2. Background of Evolutionary Theories

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Writing the earth’s history Geology as a modern science started at the hands of the naturalists in the 17th century, but did found its mark in the next two centuries. The naturalists noticed that the earth has been changing for a long time. There were controversies in deciding over the process of those changes. There were those who thought that the earth’s surface was totally under a turbid ocean of water and the rocks have gradually formed by sediments settling layer by layer. These scientists were known as Neptunists (Neptune being the God of sea). They could not answer all the questions raised by this model such as great variation of in thickness of layers, and also the ultimate fate of all that water. The opponents of Neptunists were Vulcanists (Vulcan being the God of fire) who in general assumed that earth’s central heat was responsible for most geological activities. It is this theory which ultimately prevailed.

It was now realized that similar rocks could be formed at different periods of earth’s history. Older rocks lie in general at the bottom, and younger rocks at the top. The fossils, the relics of the by-gone living beings, were being discovered in between such layers. The English surveyor William Smith (1769– 1838) discovered that particular fossils are peculiar to and are found only in particular strata – methods to compile a geological history of much of Europe. This method of stratigraphy based on the study of fossils could give an accepted history of the earth’s crust and a history of the evolution of living beings.

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There was another controversy, this one between the Catastrophists and Uniformitarians. The former pointed out to the signs of sudden extinction noticed in fossil evidence and believed that earth’s history progressed through discrete catastrophies – such as earthquakes and tidal waves. There were other evidences for such catastrophy, such as vast mounds of boulder clay, gravels and boulders in wrong places. The very short life time of earth allowed by the bible (6000 years!) also supported such catastrophies, otherwise so drastic changes is not possible within such a short time.

The opposite theory was pioneered by geologist from Scotland, James Hutton (1726-97). It says that the changes in earth are gradual, and the same processes of change are active now as it always had been. The slow and steady weathering and erosion by forces like river current are wearing down the rocks. On the other hand, volcanoes and similar intrusion of molten metal are compensating this wear down. This uniformitarian theory was much strengthened by geologist Charle’s Leyll. It was now obvious that the earth is much older than what was thought.

Q. How are the fossils formed?

A. They may form in several ways. Some of these are just impression of some dead organism which originally was created on soft materials later turned hard rock. In some others, the body was actually fossilized, which means that the cells of the body took in grains of rocks which gradually replaced the perishable cells of the original body. Ultimately the whole body structure in all details became made of rocks giving an exact replica.

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In the mean time the earth’s real age and the age of those various rock layers and the fossils in those, were being measured. Various methods were used such as using the rate of sedimentation on the seabed, the cooling rate of the internal heat of earth, - all gave successively longer and longer age – to up to millions of years. Naturally the uniformitarian geologists were vindicated by this, and so were the evolutionists like Leyll. But these methods were not fully correct and these still gave too short an age. Ultimately during the 20th century the use of radioactive decay rate gave the true age of the earth to be 4.5 billion years. The similar method could date the various era represented by the stratigraphy and the fossils with the layers. We have already seen that radioactive materials emmit alpha, beta and gamma particles and decay into other elements, ultimately non-radioactive. There is a definite time, for some thousands of years, which is spent in reducing a radioactive materials to its half mass. This is called half-life. By comparing the ratio of the mass of radioactive and non-radioactive isotopes of a material normally and after decay in an old material, one can find out time elapsed in the decay process.

History of life on the earth

The fossil records show that life has changed forms at various times on earth. The scientists could date roughly when certain type of life first appeared, and also when some of them disappeared – became extinct. Comparing the time with geological time and known theories of past geological and climactic history, the scientists could put up an agreed history with the various causes and forces behind the beginning and extinctions of certain types living things. Also there is a clear indication how a certain type of life evolved from earlier types.

The earliest evidence of life in micro-fossil form comes from 3.8 billion years. Other evidences put the appearance of single-celled life as early as 4 billion years ago, the earth itself being 4.5 billion years old. Real rich fossil evidences abound from the Cambrian period beginning some 570 million years ago – because of the hard shelled marine animal species suddenly exploded at that time leaving a lot of fossils.

