33 Evolutionary History of Life

Dr. Rajesh Kumar Gautam

CONTENTS:

1. Learning outcomes: At the end of the module the reader will know

Formation of earth
Origin of life
Theory of Divine or special creation
Theory of Spontaneous generation
Theory of Cosmozoic origin
Theory of eternity
Theory of Catastrophism
Theory of Organic evolution
Synthetic theory of evolution
Pre-biotic chemistry
First Evidence of Early Life
Prokaryote to Eukaryotes
Unicellular to Multicellular
Photosynthesis
Sexual Reproduction
Geological Time Scale

Introduction

The life on the earth was originated by the process of complex physical and chemical reactions. When earth came into existence it was hot. The formation of earth crust has taken billion of years. By the combination of Oxygen and Hydrogen atom water was formed which is an essential part of life. Gradually, continuous thunder, lighting, raining has created environment for emergence of super matter having consciousness from unique combination of simple matter and energy. Atoms of carbon, nitrogen, hydrogen, oxygen, phosphorus and some other elements combined and given shape to macro molecules or polymers like RNA and DNA which have the capacity of replication and reproduction. There are many speculations about how and when life was originated. Scientific theories about origin and evolution life are based on facts and evidences.

There are different theories about the origin of life. Before knowing about these theories, let we understand about the origin of universe because it is prerequisite for origin of life. In a book entitled “A Brief History of Time – From the big bang to black holes” the famous astronomer Stephen Hawking states that the universe was originated from an explosion of dark matter condensed at a place. This explosion was so powerful that the matter was spread in the universe and different celestial bodies like stars, planets, satellites were formed. In this process, by some of the matter the Sun its planets and satellite were formed. Our Sun and solar system is a small part of a family of many such stars (Sun) and their solar system which is called Galaxy. There are many such Galaxies in the Universe, according to an estimate; their approximate number is 170 billion (1.7 × 1011) galaxies in the observable universe ( Gott III et al. 2005). Each of these galaxies has billions of stars (sun). Small or dwarf galaxies have just a few thousand (103) stars, whereas the giants galaxies have one hundred trillion (1014) stars (Uson et al. 1990). Our own Galaxy called Milky Way is estimated to contain as high as one trillion stars (Odenwald 2014). After billions of years of Big Bang explosion the planet earth came into existence. After 1.0 to 3.5 billion of years of emergence of the planet earth, the earliest form of life was originated in the form of prokaryotic cells viz. virus and bacteria. Gradually, unicellular and multicellular organisms were evolved. The main theories about the origin of life on the earth are as follows:

1. Divine or special creation Theory

2. Spontaneous generation Theory

3. Theory of cosmozoic origin

4. Theory of eternity

5. Theory of Catastrophism

6. Theory of Organic evolution

7. Synthetic theory of evolution

Divine or special creation Theory

Divine or special creation theory is one of the oldest theories about the origin of life on the earth. According to this theory life was created by supernatural power. In every religious scripture there is mythology about origin of the Universe, Earth and Life. According to Hindu mythology the world was created by God Brahma. He is considered the God of creation. He created the whole universe. The first man was Manu and the first woman was Shradha.

According to Islam and their holy Quran the Universe was created by Allah (God). Allah begins by stating that the universe and planet Earth took six “days” to create. (7:54)

According to the Bible, the world was created within six days. On the first day, God made the heaven and the earth, on the second day; He separated the sky from the water, on the third day. He made the dry land and plants, on the fourth day. He formed the sun, the moon and the stars, on the fifth day. He made the fishes and the birds and on the sixth day, He formed the land animals and human beings. The first man, Adam and the first woman, Eve were created by God.

Buddhists do not believe in any such divine or special creation. They have opinion that one does not need to know the origin of life in order to achieve enlightenment. Many Buddhists do not think about these kinds of questions as meaningful for the Buddhist goal of relieving oneself and others from suffering.

Special creation theory lacks scientific evidences, on account of which it cannot be accepted.

