rock in which the fossil is present. On the other hand in absolute dating, the age of a fossil is determined based on decay of radioactivity of a radioactive element present within the fossil. In this consideration determination of age by uranium-lead method and 14C-12C method are most important. It is noteworthy that within the earth’s crust, there are radioactive elements.

Uranium is a radioactive element that emits radioactivity to form Lead (206Pb) which is a stable element having no radioactivity. 1% uranium takes about 66 million years to be transformed into Lead (206Pb). Therefore, based on the ratio of uranium and Lead (206Pb) present in the rock, its age may be determined. Besides this, 14C present in a rock may be converted into non-radioactive 12C through radioactive decay. Therefore, on the basis of the ratio of 14C to 12C in a rock, its age may be determined. In this respect, it is noteworthy that the half-life of 14C to form 12C is about 5568 years. However, a rock older than 50000 years can hardly be determined by the 14C-12C method of rock dating.

Types of Fossils:
Fossils may be of various types and they may be described in the following way:

1. Body fossil:
The petrified hard parts of the body of an organism are known as body fossils. The hardest parts of the body, teeth or bones are petrified & form the body fossil.

2. Sub-fossils:
Usually, the body of an organism that remains trapped in the earth’s crust is known as a sub-fossil. The body of an organism buried within the ice layer or within the pit bog forms a sub-fossil.

3. Micro-fossil:
When a body of an organism with a size of 0.5 mm or less than that is petrified, it is known as a micro-fossil.

4. Macro-fossil:
When a fossil measures about 1 cm or more, it may be observed by the naked eye. Such fossils are known as macro-fossils.

5. Unusual fossil:
When due to certain unique conditions in the environment, almost the whole of the body of an organism is preserved, it is called an unusual fossil. The mammoth discovered in Siberia or Archaeopteryx unearthed in Bavaria belongs to this category of unusual fossil.

6. Trace fossil:
Sometimes the tracks of movements of primitive forms of animals or their footprints are found to be preserved on the rock surface. These petrified symbols of animal movement are considered trace fossils. Trace fossils are also called imprints.

7. Coprolite:
The petrified excreta of animals is known as a coprolite. A study of coprolite may indicate the food habits of animals in old age.

8. Mould:
The body of an organism entangled within the earth’s crust may be decomposed and then a hollow space in the form of the shape of the organism may be produced. The hollow space after petrification develops a mould. The mould may be called a symbol of an organism.

9. Cast:
Sometimes the mould may be filled up with minerals and produce the shape of an organism. Such petrified architecture for the body of an organism is known as cast.

10. Gastrolith:
When the inner contents of the gastrointestinal tract of an animal are petrified, it is called a gastrolith.

11. Petrification:
During fossilization tissue components of animal or plant body may be replaced by minerals. Such replaced tissue or body parts in the process of fossilization is known as petrification.

12. Pseudofossils:
Sometimes within the stratified rock accumulation inorganic components may take the shape of a body part of an organism. Apparently, these accumulations may be confused with a petrified part of the body of an organism. Because these are not true fossils, they may be designated as pseudofossils.

B. Evidence from Comparative Anatomy

Comparative study of the organ systems in different vertebrates indicates that they are constructed on the same structural plan as well as exhibit a kind of gradual complexity of the organs. Examples from the comparative anatomy of Vertebrates provide sources of evidence for organic evolution.

1. The vertebral column in all vertebrates originates from four mesodermal masses in each somite. The vertebral column is composed of vertebrae, each having its centrum, neural arch, neural canal, neural spine, transverse processes, and articular processes.

2. The heart in all vertebrates consists of chambers like the auricle, ventricle, sinus venosus, conus arteriosus, etc. In the vertebrate series, the structural plan shows an evolutionary change. There is a gradual compactness of chambers from the serial arrangement in fishes to overlapping chambers in amphibia, reptiles, birds, and mammals. The main purpose is to prevent a mixture of oxygenated and deoxygenated blood by providing partitions of the chambers. In amphibians and reptiles, the heart is three chambered but the ventricle is incompletely divided into two in reptiles.

