Anatomical Evidence of Evolution

Evidences for evolution

Various theories of evolution explain that the life originated on earth, as simple organisms (like ameobae), from organic substances. Later these simple forms got specialized and diversified to suit their diversified habitats and habits. Fossil records support the occurrence of organic evolution to a large extent. Apart from these direct evidences many fields of biology like comparative anatomy, embryology, physiology and zoogeography also give positive evidences in support of evolution

(A) The evidences from comparative anatomy :   General similarities in structures are seen between groups of organisms. These similarities prove that the groups are related to each other. The degree of relationship is measured by the degree of similarities they possess. For example, a reptile is more closely related to mammal than an insect. These comparisons also show the different plans of organisation in various groups. A basic plan of organisation could be seen when organs of different
vertebrates are compared. The fore limb of a frog, a reptile, a bird and several mammals like bat, man, horse, cow and whale have striking similarities in structure with modifications to suit their mode of life.

The fore limbs of these vertebrates are made up of similar bones such as humerus, radius, ulna, metacarpels and phalanges. These components show only little variations in these organisms. Thus, they help frogs in hopping, the reptiles in crawling the birds and the bats in flying, the whales in swimming, the horses in running and so on. Similarities are seen not only in the construction of bones but also in the muscles and nerves. Such structures with similar fundamental plan and different functions are termed as homologous organs. Thus homology effects divergent evolution. The brain of man resembles that of other vertebrates. The olfactory region, the cerebral hemispheres, pineal body, optic chiasma, optic lobes, cerebellum, medulla oblongate and spinal cord are found in all vertebrates. Organs like heart, eye, ear and kidney of various vertebrates, also show homology. Such comparisons lead us to conclude that they could have descended from a common ancestor.

Vestiges:   Some organs which are well developed and functional in the ancestral types have lost their functions in their descendants. Such organs are found to be in the process of disappearance. Hence they are reduced and remain as remnants. This might be because they were not used for a long time. A change in the environment has made them useless in the present environment conditions. Such organs which are apparently of no use to the possessor and hence not necessary for their existence are called as vestigial organs. These vestigial organs serve as evidences for the process of evolution in organisms.

Nearly 180 such vestigial organs can be cited in man. Some of them are, reduced caecum, vermiform appendix, coccyx wisdom teeth, nictating membrane of the eye, body hair and the muscles to move the ears, lobes etc.,

Human vestigeal organs
a.Vermiform appendix   This is the remnant of a blind pouch, the caecum which is large and functional in the digestive system of the herbivorous animals like rabbits. Herbivorous food constituents like cellulose requires a long duration of time for digestion. Hence it is stored in the caecum for a long time. The process of digestion is carried out with the help of the intestinal bacteria that are lodged in the caecum of these herbivores. A gradual change in the diet, containing more meat and less cellulose has made caecum less important. There by, it gradually became smaller and appears as a vestige, in man.

b. Coccyx     The tail which is so useful and characteristic of lower vertebrates has lost its function in human. It is represented as coccyx. The coccyx vertebrae are originally meant for attachment tail muscles. This is because of the change in our mode of life from arboreal to terrestrial

c. Wisdom tooth :    The last molars of upper and lower jaws are called as wisdom teeth. This molar tooth is functional in primates. In humans, it differs from others in their shape, size and the time of eruption. In some it may not erupt at all. Many vestigial structures are much more fully developed in the embryos for example the embryos of the whale is densly clothed with hair. In their adults, they are represented only near the mouth and the eyes

(B) Evidences from embryology  Embryological studies are of considerable importance as they give
ample proof for the theory of organic evolution. The different stages in the development of individual organs are collectively known as ontogeny. The study of the evolutionary history of species or race is called as phylogeny. The first stage in the ontogeny of an organism is the egg stage which may be considered corresponding to the protozoan ancestors. In all the vertebrates we are able to notice that cleavage starts only after fertilization. Then it develops into a mass of cells called the blastula. The blastula is converted into a gastrula by the process called the gastrulation. During gastrulation different germ layers ectoderm, endoderm and mesoderm get segregated to attain a triploblastic nature. In further development the embryos of different vertebrates greatly resemble each other. All of them including the human embryo possess tails and gill slits. In the earlier days of the development, it is difficult to distinguish a human embryo from that of a bird or an amphibian. These differences are less in the case of groups which are very close to each other. A fish embryo very much resembles an amphibian embryo than a mammalian embryo. The pattern of further development of various organs in the embryos of various vertebrates show striking similarities.

a). Development of kidneys In all vertebrates the kidney develops as a set of tubules or nephrons. The embryonic kidney of all vertebrates consists of pronephric tubules. How ever, they do not form the adult kidney. The mesoanephros appear and form the adult functional kidney in the fishes and amphibians. Still another set namely the metanephros develops posterior to the mesonephros in reptiles, birds and mammals and constitute the functional kidney . This similarity in origin of the kidneys, in the various vertebrate groups has led to infer that these three sets of tubules probably represent the single kidney of the common vertebrate ancestor.

b). The development of heart:   In all the vertebrates, the heart begins as a pair of tubular structures,
the endocardial tubes which later fuse to form the primitive tubular heart. This simple straight tubular heart becomes ‘S’ shaped. Later, by the development of a partition it becomes two chambered as in the case of the adult fishes. The two chambered heart becomes three chambered by the development of the interauricular septum a condition found in the adult amphibians. The three chambered heart later gives rise to the incipient four chambered heart as in adult reptiles. This is effected by the formation of an incomplete interventricular septum. Finally at the level of the birds and mammals the heart becomes completely four chambered. This course of development can be traced very clearly during the development of the heart in a mammal. This type of development indicates a common ancestry for all the vertebrates.