Theory of Spontaneous Generation (Abiogenesis or Autogenesis)

Theory spontaneous generation or Abiogenesis or Autogenesis states that the life originated from nonliving things in a spontaneous manner. This concept was held by early Greek philosophers like Thales, Anaximander, Xanophanes, Empedocles, Plato, Aristotle, etc. In ancient Egypt, it was believed that the mud of the Nile could give rise to frogs, toads, snakes, mice and even crocodiles when warmed by the sun.

Van Helmont (1577-1644) held that human sweat and wheat grains could give rise to organisms. He placed a dirty shirt in a receptacle containing wheat bran and found that after 21 days the gases from the shirt and wheat had formed living mice.

Louis Pasteur, a French scientist took broths in a long necked flask and then he bent the neck of the flask. He boiled the broths in the flask to kill any microorganisms that might be present in them. The curved neck acted as a filter. If the flask with ‘swan neck’ (curved neck) is kept for months together, no life appeared, as the germ laden dust particles in the air were trapped by the curved neck which serves as filter.

If the swan neck was broken off, the broths developed colonies of moulds and bacteria. Thus, he showed that the source of the micro-organisms for fermentation or putrefaction such as for milk, sugar and wine, etc., was the air and the organisms did not arise from the nutrient media.

Thus Louis Pasteur (famous for “Germ Theory of Disease and Immunology”) finally disapproved abiogenesis and proved biogenesis

But according to biogenesis, life originated from pre-existing life which does not explain the origin of life. So abiogenesis is also disapprovedTheory of Cosmozoic Origin:

This theory was proposed by Richter (1865). According to this theory, ‘protoplasm’ reached to the earth in the form of spores or germs or other simple particles from some unknown part of the universe with the cosmic dust, and subsequently evolved into various forms of life. Helmholz (1884) speculated that ‘protoplasm’ in some form reached the earth with falling meteorites.

Arrhenius (1908, Nobel Prize Winner of 1903 in Chemistry) postulated the Panspermia Theory and stated that organisms existed throughout the universe and their spores etc., could freely travel through space from one star to the others. But, experiments have proved that Living matter cannot survive in the extreme cold, dryness and ultra-violet radiation hence the theory of cosmozoic origin was also discarded. This theory also fails to explain how the life originated, inspite, it advocated that the life had travelled from one to another planet.

Theory of Eternity of Life:

Theory of eternity states that there is no beginning of life and it will never end. It was as it is, and it would remain as it is. Some of the Indian religion for example: Jainism and Sikhism also have similar concept about the life. They believe that life is eternal, it has no beginning, it will never end.

Theory of Catastrophism:

Georges Cuvier (1769-1832), Father of “Modern Palaeontology” and Orbigney (1802- 1837) were the chief advocates of this theory. According to this theory, cataclysms (great destruction) or catastrophic (concerning disastrous event) revolution occurs upon earth from time to time which completely destroys all organisms (living beings).

New organisms, then, suddenly form from inorganic matter. Each creation consists of life quite different from that of the previous one. In fact, this theory is merely a modification of theory of special creation. This theory is also not accepted.

Theory of Organic Evolution

Charles Darwin (1859) in his book entitled ‘Origin of Species’ stated how new species evolved from pre-existing ancestors by the process of natural selection, which operates through struggle for existence and survival of the fittest. And, in this way, only few individuals, who are best, got chance of reproduction. This process repeated in every generation, and after many generations the progeny of distinct characteristics evolve as a ‘new species’. The theory of organic evolution got wide recognition among the scientists.

Synthetic theory of evolution

a new theory of evolution known as mutationism, which essentially did away with natural selection as a major evolutionary process. According to de Vries (joined by other geneticists such as William Bateson in England), there are two kinds of variation that take place in organisms. One is the “ordinary” variability observed among individuals of a species, which is of no lasting consequence in evolution because, according to de Vries, it could not “lead to a transgression of the species border even under conditions of the most stringent and continued selection.” The other consists of the changes brought about by mutations, spontaneous alterations of genes that yield large modifications of the organism and gave rise to new species: “The new species thus originates suddenly, it is produced by the existing one without any visible preparation and without transition.”