In mammals and birds, the heart is four-chambered and there is a complete separation of oxygenated and deoxygenated blood. The single-circuit heart of fishes gradually changed to the double-circuit heart of birds and mammals in the course of evolution through the transitional stages of the hearts of amphibians and reptiles due to the change of habitat and due to specialization of the aerial respiratory system.

3. The aortic arches in different vertebrates also show a gradual change in the reduction of the number of arches. The variation and modification of aortic arches in different vertebrates are due to the specialization of the respiratory system and heart. In all vertebrates, the adult arches develop from six embryonic arches. There were six pairs of aor¬tic arches in the ancestral plan. The first and second pairs degenerated in the course of evolution and in fishes four pairs are present.

In amphibians and reptiles, 1st and 2nd pairs are already absent. 3rd pair develops as a carotid artery, 4th pair develops into left and right systemic arches, 5th pair is again lost and degenerated and the 6th pair develops into left and right pulmonary arteries. In birds and mammals, all are similar to that of reptiles except the 4th pair of aortic arch which in birds right systemic arch persists and left systemic arch is degenerated and lost, whereas in mammals left systemic arch persists and right systemic arch is lost.

4. Study of the anatomy of the brain in different vertebrates also shows progressive evolutionary changes. From fish to mammals the brain, consists of similar five parts or lobes, as-Olfactory lobes, Cerebral lobes, Optic lobes, Cerebellum, and Medulla oblongata. From fishes to mammals, through the process of evolution, gradual changes occurred like a gradual increase in the size of the Cerebrum. In fishes, cerebral hemispheres are smaller but in mammals, it is much enlarged, convolutions cerebellum are gradually increased from fish to mammal whereas the size of the olfactory lobe has been gradually reduced from fish to mammal.

5. Homologous Organs:
In vertebrates, there are certain organs that are functionally different but have similar structural organization origins. These organs are known as homologous organs. For example, the forelimbs of horses, wings of birds, wings of bats, and flippers of whales serve different functions but have similar organization and identical origins. The Forelimb of the horse is used for running on the ground, the wings of birds and bats are used for flying and gliding, and the flipper of whales is used for running. All these organs in vertebrates are modifications of the forelimb for different functions but have similar structural organization.

The organs internally contain the bones such as the humerus, radius, ulna and phalanges, and associated muscles. Hence, they have also the same developmental history. On the basis of the comparative analysis also we may get supportive evidence in favour of biological evolution. Leaves of pitcher plants, Venus fly traps, cactus, and poinsettia are of different shapes and functions. But they are homologous structures and all share a common ancestor. Thorns in Bougainvillea or citrus and tendrils in Cucurbita are homologous organs and they serve different purposes.

6. Analogous Organ:
In this context, it is worth mentioning that in animals there are certain organs that serve similar functions, but they are completely different in their organization and have different origins. These organs are called analogous organs. For example, the wings of insects and wings of birds are analogous organs, because both are used for flying, but they have dissimilar origins.

Analogous organs exhibit a convergent evolution only and have no relation with evolutionary lineage. Therefore, analogous organs cannot satisfy the process of evolution.

Differences between Homologous and Analogous Organs:

Homologous Organ	Alanologous Organ
Organs are similar in structure and origin but different in function.	Organs differ in structure and origin but with similarities in function.
Organisms bearing these structures are evolutionarily related.	Organisms bearing these structures are evolutionary different.
These organs differ in outlook.	These organs are similar in outlook.
The organs contain similar internal structures.	The organs do not show similarity in internal structures.
Stages in the development are similar.	Stages in the development are dissimilar.
Example: the Forelimb of a frog, the hand of a human, the wings of a bat, and the forelimb of a horse.	Example: Wings of butterflies, bats, and birds.

7. Vestigial Organs:
In animals especially in higher vertebrates, there are certain organs that appear to be rudimentary and non-functional and these organs are called vestigial organs. The similar organs in lower forms of animals are well organized and functional. The vestigial organs may be presented as supportive evidence in favour of biological evolution. In humans, several vestigial organs may be encountered such as the vermiform appendix, muscles in the ear, coccyx, etc. The vermiform appendix is a small diverticulum in the form of a finger that is of no use to man and therefore, removal of this organ from the body does not create any problem. But this organ in herbivores is very much associated with digestion and cannot be ignored in any way.