The table given here shows the scientists’ designation of geological era and the conjectured types of living organism that abounded during that area. There have been some mass extinctions in this history namely at the end of Ordovician era (500 million year ago), At the end of Devonian era (345 million years) Permian-Triasic (220 million year), end of Triasic (190 million year), and the last one Cretaceous-Tertiary (65 million year). The table also shows what major type of life appeared in a particular era and dominated the era. Thus life started with single celled organism, and then various invertebrate animals evolved in oceans and dominated. Vertebrates first appeared and dominated as fishes, from which amphibians evolved which came to the land along with insects. The reptiles, dinosaurs, birds, mammals followed and dominated during various era.

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The crater created by the meteor that brought about Cretaceous-Tertiary mass extinction (Yucatan, Mexico). 65 million years ago

Every one of these mass extinctions put an end to a big percentage of all species which were living then (in some cases as high as 80-90%). The last one was the death blow to dinosaurs which were the predominant species for long up to that time. Each one created new opportunities for new types of species to evolve, for example the mammal domination after the last one. These mass extinctions were caused by climate change, sea level rise, meteorite strike, or wide scale volcanic eruptions or a combination of several of these. There is a very clear evidence of the cause of the last one – a huge meteorite struck in Yucatan, Mexico.

The ocean of Cambrian-Ordovician age500 million years ago

The progress of early vertebrates370 million years ago

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lobe-finned fish

fish amphibians

Reptiles and insects in carboniferous forests300 million years ago

The age of reptiles: Trasic and Jurassic age190 million years ago

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The ferocious carnivorous dinosaur: Tyrannosaurus (Jurassic age)

The huge herbivorous dinosaurs: Brachiosaurus (Jurassic age) 150 million years ago

Now scientists are warning that we are going through another mass extinction – the sixth one. The rate of extinction of species give such indications. The reason for this one is not however natural, but manmade through the destruction of habitats of the biodiversity. This might bring about a serious consequence for ourselves as a species.

From the fossil records we could follow the changes in the living things. In some cases this has been marked very vividly, for example in the evolution of horse and elephants. The earliest horse lived 50 million years ago with the size of dog and different number of toes and other differences with modern horse. The earliest elephant did not have tusk or trunk. These things gradually

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developed over millions of years towards the modern elephant. The discovery of Archaeopteryx resembling a bird as well as a dinosaur/reptile gave rise to the theory that modern birds evolved from dinosaurs, or both dinosaurs and birds have a common origin in even older reptile. One of the first species which resembled human (homonids) have been identified as Australopithecus Aferensis in Afar areas of Ethiopia in Africa (~ 5 million years) Discovery of a reasonably complete fossil of a female (who was named Lucy) showed that she had many ape-like characteristics, but a few human ones too such as standing erect, and multiple use of hand. This is regarded as a possible missing link in the divergence of human line from the ape line.

Lucy Human-like foot-print from Lucy’s species

Scientist found foot-prints thought to be from Lucy's species more resembling man than ape. Then hominid species more resembling man with bigger brains than Lucy were discovered – Homo habilis, Homo erectus etc. all originating in Africa (~2 million years) – and erectus after spreading outside Africa. Other more advanced species originated in Africa.

Homo erectus 8

One of the latest was Homo sapiens, which is our species– first appearing around 150 thousand year ago. They spread out from Africa approximately 70 thousand years ago gradually populating all continents.

Early Homo sapiens skull fossil

Homo sapiens stone tools (~ 20,000)

Cave painting ~ 20,000

Homo sapiens ~20,000 years

During the 19th century and later, this history of life in earth was gradually revealed through fossil findings and other researches. At the later part of the 20th century this history in general has been on a much more solid foundation by the rapid advance of molecular biology and the wonderful use of DNA research in tracing the history of living beings.

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Q. What is the place in earth where Homo sapiens first evolved? What is the place where first humanlike species evolved? How are we sure about that.

A. The answer is East Africa for both the questions. Homo Sapiens first evolved here some 150 thousand years ago. The first humanlike species evolved here some 5 million years ago. Both fossil evidence and DNA evidence confirmed this origin.

Q. Though life has been in existence for some 4 billion years we have good continuous fossil evidence for history of life a only for last million years or so. Why is that?

A. Roughly at that time called Cambrian Age there was an explosion of fossils called Cambrian Explosion. There were abundant fossils of jelly fish, sponges, starfish, shelled marine animal, sails. Before this era animals were mostly soft bodied and had not left much of fossils. Therefore evidences were scanty for history of life before thus.