The formation of Earth

Earth was formed around 4.54 billion (4.54×109) years ago by accretion from the solar nebula. Volcanic outguessing probably created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. One very large collision is thought to have been responsible for tilting the Earth at an angle and forming the Moon. Over time, the planet cooled and formed a solid crust, allowing liquid water to exist on the surface.

The first life forms appeared between 3.8 and 3.5 billion years ago. The earliest evidences for life on Earth are graphite found to be biogenic in 3.7-billion-year-old meta sedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48-billion-year-old sandstone discovered in Western Australia. Photosynthetic life appeared around 2 billion years ago, enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose. During the Cambrian period it experienced a rapid diversification into most major phyla. Although more than 99 percent of all species that ever lived on the planet are estimated to be extinct,there are currently 10–14 million species of life on the Earth.

Pre-biotic chemistry

Before origin of primordial form of life, there were different elements in gaseous, liquid or solid state. How abiotic substances transformed into biotic form or cell during early state was initially based on speculations, but later it was established on the basis of scientific experiments. In the periodic table there are more than 115 elements. But, only some selected elements are basis of life chief of them are: hydrogen, oxygen, nitrogen, phosphorus, carbon etc. In the beginning, the crust of earth was hot, whole atmosphere were unsuitable for present day life. There were continuous thundering, lighting and raining. Because of such physical environment, the chemical substances available were bonding with each other and making different compounds which were essential for life, for example: two gases

oxygen and hydrogen bonded together and formed a liquid i.e. Water (). Water can be considered as the hydride of oxygen or the oxide of hydrogen. Water is one of the most abundant compounds in the universe. Life in the solid state would be difficult, as diffusion of metabolites would occur at an appallingly slow pace. Conversely, it is improbable that life in the gas phase would be able to support the stable associations required for the propagation of genetic information, and large molecules are generally non-volatile. Thus it would appear that life would need to exist in a liquid medium. The question then becomes what solvent molecules are prevalent and able to exist in the liquid phase over the range of temperatures where reasonable reaction rates might proceed while at the same time preserving the integrity of the solute compounds. The high temperature limit is set by the decomposition of chemical compounds, while the low temperature limit is determined by the reactivity of the solutes. Water has the largest liquid stability range of any known common molecular compound at atmospheric pressure, and the dielectric constant of water and the high heat capacity are uniquely suited to many geochemical processes (Miller and Cleaves 2007).

There are no other elements besides carbon that appear to be able to furnish the immense variety of chemical compounds that allow for a diverse biochemistry. Carbon is able to bond with a large variety of other elements to generate stable heteroatomic bonds, as well as with itself to give a huge inventory of carbon-based molecules. In addition, carbon has the exceptional ability to form stable double bonded structures with itself, which are necessary for generating fixed molecular shapes and planar molecules necessary for molecular recognition. Most of the fundamental processes of life at the molecular level are based on molecular recognition, which depends on the ability of molecules to possess functional groups that allow for weak interactions such as hydrogen bonding. Carbon appears unique in the capacity to form stable alcohols, amines, ketones, and so on (Miller and Cleaves 2007).

One carbon and four hydrogen atoms were bonded and made a compound called Methane(). In the same way, hydrogen and nitrogen atom bonded and formed Ammonia. Carbon dioxide and carbon monoxide were made by bonding of carbon and oxygen atoms. These chemical compounds further combined and made acids like: Formic acid, acetic acid, urea etc. further combination of these compounds formed amino acid viz. Alanine, Valine, Methonine, Glutamic acid, Lactic acid and so on; which can be further understood by following diagramme.

Figure: Diagrammatic representation of pre-biotic chemistry.

