Biology Class 11 Notes - Origin of earth, Darwinism, Neo Darwinism

Origin of Earth

According to Nebular hypothesis about 10000 to 20000 millions of years ago . There was a huge mass of highly condensed cosmic material. It exploded (Big bang) to form many nebulae . From one of the nebulae our solar system evolved . Sun evolved from the dense central part and planets evolved from the less denser peripheral part.
                Our earth evolved about five millions years ago. In the beginning it was the spinning ball of hot gases. Its temperature was about 60000c . Gradually, it cooled down and heavy elemnt like Fe, Ni formed the central core and lightest elements like H, C, N, He etc formed the atmosphere but there was no free oxygen therefore the atmospheric condition wa s reducing.

Origin of life

                Different philoshophers and scientist have proposed different explanation for the origin of life . The main theorys for the origin of life are

1.       Theory of spontaneous generation :

According to the Nebular hypothesis, all the living organisms originated spontaneously or automatically from the non-living materials and they remain unchanged . For example : Aristotle proposed that fish, frog, insect etc were developed from the mud filth, tapeworm developed from the excreta of animals.
                But the theory of Abiogenesis was experimently rejected by Fransisco Ready and Lois Pasteur and they proposed theory of biogenesis (i.e. lifeoriginated from life).

2.       Theory of special creation :

According to this theory, all the living organisms were created by super natural power (God) in the form as they are present today either at a time or at regular interval.

3.       Theory of panspermia or cosmozoic theory :

According to this theory life came on the earth from other planets in the form of seeds and spores.

4.       Theory of catastropism and sudden origin of life :

Many violent and destructive changes occured on the earth time to time which destroyed existing life and formed inorganic materials life originated suddenly.

5.       Modern theory or chemosysthetic theory :

It was proposed by Oparin and supported by Haldane. It states that the primitive life originated in the water bodies on the primitive earth by chemical evolution through a series of chmical reactions. According to this theory , atom combined to form molecules which reacted to form simple organic compounds like alcohol, aldyhides, fatty acids,
lycerol, simple sugar, amino acids, purines, pyrimidines, etc. These simple organic compunds reacted to form complex organic compounds like polysaccharides, proteins, fats, nucleotides, nucleic acids etc . which accumulate to form coacervates aand the coacervates finally organised to form living cells.
                Oparin explained his thory in his book “The Origin of life on the earth” published in 1936 A.D. This explanation of origin of life can be described stepwise as follows :

A.      Chemogany: (chemical evolution)

Conditions on the primitive earth were such which favoured the chemical evolution. When the surface temperature of the earth was 1000c , it atmosphere had nitrogen in the form of NH3 , carbon in the form of CH4 and oxygen in the form of H2O vapour but there was no free oxygen. So, the primitive atmosphere was reducing.
                As the earth cooled down it developed a solid earth crust which later formed allevations and depressions.
                The atmospheric water vapour condensed and finally come on the surface as rain. Rain water collected in the depressions dissolved the minerals and finally organised to form large water bodies called oceans.
                The minerals and simple molecules present in the water of ocean combined and recombined by various ways to form the simple organic compounds like alcohol, aldyhides, fatty acids, glycerol, simple sugar, amino acids, purines, pyrimidines, etc.The energy for the reaction was provided by the heat of the earth and the electrical energy produced during lightening.
                The simple organic compounds through the process of polymerization fromed complex organic coumpunds like polysaccharides, proteins, fats, nucleotides, nucleic acids etc .
                The complex organic compounds accumulated to form colloidal shell like aggregates called coacervates having self – duplicating molecules and capable of growth and division.

B.      Biogany ( Formation of Primitive lives):

 Some of the proteins of cacervates started acting as enzymes (bio – catalyst). Nucleic acid associates with the proteins to form nucleoproteins which had unique quality bof duplication and behaving like chromosomes. The self duplicating capacity in nucleo proteins is due to the nucleic acids , which carrys hereditary information. Thus, nucleic acid had shown the first sign of life. Later , a thin walled cell like structure developed called protobionts or eobionts. Eobionts gave rise to prokaryots and eukaryots.

