HISTORY OF LIFE ON EARTH, Cheat Sheet of Earth science

Download HISTORY OF LIFE ON EARTH and more Cheat Sheet Earth science in PDF only on Docsity! MODULE - 5 The Living World 21 History of Life on Earth SCIENCE AND TECHNOLOGY Notes 20 HISTORY OF LIFE ON EARTH It is a fascinating experience to look-up at the sky on a clear night. Have you not wondered while looking up, when and how our planet earth came into existence? Or how life began and such diverse forms of life that we see around have evolved? These are some of the mysteries that scientists have tried to answer. In this lesson you shall learn how earth was formed, about theories explaining origin and evolution of life forms on this planet. The story will continue upto evolution of humans on earth. OBJECTIVES After completing this lesson, you will be able to: • describe the physical conditions of primitive earth; • discuss the theory of origin of life (Oparin’s theory) and relate it to the changing environment on earth; • become aware of Darwin’s major contributions; • modify Darwin’s thought to incorporate it in Neo-Darwinism; • identify the levels of organic evolution; • list the evolutionary events in the history of life; • trace stages of human evolution through time. 20.1 PHYSICAL CONDITIONS OF PRIMITIVE EARTH The physical conditions on primitive earth were not congenial for life. The earth was extremely hot-a ball of hot gases. SCIENCE AND TECHNOLOGY MODULE - 5 History of Life on Earth The Living World 22 Notes 20.1.1 Solar system and the formation of planet Earth The universe around us is so enormous that it is difficult even to imagine its dimensions. In a far corner of the Milky Way Galaxy, (one of the billions of galaxies comprising the universe), sits our solar system, like a tiny sand particle on a vast sandy beach. Within this system, Earth, the planet on which we live, is one of the planets revolving around a medium-sized star we call the Sun. ACTIVITY 20.1 Use any general knowledge book/geography book/science book/book on environment or use the internet to obtain a picture of earth and the other planets in the solar system. Highlight location of earth with a pen and observe it with respect to the sun. The whole universe formed probably 12 to 14 billion years ago (one billion = 109 or 1,000,000,000) as a result of a ‘Big Bang’ and subsequent expansion. Our solar system came into existence 5-7 GY ago (GY = Giga Year). In its initial stages of formation (4.5 GY) earth was impacted by another planet that caused the spin (that gave us day and night) and tilt (that gave us seasons) of our planet and also led to the formation of moon. For nearly 700 million years (up to 3.8 GY ago), earth experienced frequent and catastrophic bombardment by meteorites of different sizes. Gradually earth’s crust solidified although volcanoes kept on spewing out harmful gases. These gases accumulated and combined to form methane, ammonia and hydrogen cyanide. These three gases, all lethal, along with minor gases like carbon dioxide and carbon monoxide, formed the atmosphere of the primitive earth. Thus, the prebiotic (before life arose) atmosphere was so unlike the present one. Note that there was no oxygen then, a gas so essential for nearly all living organisms. ACTIVITY 20.2 Select 5 friends, each one of you represent one of the 5 stages in the evolution of our planet- earth such as (i) our universe (ii) our galaxy- milky way, (iii) our solar system (iv) planet earth (v) India on this planet. You may dress yourselves in ways that would convey some important information/details about the “characteristics” that each one of you is going to represent. (You can take the help of internet or books or yours elders). Here some graphical representations are given below for your use. You may enlarge the graphics, select the appropriate one for the parts MODULE - 5 The Living World 25 History of Life on Earth SCIENCE AND TECHNOLOGY Notes multicellular organisms with a variety of body plans and life styles,of all those invertebrates and higher plant groups that you are familiar with. Biologists call this period the ‘Cambrian explosion’. (See box I) BOX- I The geological time scale ERA PERIOD EPOCH AGE (Millions of years) MAJOR EVENT Recent Historic time Quaternary Pleistocene 1.8-0.01 Ice ages; Humans appear Pliocene 5-1.8 Ape-like ancestors of humans Miocene 23-5 Continued radiation of mammals and angiosperms Oligocene 34-23 Origin of most mammalian orders Eocene 57-34 Angiosperm dominance and increase in mammalian diversity Cenozoic Tertiary Paleocene 65-57 Major radiation of birds and mammals Cretaceous 144- 65 Flowering plants appear Dinosaurs extinct at the end Jurassic 208-144 Dinosaur dominance First birds Mesozoic Triassic 245-208 Gymnosperm dominance First dinosaurs and mammals Permian 286-245 Radiation of reptiles Carboniferous 360-286 Extensive vascular tree forests, origin of reptiles Devonian 408-360 First amphibians and insects Silurian 438-408 Colonization of land by plants Ordovician 505-438 First vertebrates (jawless fishes) Paleozoic Cambrian 544-505 Origin of most invertebrate phyla 700 Origin of animals 