The Miller-Urey experiment

In the 1930s, Oparin and Haldane independently suggested that ultraviolet radiation from the sun or lightning discharges caused the molecules of the primordial atmosphere to react to form simple organic (carbon-containing) compounds. This process was replicated in 1953 by Stanley Miller and Harold Urey, who subjected a mixture of H2O, CH4, NH3, and H2 to an electric discharge for about a week. The resulting solution contained water-soluble organic compounds, including several amino acids (which are components of proteins) and other biochemically significant compounds.

First evidence for early life

The earth itself was formed about 4.6 billion years ago. The oldest clear evidence of life—microfossils in ancient rock—are 3.5 billion years old. The origin of life seems to have taken just the right combination of physical events and chemical processes. Alexander Oparin, a Russian chemist, proposed that life must have arisen from nonliving matter under a set of very different environmental circumstances sometime in the distant history of the earth. His theory is known as primary abiogenesis. Primary because all living cells are now known to come from previously living cells, except in that first case. At the same time, J. B. S. Haldane, a British geneticist, was also independently espousing the same views. Oparin stated that in order for cells to evolve, they must have had some means of developing chemical complexity, separating their contents from their environment by means of a cell membrane, and concentrating materials within themselves. He termed these early, chemical-concentrating bubbles like structures protobionts.

What do we know about the earliest life-forms? The fossils found in ancient rocks show an obvious progression from simple to complex organisms, beginning about 3.5 billion years ago. Life may have been present earlier, but rocks of such great antiquity are rare, and fossils have not yet been found in them. The earliest identified organisms were minute and relatively featureless, and their fossils look like small rods, which are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria (Brasier et al. 2006).The earliest evidence of life appears in microfossils, fossilized forms of microscopic life. Microfossils were small (1 to 2 micrometers in diameter) and single celled, lacked external appendages, and had little evidence of internal structure. Thus, they physically resemble present day bacteria. Such organisms with this simple body plan are called prokaryotes, means “before” and “kernel,” or “nucleus.” The name reflects their lack of a nucleus, a spherical organelle characteristic of the more complex cells of eukaryotes.

It is apparent from fossil record that eukaryotes did not appear until about 1.5 billion years ago. Therefore, for at least 2 billion years—nearly a half of the age of the earth—bacteria were the only organisms that existed.

Prokaryotes to Eukaryotes

Prokaryotes are essentially single celled organism; such cells were lacking the nucleus. In the course of evolution of life, after prokaryotes or nucleus-less single celled organism; Eukaryotes were evolved which having nucleus. Initially, they were unicellular but gradually they evolved into multicellular organism.

The origin of the eukaryotic cell is considered a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The timing of this series of events is hard to determine; Knoll (2006) suggests they developed approximately 1.6–2.1 billion years ago. Some acritarchs are known from at least 1.65 billion years ago, and the possible alga Grypania has been found as far back as 2.1 billion years ago (Knoll 2006).

Unicellular to Multicellular

There are various mechanisms by which multicellularity could have evolved. One hypothesis is that a group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit. This is essentially what slime molds do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a syncytium. A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected cells in one organism (this mechanism is observable in Drosophila). A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could later develop specialized tissues. This is what plant and animal embryos do as well as colonial choanoflagellates (Fairclough, 2010).

Because the first multicellular organisms were simple, soft organisms lacking bone, shell or other hard body parts, they are not well preserved in the fossil record.

The symbiotic theory suggests that the first multicellular organisms occurred from symbiosis (cooperation) of different species of single-cell organisms, each with different roles. Over time these organisms would become so dependent on each other they would not be able to survive independently, eventually leading to the incorporation of their genomes into one multicellular organism.

The first evidence of multicellularity is from cyanobacteria-like organisms that lived 3-3.5 billion years ago (Grosberg and Strathmann, 2007). Complex multicellular organisms were evolved in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and terrestrial plants.