C.      Nature of primitive life and its evolution :

The first formed cells were anaerobic, prokaryotic and chemoheterotropic.
These started taking organic compounds from sea water as the source of energy and to make protoplasm. They started mitosis and increased their numbers . Now to face with the decreasing supply of organic compounds. Some of those evolved into chemoautotrophs and started synthesizing own organic food from simple inorganic compounds in presence of chemical energy and enzymes. For example : Nitrifying bacteria, sulphate reducing bacteria etc.
                Again now , to face the problem of decreasing inorganic compounds , some of those developed chlorophyll and started photosysthesis whcih lead to the evolution of photo autotrophs and in the process of photosynthesis O2 gas was  evolved as a byproduct so aerobic autotrophs like cyanobacteria were developed. Due to the photosynthesis and evolution of oxygen the atmosphere of the earth become oxidizing . For long period the only forms of life on the earth were bacteria, moulds , cyanobacteria, etc. Gradually eukaryots like algae , fungi, protozoans were developed.

Miller and Urey Experiment :

                Miller and Urey experimentally proved the formation of simple organic compounds from simpler molecules under reducing condition. They subjected mixture of CH4, NH3, H2O,H2 for about 1 week over an electric spark between two tungsten electrodes in a gas chamber placed under the reducing condition . CH4, NH3 and H2 were taken in the ratio of 2:1:2. They passed hot products through a condenser then chemically analyzed the products by chromatographic method. They found that many organic compounds like amino acids, purines, pyrimidines which are essential for the formation of nucleoproteins were present in the product. This experiment supports the Oparin theory of Origin of life.

Evolution:

Evolution is the gradual continuous , irreversible process of change and development of simple and primitive things to complex and organized things. Charles darwin proposed tha term organic evolution which states that present complex organisms have evolved from simple and primitive form of life through modifications also called “Descent with modification”.
                The earliest view for the evolution was based on mythology. According to Hindu Mythology, God Brahma created all living organism from his own body and all living things are made up of five basic elements air, water, fire, earth and sky.
                Aristotle put forward his taxonomic views . He explained that higher organisms were evolved from lower organisms . He arranged the organisms in a graded series from lower to higher.

Evidences for organic evolution :

1.       Morphological and Anatomical evidences :

Taxonomy based upon similarities and disimilarities among the various groups of oraganisms was made. In all the vertebrates dorso-tubular nervechord, notochord, gillslits are present at least in embryonic stage. In fish heart is two chambered and only internal ear, in amphibians heart is three chambered and car has both middle and internal parts in reptile heart is incompletely four chambered , in mammals external ear pinnae also present . This shows that all the vertebrates are interrelated and they have evolved from some common ancestors.

2.       Homologous organs :

The organs having similar structure and develpment origin but appear differently and performance or function is also different are called homologous organs.
For example: Forelimbs of vertebrates like seal , bat , horse , man etc. appear different and perform different functions but they have similar structure and arrangement of bones, muscles, nerves, blood vessels etc.
This relationship between the structure having similar structure is called homology or divergent evolution.

3.       Analogous organs :

The organs having similar functions but different structures are called analogous organs.
                For example : Wings of birds and insects.
This relationship between the different groups due to their similar funtions are called analogy or convergent evolution.

4.       Connection link :

The organisms having the character of two different groups. They proved that one group had evolved from another through gradual modifications are called connecting link.
                For example : Peripatus is the connection link between annelida and arthopoda. The arthopoda charaters are compound eyes, antennae, semi-jointed legs. The annelida characters are wormlike body, segmental nephridial excretory organ.
                Protopterus is the connecting link between fish and amphibians .
Ornithorychus and echidna are egg laying mammals. They are similar to mammals having mammary glands and are similar to reptiles because they give hard shelled eggs, abdominal testes , cloaca etc.

