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In spite of many evidences, studies and theories, we are still not aware of how the evolution actually occurs. If we go backwards at some point in the history of the earth it would have consisted of only the basic elements, the non-living materials like the air, water, sunlight etc. It is from this point that the possibility of non-living material giving rise to life forms arise. It is quite possible hypothetically, to consider the idea of a single species at the very beginning of evolutionary time almost 3.5 billion years ago. There are many theories of the exact evolution of life and the search for this knowledge continues. The central ideas of evolution are that life has a history-it has changed over time-and that different species share common ancestors.
A comparison on the common features shows that an evolutionary process and a pattern of relationships exist between species. As lineages evolve and split and modifications are inherited, the evolutionary paths of species diverge. This produces a branching pattern of evolutionary relationships. Such evolutionary change and relationships are represented in "family trees," or 'phylogenetic' trees. The trees show the relationships among organisms illustrate the idea that all of life is related. Many of the phylogenies one encounters are the "family trees" of groups of closely related species. The phylogeny shown below represents the basic relationships that tie all life on Earth together.
This tree, illustrates the idea that all of life is related and can be divided into three major groups of organisms that includes all the descendents of a common ancestor. They are often referred to as the three domains or 'clades': Archaea, Bacteria, and Eukaryota.
Since a phylogenetic tree is a hypothesis about evolutionary relationships, one would use characters that are reliable indicators of common ancestry to build that tree. One uses characters in different organisms that are similar because they were inherited from a common ancestor that also had that character. These are called homologous characters. An example of homologous characters is the four limbs of tetrapods. Birds, bats, mice, and crocodiles all have four limbs. Sharks and bony fish do not. The ancestor of tetrapods evolved four limbs, and its descendents have inherited that feature-so the presence of four limbs is a homology.
Let us look at some common indicators that form a basis for distinguishing evolutionary linkages.
The comparative anatomical studies of various types of organisms show structural and functional differences and similarities between various species. For example, the basic structure of all the flowers is the same with the whorls of calyx, corolla, androecium and gynoecium. However, each species is different in the shape, size and number of the individual members of the species. Similarly, the limb-bone pattern of all the animals with four limbs (tetrapods) is the same - pentadactyl limb. It shows modifications in the different species as shown in the diagram:
As mentioned these organs that have a common basic form but are present in different species would be termed homologous organs. Thus the wings of bat are homologous to the limbs of man.
Certain homologous organs have no function in some species. Such as the appendix in human beings which is non-functional and its homologous organ is functional in herbivorous animals. Such non-functional homologous structures are called the vestigial organs. Other examples of vestigial organs are the tail bone or coccyx in human which is homologous to the ones in the monkeys and the nictitating membrane, the reduced third eyelid that forms a transparent cover over the eyes of underwater creatures.
These homologous features are important in the evolutionary process.
A study of the comparative biochemistry shows that certain similar molecules occur in different groups of animals. This can be compared to the homologous structures.
For example, the structure of haemoglobin present in man, chimpanzee, gorilla and gibbon show remarkable similarities. Haemoglobin is a protein that has four polypeptide chains. Each polypeptide chain is made up of a fixed number of amino acids. The haemoglobin of chimpanzees differs from that of man in only 1 amino acid, of gorilla in 3 amino acids and of gibbon in 8 amino acids. This indicates a common ancestry between the various primate groups.
Comparative morphology involves the study of the external features of organisms. There are three types of evidences that comparative morphology reveals. They are:
There appear to be many structures that are similar because of the function they carry out and in their external appearance. However, these structures differ internally. Such structures are called analogous structures. For example, wings of bat and butterfly are similar in appearance and function but they are internally very different and have different origins. Similarly thorns and spines seen in plants are also analogous structures. Both are pointed structures that are protective in function. However, thorn is modification of stem and spine is modified leaf. Another example of analogous structures in plants is the tendrils of different types carrying out similar function.
The study of morphology also reveals some organisms form links between two different groups. There is further evidence that evolution has taken place and it also shows the direction taken by the process of evolution. These organisms that show the features of two different groups are called the living fossils as they provide a link between the two groups. They are as important as fossils in providing linkages and also they may have been surviving without much variation for many years.