1,500 Origin of Eukaryotes 2,500 Oxygenbuild-up in atmosphere 3,500 Origin of life Precambrian 4,500 Origin of Earth Fossils, the remains of plants, animals and lower living beings provide evidence for the sequence in which different kinds of living organisms came to exist on earth. When a fossil is collected, the age of the sedimentary rock in which it is found is determined and that age is generally taken as the time in earth’s history when that particular animal lived. Paleontologists (Scientists who study fossils)are able to reconstruct the history of life on earth from the fossils collected in sedimentary rocks of different ages. They clearly show that species and higher taxonomic groups (like angiosperms, insects and birds) evolved gradually. (See Box I) SCIENCE AND TECHNOLOGY MODULE - 5 History of Life on Earth The Living World 26 Notes INTEXT QUESTIONS 20.1 1. When did the earth come into existence? ___________________________________________________________ 2. Why did life not exist on primitive earth? ___________________________________________________________ 3. What are fossils? ___________________________________________________________ 4. What is meant by Cambrain explosion? ___________________________________________________________ 5. From the geological time scale, find out the time in million years ago(mya) when: (i) dinosaurs became extinct ______________ (ii) human evolution began ______________ (iii) flowering plants became dominant on earth ______________ BOX II CHARLES DARWIN (1809- 1882) Charles Darwin is, like Isaac Newton in Physics, a giant in Biology, whose theory of evolution revolutionized our understanding of life and its diversification on earth. Darwin was born in Shrewsbury, England on February 9, 1809. He developed a passion for nature from early childhood, a trait he probably inherited from his grandfather Erasmus Darwin. His father wanted him to study medicine at Edinburgh but Darwin did not have the aptitude for it. He also did not pursue studies to become a cleric, his father’s second choice. Darwin was offered the position of a naturalist on board the ship HMS Beagle which he accepted with excitement and enthusiasm. The voyage on HMS Beagle was a major turning point in Darwin’s life. During the five years (1831-1836) of its voyage he discovered rare fossils in Andes Mountains, collected fascinating animals and plants in Atlantic rain forests of Brazil and made observations on the geographic variation in the famous MODULE - 5 The Living World 27 History of Life on Earth SCIENCE AND TECHNOLOGY Notes Darwin’s finches on Galapagos Islands. Darwin gained from all these experiences valuable insights and scientific support for the theory of evolution he was formulating. Upon return to England, Darwin started accumulating more scientific material in support of his theory of evolution through a mechanism that he called natural selection. Darwin’s famous book on “Origin of Species” was published in 1859. Darwin died in 1882 at the age of 73. He was given state funeral and was buried in Westminster Abbey next to the grave of Isaac Newton. Scientists around the world commemorated Darwin in 2009 by celebrating his birth bicentennial and the 150th anniversary of the publication of his famous book on the ‘Origin of Species’. 20.3.1 Diversity of Life Resulted from Evolution When we explore nature, we observe that 1. There is so much diversity of microbes, plants and animals in the biosphere of our planet. 2. Many animals and plants share common features. We humans are similar to rats, horses, elephants and tigers in possessing hair and mammary glands. Further, we share features like vertebral column with birds, snakes, frogs and fishes. In fact, all living organisms have so many characteristics in common, including DNA, the hereditary molecule. 3. There is so much variation even among individuals of the same species. You can easily notice that all your classmates are not alike; they differ from each other in features like height, facial expressions and skin pigmentation. Likewise, individuals in a school of fish, tomato plants in a vegetable garden, Aedes mosquitoes in a water tank, they all show variation in some feature or the other. These three observations lead us to ask important questions. How and why did such huge diversity of life forms arise? Were the diverse life forms present from the beginning of earth’s history or did they arise gradually over a period of time? Why do even remotely related organisms have so many features in common? Is it possible because they all came from a single ancestor? Why is there so much variation within any single species? ACTIVITY 20.3 Collect the pictures of a plant, any animal and a human being from old news paper or magazines. Paste the pictures and note in the table shown below 3 features in which they are similar and three features in which they differ. SCIENCE AND TECHNOLOGY MODULE - 5 History of Life on Earth The Living World 30 Notes 20.3.3 Neo-Darwinism Although Darwin talked about heritable variation, he did not know exactly how heritable characters arose and were passed on from one