5.       Vestigial organs :

The organs which are reduced and non-functional but were functional in the ancestors and also functional in the related animals called vestigial organs.
                Example : The vestigial organs in Man are as follows :
Ø  Nictitating membrane
Ø  Verniform appendix
Ø  Coccyx
Ø  Ear muschles

6.       Palaeotological evidences :

It includes the study of fossils. Fossils are the remains of past organisms found in the sedimentary rocks. Fossils provides direct and most convincing evidences in support of organic evolution. Fossils includes hard structure like teeth, bone, goot print, shell etc. Fossils are found in different layers of rock. So, it is essential to estimate the age of the rocks in which fossils are found. To estimate the age of the fossils radioactive dating method i sused.
                The most important finding from fossils record which help to explain evolutionary line are as follows :
Ø  Most primitive forms of life are recorded in the ldest reocks.
Ø  There was existance of diverse group of organisms which are not exactly similar to the organisms that exist today.
Ø  Different fossils are formed at different time periods.
Ø  There is succession of high and more complex form of fossils from older to recnet layer of rocks.
Ø  Large groups of organisms have become extinct but some species are persisting int he same or modified form.
Ø  Some organisms whcih remained scarce in one period of era become dominat in next period of era due to adoptive radiation.

7.       Missing link :

These are those extinct organisms which had the characters of two different groups. It shows the evolution of one group from another group.
                For example : Archaeopteryx
Fossils of Archaeopteryx was discovered by Wagnar in Germany. It was about the size of crow and had characters of both reptiles and aves. The reptil charaters are teeth in jaw , claws in the fingers , a long tail etc. and aves characters are feathers, wings,etc.
                These structure suggests that birds are evolved from reptiles like forms which become extinct. These fact explains the statement – “Birds are glorified reptiles “.

8.       Phylogany :

It is the evolutionary history of animals. Now, the scientist have collected the complete series of fossils of many animals . For example: Elepant, camel, horse etc. On the basis of these fossils we are able to construct the philogeny of many animals.

Phylogany of horse :

The evolution of horse was started in North America about 60 millions years ago. The ancestor of modern horse was Eohippus. It was about the size of a fox with short limb , four toes in the fore limbs and three toes in the hind limbs . The probable of descent passes form Eohippus through Mesohippus , Miohippus, Merychippus, Pliohippus to the modern horse Eqqus. The changes took place during this evolution are as follows :
Ø  Enlargement in size.
Ø  Reduction in the number of side toes.
Ø  Enlargement of third digit , elongation in the limbs, fore head and neck also.
Ø  Development of premolar and molar teeths.

9.       Embryological evidences :

It is based on the camparative study of various animals. It shows that –
Ø  The early development of all triphoblastic and diploblastic animals are similar.
Ø  The embryo of all the vertebrates consists of gillslits, notchord, simple two chambered heart lateron these organs develped into different forms. This shows that all vertebrates have evolved from some common ancestors.

10.   Recapitulation theory or Biogenetic law :

It states that “Ontogeny repeats Philogany” which means that an individual during its own develpment passes through their evolutionary history.
For example : Fish like characters like fins , gills, tale with tale fins are present on the larva of a frog (i.e. Tadpole). This shows that amphibians have evolved from fish like ancestors.

11.   Physiological and Biological evidences :

Ø  Study of protoplasm in different organisms shows that it is made up of similar molecules like carbohydrates, nucleic acids, fats etc.
Ø  The similar enzymes of different animals are simlar in structure and mode of fraction.
Ø  The chromosomes of all the organisms are basically made up of nucleic acid and proteins.
Ø  Blood groups A, B , AB and O are found in the human beings , blood groups A and B also found in Apes. Thish show that man are very much simlar to Apes.