c). The development of tooth:    In the same way the appearance of tooth buds in the embryos of birds indicates their descent from the ancestors (reptile like forms) which had teeth. The German Biologist Ernest Von Heackel was the first to notice the stricking similarities between the stages of ontogeny and those of phylogeny of a particular race. He propounded the famous law known as the ‘Biogentic law’ or the theory of Recapitulation. This law states that the ontogeny briefly repeats or recapitulates phylogeny or the evolutionary history of the race. In other word ontogeny repeats phylogeny, which mans the ancestral characters reappear in the developmental stages of an individual. Hence this law tries to explain why a tadpole of a frog resembles a fish, the caterpillar of a butterfly resembles a worm. Though Hackel’s theory is accepted by many scientists a few raised objections against his law
(1). It is found that the order in which the characters appear in phylogeny are not faithfully reproduced in ontogeny. For example in animal phylogeny the teeth evolved before the tongue but in the mammalian ontogeny the tongue appears earlier than the teeth

(2). Embryological development is not a precise record of ancestral evolution. An young mammal never looks like an adult fish or reptile. But it resembles only the embryos of the reptiles or fishes.

(3) The developmental history is too much precised when compared with the evolutionary history. Hence it is concluded that an animal does not recaptulate the adult stages of any of its ancestors. An amphibian has not evolved from an adult fish by proceeding beyond that stage. But it diverges as an embryo along new lines during which stage the limbs are well developed and gills degenerate. Similarly the human embryo recapitulates the embryonic history and not the adult history.

(C) Evidences from physiology:   The studies on physiology and biochemistry prove that the process
of evolution has occurred biochemically. The protoplasm which is the basic unit of life has the same quantitative and qualitative attributes in all living organisms.

a) Similarity of biomollecules   The nucleic acid which forms the basic hereditory material for all
the diversified groups strongly support the common ancestry. The various enzymes and the hormones of the diversified groups support the same view point. For example, trypsin, the proteolytic enzyme is found through out the animal kingdom. The enzyme amylase could be seen from poriferans up to the human. The hormones of different vertebrates are also wide spread in animals. The hormones seem to be simliar among the invertebrates. This could observed among the vertebrates too. The non-specific nature of the hormones could be proved by an experiment done on amphibians. When the thyroid glands of a tadpole are removed it remains as a tadpole and never metamorphos into an adult frog. When these tadpoles are fed upon mammalian thyroid tissue they metamorphose in to a frog. Like wise the hormones from the thyroid gand of a cattle can be effectively used in the treatment of thyroid deficiencies. The haemoglobin crystals of various vertebrates show variation. But the haemoglobin crystals belonging to animals of the same genus are more or less identical.

b) Similarity in Host parasite relationsip:   The protozoan parasite that causes malaria in birds cannot cause the fever in human as their physiology is different. The virus that causes polio in human can cause the same disease in monkeys and apes but not in other vertebrates.

c) Similarity in blood group:  The anthropoid apes and man have A, B, O and AB blood groups.
A disease called gout occurs in both because of the accumulation of uric  acid in the blood vessels of the joints.

d). The serological tests:  The serological tests are conducted in closely related organisms. It is
found that they have similar type of antigen. For instance, the blood of a dog was injected into a rabbit. The antigens in the rabbit stimulated it to produce antibody. Sufficient time was given for the antibodies in the rabbit to attain a high level of concentration. After that the serum of the rabbit (anit dog serum) was extracted. When it was mixed with the serum of the dog, the antigen-antibody reactions took place. This made the serum cloudy. This reaction will not happen if the rabbit is not immunized.

If the antidog serum is mixed with the serum of the wolf, precipitation occurs but not to that extent as in the case of the dog When the same, test is conducted with the fox and the cat (all coming under the order carnivora) the same degree of relationship could be shown with the degree of precipitation. When it is done with the cat no precipitation occurs showing that cow is very distant from the carnivores. Such biochemical tests apart from providing a sound evidence for evolution they solve many taxonomical problems too.

(D) Zoogeographical evidence:   Geographical distribution of animals and plants provide ample
evidence for the fact of organic evolution. Biogeography is the spatial distribution of organisms upon the earth’s surface at various periods. The organisms belonging to the same species when happen to grow in different areas with different climatic conditions may undergo adaptations to suit their habitat. In course of time they may be modified so as to call them as different species.This could be explained with the camels. The Arabian one humped camel living in the vast deserts is highly adapted for the desert life by storing water in the (pouches of the) stomach, food in the hump and having broad heavily padded feet to hold grip in the loose sand. On the other hand the two humped camel in the colder regions of central Asia, has a coat of shaggy hair which insulates its body against the extremely cold climate. Its feet are not padded but quite hard enabling the animal to move about more freely on the rocky terrains of central Asia. From these two examples we can understand that the differences are due to different environments for several generations. Charles Darwin during the course of his voyage in the ship ‘HMs Beagle’ observed the flora and fauna of Galapagos islands which is 600 miles away from the west coast of south America, The inhabitants of each island differed from those of other islands as well those of the main land. Each one of these islands had their own type of giant tortoise, which was different from those of the main land as well as from those of others. We may suggest that these are the migrants from the main land or from other landmasses.

The famous Darwin’s finches (13 species of finches) inhabiting different islands exhibit special adaptations for their particular modes of life. Those that fed on insects had long slender beaks by which they could pick up small insects from small crevices and pierce them. On the other hand
plant eaters had short strong beaks, which were useful in breaking open hard nuts. Those living on islands covered with remains of volcanic eruptions possessed black feathers, which merged with the surroundings. Those living among green, had green feathers, which blended well with the background. Those inhabiting pebbly beaches possessed speckled gray plumage. And hence they had a wider range of adaptive radiation.