Geological time scale

The evolution of life can be better understood by the help of Geological Time Scale. As you know our calendar is merely 2015 years old. For chronological presentation of incidents older than 2015, the birth of Jesus Christ is being used as landmark; for example, 300 BC or 1000 BC, which means 300 or 1000 years before from the birth of Jesus Christ. This has a limit. To describe the timing and relationships between events that have occurred throughout Earth’s history system of chronological measurement that relates stratigraphy to time is used known as Geological Time Scale. Evidence from radiometric dating indicates that Earth is about 4.54 billion years old. For convenience and better understanding of evolution of life a compact Geological Time Table is presented which is self explanatory.

The history of earth is divided into eras, epochs and periods- the oldest era was Archeozoic which was 3600 million or 3.6 billion years old, no recognizable fossils were recovered from the strata of this era. The Archeozoic was followed by Proterozoic which was 1600 million or 1.6 billion years old during this era primitive algae, protozoan, molluscs and invertebrates were evolved. This was followed by Palaeozoic era, which was 280 million years old and divided into seven Periods: Permian, Carboniferous, Devonian, Silurian, Ordovician and Cambrian which was 600 million years old. During this era the life evolved from aquatic to terrestrial. Amphibians and terrestrial plants were evolved during Devonian and Ordovician period; followed by reptiles and birds during Carboniferous and Mammals during Permian.

The Mesozoic era is divided into three period viz. Cretaceous, Jurassic and Triassic. During this era giant reptiles like dinosaurs, birds and archaic mammals were evolved. This era was 135 million years old. After that Cenozoic era was started which is still continuing. It was divided into two periods and seven epochs. During this era, most of the species of present day plants and animals were evolved. Ancestors of modern man like Australopithecus were evolved during Miocene epoch followed by Homo erectus known as Pithecanthropus erectus, Sinanthropus pekinensis, Homo Narmdadensis and their contemporary archaic hominids were evolved during Pliocene and Pleistocene, 2-6 million years ago. The ancestors of modern man were evolved just 2 to 2.5 lakh years ago.

Photosynthesis

In the history of evolution of life, Photosynthesis is a great landmark. It is a process used by plants and other organism to convert light energy, normally from the Sun, into chemical energy that can be later released to fuel the organisms’ activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis was given. Photosynthesis arose and oxygen began accumulating in the atmosphere about 3 billion years ago. Today, all green plants are autotrophic because of photosynthesis. Animals are dependent on the plants, for source of energy. A type of organism known as cyanobacteria evolved some 3 billion years ago. Cyanobacteria were capable of photosynthesis. Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old (Davis 2004).

Sexual reproduction

Sexual reproduction is a biological process that creates a new organism by combining the genetic material of two organisms by means of meiotic cell division. Sexual reproduction is prime method of reproduction for the vast majority of macroscopic organisms, including almost all animals and plants. The first fossilized evidence of sexual reproduction in eukaryotes is about 1 to 1.2 billion years ago. Why sexual reproduction developed and why it is maintained? The reasons include fighting the accumulation of deleterious mutations, increasing rate of adaptation to changing environments, dealing with competition or as an adaptation for repairing DNA damage and masking deleterious mutations. Ultimately, it is a means of genetic variation; and provided way for biological evolution of life.

Modern form of life

The Cambrian Explosion is the term given to the time period between 570 and 530 million years ago when most modern groups of animals evolved. The Cambrian Explosion refers to an unprecedented and unsurpassed period of evolutionary innovation in the history of our planet. During the Cambrian Explosion, early organisms evolved into many different, more complex forms. During this time period, nearly all of the basic animal body plans that persist today came into existence.

First Vertebrates

The first back-boned animals, also known as vertebrates, evolved about 525 million years ago during the Cambrian Period. The earliest known vertebrate is thought to be an animal that have had a skull and a skeleton made of cartilage. Today there are about 57,000 species of vertebrates that account for about 3% of all known species on our planet. The other 97% of species alive today are invertebrates and belong to animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.