Q. There have five mass extinctions of species during the last 550 millions of the history of life in earth. What have been the impacts of these mass extinctions.

A. Great turmoil for life which became precarious across the species. There have been permanent extinction for majority of the species, severe reduction in many, whole families have become extinct. This also had some positive effect on some species opening up new opportunities for them and thrive. For example the extinction of all the dinosaurs during the last mass-extinction 65 million years ago opened up opportunities for mammal to become dominant, and among them later for primates and hominids to thrive and eventually man to become the most dominant species.

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Classification of living things

Carl Linnaeus (1707-78), a Swedish botanist, took upon himself the task of classifying all plants and animals. He constructed the classification pattern from the most obvious relationship visible between species. He grouped most similar species into a higher category called genus (plural: genera), and grouped the genera themselves by even more basic resemblance (order). Linnaeus chose his criterion rather artificially based on a simple important character. For example for plants he used flower parts – the number of pistils and stamens to decide higher levels of division ‘classes’ and ‘orders’. Modern biologist, however added one more division ‘family’ after genera before going to ‘class’, ‘order’ and then ‘phylum’ and ‘kingdom’.

Linnaeus’ another innovation was the binary system of scientific naming – that identifies each species by two Latin (or pseudo Latin) names – the first one gives genus and the second one species. Thus the common house cat is identified as Felis domestica. This is the scientific name used by all. Similarly human has name Homo sapiens; China rose (Joba): Hibiscus rosasinensus; Mango: Mangifera indica; Rice: Oryza sativa. For the house cat species is domestica, genus Felis, Family Felidae, Order Carnivora, Class Mammalia, Phylum Chordata, Kingdom Animalia. Compare it with human as a species. Species sapiens, Genus Homo, Family Homonidae, Order Primate, Class Mammalia, Phylum Chordata, Kingdom Animalia. All living things thus are divided into three kingdoms – i) Protist, ii) Plants and iii) Animals. Protists are bacteria and other single-celled organisms. The table below shows how several animals are grouped together.

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Classification implies both relationship and changes. It shows that living things are alike in many ways and indicates relationships. It also furnishes a basis for the idea that the first living things were simple and that as time passed living things became more abundant and complex; as evidenced by chronologically patterned fossil sequence.

Q. Why did scientists divide all living things into 3 kingdoms?

Q. Why did scientists use Latin names for classification? Why they sometimes use pseudo Latin? e.g. Tenualosa ilisha, Puntius ticto (Puti fish).

Q. What is the meaning of belonging to the same species? How can so many different variety of individuals are within the same species?

A. Close resemblance and fertile procreation between the members are the main criteria to belong to the same species. Even then external appearances may vary a lot between individuals of the same species.

Q. From the classification of living things and the family tree it builds what would be the immediate impression one would get about the relationships of various living beings?

A. They resemble each other – very closely with some, less closely with some, distantly closely with others – indicating a commonality. It is as if many are variations on a common template.

The external resemblances in body plan, appearances, and behaviors are there for anybody to see. Some anatomical resemblances may be more subtle. More recent discoveries at the level of DNA makes the closeness even more pronounced to the extent that even the higher animals have the same DNA code as the bacteria indicating their common origin.

Q. How does the cell theory too creates an essential commonality throughout the whole living world?

A. Cells are the basic unit of all living things from unicellular bacteria to complex animals. Though such diverse organisms differ a lot when considered as a whole, the cells themselves are very similar across the whole living world with their cell membrane, protoplasm, nucleus, DNA etc. Cells are all constituted remarkably similarly. The similarly is even more striking when it comes to the codes carried by DNA in the cells.

Q. There are many species which are now extinct, and we know about many of these because of their fossils. These are also duely named and tried to be placed in the classification of living things. Why is this done?

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A. This is done to try to get the whole picture of the tree of life – even those branches which became extinct. Example: Till some 30,000 years ago there were more than one species of human beings including our own. Apart from Homo sapiens, there were Homo neanderthalis and Homo erectus till comparatively recent time. Before that some two million years ago there were Homo habilis, and some 5 million years ago Australopithecus afaransis (the species of famous fossil ‘Lucy’ ) who are thought to be one of the first hominids. All those except for Homo sapiens are now extinct.

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