For example, some living fossils are:
Duck-billed platypus, an egg-laying mammal that forms a link between reptiles and mammals; amphibians that show link between fishes and reptiles; lungfish that shows link between amphibians and fishes.
The principle of adaptive radiation is based on two evolutionary mechanisms:
- Divergent evolutions
- Convergent evolutions
Divergent evolution involves the homologous structures. A group of organisms modify a common homologous structure to perform different functions. An example is the modifications of the pentadactyl limbs by seals and otters for swimming, bats for flying, and by homosapeans for manipulation.
Another example of divergent evolution is the modification of the basic mouthparts of the insects into the different types - sucking, biting, etc.
It is because of this adaptive ability that the insects are so widely present and are of so many varieties.
Convergent evolution involves analogous structures. Analogous structures are those that show no similarity in the internal structure or anatomy. However, they carry out similar structural or physiological functions. For example, birds and bats both have wings, while mice and crocodiles do not. Does that mean that birds and bats are more closely related to one another than to mice and crocodiles? No. When we examine bird wings and bat wings closely, we see that there are some major differences.
Bat wings consist of flaps of skin stretched between the bones of the fingers and arm. Bird wings consist of feathers extending all along the arm. These structural dissimilarities suggest that bird wings and bat wings were not inherited from a common ancestor with wings. This idea is illustrated by the phylogeny below, which is based on a large number of other characters. Bird and bat wings are analogous-that is, they have separate evolutionary origins, but are superficially similar because they evolved to serve the same function.
Haeckel (1834-1919) formulated that 'ontogeny recapitulates phylogeny'. This means that the developmental stages that an organism goes through repeat the evolutionary history of the group to which the organism belongs.
Though it does not explain the mechanism of evolution, the study of the embryos and their development reveal that at the embryonic stages there are features that are absent in the adult form. These features are common to a group of organisms. For example, at a particular stage, all the vertebrate embryos possess a single circulation with two-chambered heart showing no separation into right and left halves.
As the development progresses, the embryos change according to group they belong to - fish, amphibian, reptile, bird or mammal. The above situation is retained only in the fish and all other vertebrates deviate from this.
Given below is another example of the embryological evidence that certain animals have a common ancestor:
Study of fossils is called paleontology. Fossils are defined as preserved remains of a living organism that existed on earth a long time ago. Fossils are formed by the preservation of the remains of the organisms that existed in the earlier days. The dead bodies of the organisms are immediately covered by materials which do not allow decomposition at a fast rate. This preserves the dead bodies and forms them into fossils. The existence of dinosaurs has come to be known only through fossils.
In addition to knowledge about the extinct forms, fossils also provide links between two groups indicating that perhaps one species evolved from the other. For example, a fossil called archaeopteryx show features of both reptiles (teeth) and birds (wings).
Thus it provides a link between birds and reptiles.
Entire Organisms
In rare cases, the entire organism may be preserved. This may happen if the dead bodies of the organisms have been frozen into ice during glaciations, encased in hardened resin or tar or trapped in acidic bogs (bogs are marshy areas where acidic conditions prevent any bacterial or fungal decomposition of the bodies).
Hard Skeletal Remains
The skeleton or its parts get preserved with the living matter completely disintegrating.
Moulds and Casts
The skeleton remains for sometime and the sediments harden around it. The soft parts disintegrate and the gaps are filled by fine materials. The skeletal parts also dissolve leaving behind only the moulds and casts. Many details are preserved.
Petrifactions
They are similar to casts in formation but are formed by mineral deposits such as silica, calcium carbonate, etc. Finer details are also preserved.
Impressions
They are only impressions of the remains of the organisms on the fine-grained sediments.
Imprints
They are footprints, trails, tracks and tunnels made by various organisms that are rapidly baked and filled with deposits.
Coprolites
They are faecal pellets that contain evidences of the food eaten. For example, teeth and scales.
The oldest fossil-bearing rocks contain fossils of few organisms that were simple. However, the younger rocks contain more fossils and these fossils show more complex structure indicating that the complex organisms originated from simpler organisms.
One of the best examples of evolutionary record (phylogeny) is of the horse. Almost the complete fossil record was found in North America. The phylogeny of the modern horse can be depicted as follows:
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