The ear muscles in the lower form of vertebrates help to move their pinna in different directions but the same in human ear cannot help in moving the ears. Coccyx is a structure formed of a combination of 3-5 degenerated caudal vertebrae in man, on the other hand in lower mammals well-formed tails with distinct caudal vertebrae are not at all vestigial structure. Greatly reduced wings of kiwi and ostrich are vestigial organs. In whales, the hind limbs become abortive and are represented by bony remnants which are hidden inside. Thus vestigial organs may be evidence in favour of biological evolution.

Divergent Evolution Vs. Convergent Evolution:

Divergent Evolution	Convergent Evolution
1. Development of different functional structures from a common ancestral form is called divergent evolution.	1. Different species evolve similarities in their characteristics and functions due to the adaptation to a particular environment.
2. Homologous organs show divergent evolution.	2. Analogous organs show convergent evolution.
3. Thron of Bougainvillea and tendril of Cucurbita are homologous structures but their functions are different.	3. The wings of birds and insects are different structurally but perform the same function.

C. Embryological Evidence

In animals especially in higher forms development starts after fertilization. Fertilization results in the formation of a zygote which gives rise to an organism following cleavage, gastrulation, organogenesis, etc. A study on the development of organisms leading to the establishment of the complete body of the organism is known as embryology. In evolutionary history birds and mammals originated at the final event in evolution and this history of evolution of a mammal or a bird may be supported by the developmental history of the organism. Based on this fact Ernst Heinrich Haeckel (1834-1919) proposed the Recapitulation theory which states that ontogeny repeats phylogeny. This means that the developmental history of the organism advocates its phylogenetic history.

In the history of evolution, human appears to be the latest creation in nature. The ontogeny or developmental history of man indicates that a man starts his life from a zygote which is a single-cell living body comparable to a unicellular organism like protozoa. Then through cleavage, it produces a morula or blastula which is a multicellular body without the formation of tissue structure and this stage may be comparable to a parazoa identical to sponges. In the next step, gastrulation converts the blastula into a gastrula which appears as an elongated body with a gastrovascular cavity. The gastrula at this stage is comparable to a cnidarian organism like Hydra.

Further development results in the formation of somites in the body with coelom inside the body similar to an annelid organism. As the development advances more and more a body form containing a vertebral column and gill slits is developed when the body may be comparable to a fish-like organism. This eventually develops the embryonic body identical to the small human body. Thus ontogeny may be correlated with phylogeny and this may be indicated in the following way.

Evidence in Support of Recapitulation Theory

A large number of embryologists confirm Haeckel’s view on recapitulation and put forward many pieces of evidence in support of this theory as:

• Early embryos of different vertebrates show close similarity.
• The heart in the vertebrate series is built on a common basic plan.
• In the development of kidneys in vertebrates, there is a succession of Pronephros, Mesonephros, and Metanephros.
• Presence of gill slits in all vertebrate embryos.
• In all vertebrate embryos, there are six pairs of arterial arches.
• Almost all crustaceans pass through Nauplius larval stage.
• Mollusks and annelids pass through Trochophore larval stage.
• Most of the coelenterates have Planula larvae.
• Many supporters of Haeckel like Weismann, Macbride, Graham Kerr, and Agassiz advocated Haeckel’s views.

Modern Views of Recapitulation Theory

At the present time, recapitulation theory is no longer regarded to be correct. Striking similarities of embryos of different animals do not reflect the repetition of phylogenetic stages, but embryonic structural parallelism may be regarded as a reflection of characters not of the adults but of the embryos. Experimental embryologists have altogether denied the theory. Needham has regarded recapitulation as a phenomenon and not as a law. However, early embryonic similarity among many animals may be taken as evidence in support of biological evolution. In vertebrates starting from fish to mammals, the embryonic stages indicate a similarity in their organization which may suggest their common origin.