generation to another. This is because Darwin was not aware of the principles of genetics developed by Mendel a few years before the publication of his book ‘Origin of Species’. Incorporation of Mendelian genetics into Darwinian theory later by evolutionary biologists led to the emergence of Neo-Darwinism. Further refinements in the light of advances in population genetics and other areas of biology led to the modern synthetic theory of evolution. It is to Darwin’s credit that his basic theory of evolution by natural selection finds support even in the most recent developments in molecular biology. Read Box III to learn how natural selection acts on variation to produce new species. Also see figure 20.2 BOX-III Natural Selection in Action Although Darwin and many scientists of his time were convinced that natural selection is the right mechanism by which species evolve, they could not prove it experimentally or otherwise. It was felt that since any visible modification in a species evolves slowly over a long time, the effects of natural selection cannot be demonstrated easily. But, now we know that it can certainly be demonstrated, as you can see from the following examples: 1. Industrial melanism in peppered moth (Biston betularia) (see Fig.20.2) Fig: 20.2a The typica (t) and carbonaria (c) forms of Biston betularia on the light- coloured trunk of a birch tree 20.2 b. The typica (t) and carbonaria (c) forms of Biston betularia on the soot- darkened trunk of a birch tree MODULE - 5 The Living World 31 History of Life on Earth SCIENCE AND TECHNOLOGY Notes Peppered moth is a common moth in England and it occurs in two varieties- a light-coloured variety called typica and a dark-coloured carbonaria. When these moths rest on the light-coloured trunks of trees, the typica moths get nicely camouflaged and cannot be spotted by birds that catch and eat them. But the carbonaria moths being dark against the light background of the bark stand out and are be easily located by the birds and captured. Because of this, the carbonaria moths suffered more predation and therefore always remained in very low numbers in the population. Then the industrial revolution in England in the mid 19th century brought in many coal-based industries and the resulting soot started depositing on the trunks of trees on the countryside. Soon after, scientists noticed that the numbers of typica moths started going down drastically while those of the carbonaria forms increased. How did it happen? The soot deposits made the bark black and against the dark background, the carbonaria forms now had the advantage of camouflage whereas the typica moths became more and more vulnerable to bird predation. Consequently, the carbonaria form increased in numbers while typica numbers decreased. This is natural selection in action. In the industrialized England the dark carbonaria variety of the moth had the selective advantage because they escaped from bird predation more often and lived to leave more offspring for the next generation. Only the best adapted live and leave their genes in the next generation. 2. Evolution of insecticide-resistant mosquitoes In our desperate efforts to control pest and disease-carrying vector insects, we have been spraying pesticides like DDT in large and larger quantities, but have not been able to eliminate them. When we spray a poisonous chemical on a population of mosquitoes, surely many of them die. But in any population there is variation for resistance and a few genetically resistant individuals survive the spraying. They breed and produce resistant offspring. In the next generation the proportion of pesticide-resistant individuals is higher. As spraying practice continues, the entire population becomes resistant in a few generations and thus a genetically distinct variety of mosquito has evolved on whom DDT has no effect. INTEXT QUESTIONS 20.2 1. Who is Charles Darwin? Name his famous book on natural selection. ___________________________________________________________ 2. Mention his two major contributions. ___________________________________________________________ SCIENCE AND TECHNOLOGY MODULE - 5 History of Life on Earth The Living World 32 Notes 3. What is the function of Natural Selection? ___________________________________________________________ 4. What is meant by Neo Darwinism? ___________________________________________________________ 5. Name the evolutionary mechanism that causes organisms to evolve. ___________________________________________________________ 20.4 LEVELS OF ORGANIC EVOLUTION With progress in various fields of Biology, the theory of evolution by Natural Selection became more and more acceptable. The unit of evolution, in the modern synthetic theory of evolution, is the population. It is the population which evolves and not the individual. Variation occurs at the genetic level through mutation and sexual reproduction in the “gene pool” of the population (gene pool means all the different genes in a population of individuals). Natural Selection causes greater reproduction of the variant genes having adaptive advantage. Evolution at the level of the hereditary material or genes that is the gene pool of a population is termed microevolution. Populations of a species differ due to microevolution. Macroevolution or adaptive radiation is the evolution and diversification at the level of species and genera. eg Dinosaurs evolved as runners, fliers, swimmers due to macroevolution or adaptative radiation. 