Early Embryonic Similarity

It has been observed that the early stages of the embryo of different vertebrates resemble very much. The early embryos of fish, frogs, lizards, chick, or rabbits exhibit remarkable similarity and it is difficult to identify a human early embryo from that of a frog, chick, or rabbit. These similarities in the embryonic stages of different vertebrates are due to:

• Similar body structure and plan like the presence of gill clefts, tail, rudiments of the eye, etc.
• A similar somite along the vertebral column is formed by the replacement of notochord.
• The similarity in limb development in a tetrapod.
• The common pattern of development with resemblances in the embryonic structures of different vertebrates is due to the inheritance of such characteristics from an ancestor common to all related animals.

‘Retrogressive metamorphosis’ means degenerative changes where an active larva transforms into a sedentary adult.
e.g., a Urochordate larva bears all advanced characteristics of Chordata but after metamorphosis, it loses.

D. Molecular Evidence for Biological Evolution

From tiny bacteria to higher plants, from protozoa to mammals, the structural and functional unit of all living forms is the cell. The major constituents of cell throughout the living world is the same for all. Yet life exists on this earth in spectacularly diversified forms. Living beings are fantastically diverse in structure, behaviour, habitat, physiological actions as well as in reproduction. The structure and functions of all these diverse life forms are determined by a marvelous macromolecule-the gene or DNA-the composition and structure of which is uniform from virus to mammal. The gene is the ultimate living particle of life. Through the process of evolution, the molecular structure of life has been conserved in biological evolution.

1. Uniformity of Cell Architecture:
Starting from bacteria to man and all plants have one thing in common, all of their body is made up of cells. There is a tremendous uniformity in the structure of cells and cell organelles. All of them have similar natures of the cell membrane, ER, mitochondria, ribosomes, Golgi body, and nucleus. Their functions in all different organisms are the same.

2. Chemical Evolution of Life:
About 3800 million years ago in the primitive earth, life first originated through chemical evolution by trial and error method of mixing and remixing of elements like carbon and hydrogen to form hydrocarbons and thereby gradually many organic compounds like acetic acid, glycol, glycolic acid, and many other amino acids were formed. They again joined to form proteins, and proteins are linked with nucleic acids to produce nucleoproteins which are the molecular precursors of life.

3. DNA the Molecular Life:
These macromolecules are known as a gene that replicates and synthesizes proteins in all living forms from the beginning of life through the past ages upto the recent. The molecular mechanism of DNA replication, protein synthesis, and cell division is similar in all life forms which clearly exhibits its evolutionary pathway through the ages.

4. Universality of Molecular Structure of DNA and its Function:
In all life forms the molecular structure of DNA or RNA is the same. DNA is a polymer, known as polynucleotide and each of its monomers is a molecule of nucleotide. Each nucleotide molecule consists of one molecule of purine or pyrimidine base, one molecule of ribose or deoxyribose sugar, and one molecule of phosphoric acid. DNA is a double helix and this double helical structure is universally present in all eukaryotes. The structure has been formed at the dawn of life and is still persisting. The unique function of DNA duplication during cell division in all life forms is amazing and its mode of function is similar in all. This has been achieved in the cell at the time of origin of life and this property has been propagated generation after generation to maintain the thread of life through ages of evolutionary progression.

5. Variation of DNA and Diverse Life Forms:
Morphological, anatomical, or physiological traits or characteristics of organisms are determined by the expressions of one or more genes that are of DNA material. The different genes are only variations of genetic material or DNA. The variations of the DNA molecules are due to changes in base sequences. These variations can occur by mutation under the stress of the environment physically or chemically. Small variations gradually accumulate in the population, and that may lead to the origin of new species. A vast number of species existing on the earth have all originated from preexisting other species by variations of DNA molecules, caused by mutation.

Base sequences in DNA are different in different species and even differ from individual to individual. The degree of similarity between the DNAs of two species can be estimated by the pairing property of DNA strands. It has been observed by many scientists that relative similarities exist between human DNA and the DNA of other vertebrates. Similarities appear more between humans and apes and less in distantly related groups.

6. Uniqueness of Protoplasm in All Living Forms:
The chemical composition of all living beings is limited to almost four elements as carbon, hydrogen, oxygen, and nitrogen. These are combined to form three major organic compounds carbohydrates, proteins, and lipids. These compounds along with some inorganic compounds and elements are suspended in water inside the cell as colloid forms. Protoplasm that exists as the physical basis of life in all cells of all organisms is composed of all these chemical components.