20.5 MAJOR EVENTS IN THE HISTORY OF LIFE As we mentioned earlier, all the different life forms that we see on earth now evolved only gradually. Radiometric dating of geological (earths’) strata and detailed study of the fossils found in them help us in reading important ‘chapters’ in the history of life on earth since its origin 4.5 billion years ago. Geologists have given names to different periods of this history (see Box I). You may recall that microscopic, unicellular prokaryotes were the exclusive life forms on earth for nearly three billion years. Dinosaurs ruled the earth for nearly 150 million years and became extinct 65 million years ago. Recall from sub- section 20.2.3, the appearance of eukaryotes and the Cambrian explosion. If you could have traveled 200 million years back in earth’s history, you would have found neither birds nor flowering plants anywhere on earth! When did we humans arrive on this planet? Just 2 million years back (see the box on geological time scale)! If you consider the 24 hours geological clock with the MODULE - 5 The Living World 35 History of Life on Earth SCIENCE AND TECHNOLOGY Notes • Life originated on earth in the distant past from chemical compounds through a series of chemical changes that occurred in water (chemosynthetic theory) proposed by AI Oparin and Haldane. • Large molecules like proteins were formed which got together surrounded by membrane to produce the precursors of primitive cell. Thus unicellular organisms, came into being. • It has, however, not been possible to create a cell in the lab. • In the geological era called Cambrian, there was a great almost explosive diversification of multicellular organisms of different shapes, sizes and functions. This is known as Cambrian explosion. • Evolution is formation of complex organisms through change from simple ancestral types over the course of geological time. • Darwin’s two major contributions were the idea of : (i) shared ancestry and (ii) natural selection as mechanism of evolution. • According to Darwin, organisms produce more offspring than can survive because environmental resources are limited. • In the organisms’struggle for existence, those with advantageous (variations) characters survive and reproduce to leave more offspring while the disadvantageous variants are eliminated. This is known as natural selection. • With progress in genetics sources of variations were discovered and Darwin’s original theory of natural selection was modified as Neo-Darwinism or modern synthetic theory. • From the time of origin of earth (4.5 billion years ago) to the present, the entire period is divided into different eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic. • Major events of evolution were: Cambrian explosion and advent of mammals in the Cretaceous era. • Human evolution began approximately 1.5 million years ago and main stages were Australopithecus, Homo erectus and Homo sapiens. TERMINAL EXERCISES 1. What were the primitive conditions on earth. Tell your friend/cousin/ colleague. Then ask your friend to name the gas which was absent without which today no life can exist. 2. What are the main points of Oparins’theory of Origin of life? Make a five point quiz on it. 3. Mention Darwin’s two major contributions to evolutionary ideas. SCIENCE AND TECHNOLOGY MODULE - 5 History of Life on Earth The Living World 36 Notes 4. Write a note on NeoDarwinism. 5. List the five major events during geological time period beginning from origin of life. You may begin with origin of animals. 6. State major trends and stages of human evolution. Do you think humans are still evolving? Write five sentences to justify your response. 7. Earlier groups of animals became extinct due to natural happenings. Today, how is it that wild animals have become endangered and are heading towards extinction. 8. Write a ten sentences conversation between your father and yourself justifying the need for conservation of animals living in our forest. ANSWER TO INTEXT QUESTIONS 20.1 1. 4.5 to 5 billion years ago. 2. Too hot and only certain gases present and reducing atmosphere. 3. Remains of organisms who lived in the past. 4. The time 600 millions years ago when sudden formation of different groups of invertebrates on the earth took place. 5. (i) 144-65 mya (ii) 1.5-2 million years ago (mya) (iii) 57-34 mya 20.2 1. Founder of theory of evolution. 2. (i) all organisms related through ancestry (ii) Mechanism of evolution is Natural Selection 3. Fittest individuals in a population survive and reproduce to leave the fit genes in the next generation. 4. Darwin’s theory modified in the light of progress in genetics. 5. Natural Selection 20.3 1. 1.5 to 2 million years ago 2. Australopithecus 3. Homo sapiens 4. Apes 5. Australopithicus