7. Chromosomes – Number and Structure are Species Specific:
The essential component of the nucleus in each eukaryotic cell is the chromatin or hereditary material that organizes into chromosomes. The chromosomes have fairly constant chemical components as being composed of DNA and protein and mostly Histone proteins. The chromosomes are the carrier and bearer of hereditary materials and they are filamentous but become shortened, condensed, rod-shaped, V-shaped, or J-shaped at the metaphase and anaphase stage of cell division. These chromosomes’ shape, banding pattern, and numbers are very much fixed and species-specific. The architecture and number of chromosomes of a species are conserved for an unlimited number of generations after which they may or may not be changed by mutation and give rise to variation of the species and that variation is intensified through generations to form a new species in the process of evolution.

Percent of Chromosome Bands in Some Organisms

1. Chimpanzees	99+%
2. Gorillas	99+%
3. Orangutans	99+%
4. African Monkeys	95+%
5. Domestic Cats	35+%
6. Mice	7+%

8. Molecular Evidence from Biochemical Processes, Respiratory Pigments, Hormones, Enzymes, and Blood Proteins:
The strongest molecular evidence of common ancestry comes from the biochemical processes occurring during metabolism in all living beings from bacteria to man and in all plants and animals. For example,
(a) The process of protein synthesis, biosynthesis of various organic molecules, and the same biochemical reactions during respiration by catabolism of organic substances.
(b) The energy in all living forms is released by the biological oxidation of glucose and is stored in ATP.
(c) In various biochemical pathways, various steps are regulated by the same enzymes in all living beings.
(d) Cytochrome-C is a respiratory pigment that takes part in the electron transport system and is present in all eukaryotic cells. It is formed by 104 amino acids, and cytochrome-C molecules are identical in chimpanzees and humans. But it differs from Neurospora Cyt-C in 44/104 amino acid position.

Some Organisms and their Number of Amino Acids:

Organisms	Number of Amino Acid
1. Chimpanzee	0
2. Rhesus Monkey	1
3. Rabbit	9
4. Cow	10
5. Pigeon	12
6. Bull Frog	20
7. Fruit Fly	24
8. Wheat Grain	37
9. Yeast	42

(e) Analysis of Insulin molecules shows that beef, sheep, pig, whale, horse, and rabbit insulin differ only in one to three amino acid positions. Beef insulin is very similar to that of humans.

(f) Hormones of different animal groups are identical. The thyroid hormones of all vertebrates are identical and interchangeable. Beef thyroid can be used in human thyroid deficiency and even the human thyroid can induce metamorphosis in frogs. Antidiuretic Hormone is a peptide hormone, eight amino acids long chain that signals the kidneys to conserve water. Its sequence differs only slightly between major groups of vertebrates.

Some Organisms and their Amino Acid Sequence:

Organisms	Amino Acid Sequence
1. Pig	cys-tyr-	phe	-gln-asn-cys-pro-	lys	-gln
2. Human	cys-tyr-	phe	-gln-asn-cys-pro-	arg	-gln
3. Fish, Amphibians, Birds	cys-tyr-	ile	-gln-asn-cys-pro-	arg	-gln

(g) Haemoglobin is a conjugated protein found in blood and at the molecular level, it contains two a and two b chains. Each a chain has 141 amino acids and each b chain has 146 amino acids, b chain of haemoglobin of humans and gorillas differ in one amino acid, of human and pig in 10 amino acids, and of human and horse in 26 amino acids. It is thus evident that the gorilla is the nearest relative of humans and both have evolved from a common ancestor.

(h) Comparative Seroiogg and Precipitation Method of Relationship Test:
Blood proteins of different mammals are similar but different from other vertebrates. To find out the inter-relationship of different animal groups, serological tests by precipitation method were introduced by Dr. Notital. The serum is collected from human blood and is injected into rabbits at definite intervals for a definite number of doses. Then serum is preserved from that rabbit and is known as “antihuman serum”. This antihuman serum is added to the blood of other animals and the degree of clotting is noted. The clotting is more in closely related animals and reduces as the relationship becomes more distant.