The first land vertebrates evolved about 360 million years ago. Prior to about 360 million years ago, the only living things to inhabit terrestrial habitats were plants and invertebrates. Then, a group of fishes know as the lobe-finned fishes evolved the necessary adaptations to make the transition from water to land.

Amphibians, Reptiles and Birds

Between 300 and 150 million years ago, the first land vertebrates gave rise to reptiles which in turn gave rise to birds and mammals. The first land vertebrates were amphibious tetrapods that for some time retained close ties with the aquatic habitats they had emerged from. Over the course of their evolution, early land vertebrates evolved adaptations that enabled them to live on land more freely. One such adaptation was the amniotic egg. Today, animal groups including reptiles, birds and mammals represent the descendents of those early amniotes.

Genus Homo

The genus Homo first appeared about 2.5 million years ago. Humans are relative newcomers to the evolutionary stage. Humans diverged from chimpanzees about 7 million years ago. About 2.5 million years ago, the first member of the genus Homo evolved. Our species, Homo sapiens evolved about 500,000 years ago. Anatomically modern humans were evolved 250,000 years ago.

Epilogue

It is very clear from above description that the life on this planet was originated in distant past by the process of complex physical and chemical reactions in a particular kind of environment in which abiotic or prebiotic material were transformed into primordial cells or unicellular organism. Little more than a billion of years, the earth was populated by unicellular organism only, then by course of evolution, organism like cyanobacteria were evolved; who have the capacity of photosynthesis. These Prokaryotes were evolved into Eukaryotes and multicellular organisms. Thereafter, by means of meiotic cell division and sexual reproduction the genetic variation was maintained life evolved into its present day shape from aquatic to amphibians, reptiles, terrestrial species, birds, mammals, primates, apes and humans.

Life have certain characteristics e.g. mobility, sensitivity, growth, development, reproduction, regulation and homeostasis. The basic components of all organisms are same. They all have some of common chemical elements; these are hydrogen, oxygen, nitrogen, carbon etc. Basic internal cellular structures of all organisms are same. The hereditary material i.e. DNA and RNA and its structure are also same in all living organism.

Today, because of scientific advancement many impossible things are possible, but still we are unable to create life or a cell from the known matter from which a cell is made. It doesn’t carry the meaning that the life has supernatural origin. The reality is that after all scientific advancement we are unable to create the environment by which the earth has pass through in its distinct past, during which the pre-biotic matter were transformed into a cell and started reproduction by the means of cell division.

Many times the evolution of life is questioned by ridiculous objections. For example, which came earlier, Hen or Egg? They have their own logic to question the evolution, according to them there can’t be Egg without Hen and there can’t be a Hen without Egg. But, answer is simple first Egg came thereafter Hen, how it is? Very simple, first unicellular organism were evolved and from them multicellular were evolved. An ‘Egg’ is basically, a fertilized unicellular zygote which gradually develops into multicellular embryo then Hen.

Summary

It is very clear from above description that the life on this planet was originated in distant past by the process of complex physical and chemical reactions in a particular kind of environment in which abiotic or pre-biotic material were transformed into primordial cells or unicellular organism. Little more than a billion of years, the earth was populated by unicellular organism only, then by course of evolution, organism like cyanobacteria were evolved; who have the capacity of photosynthesis. These Prokaryotes were evolved into Eukaryotes and multicellular organisms. Thereafter, by means of meiotic cell division and sexual reproduction the genetic variation was maintained life evolved into its present day shape from aquatic to amphibians, reptiles, birds, mammals, primates, apes and humans.

Despite the basic biological, chemical, and physical similarities found in all living things, a diversity of life exists not only among and between species but also within every natural population.

Today, because of scientific advancement many impossible things are possible, but still we are unable to create life or a cell from the known matter from which a cell is made. It doesn’t carry the meaning that the life has supernatural origin. The reality is that after all scientific advancement we are unable to create the environment by which the earth had passed through in its distinct past, during which the pre-biotic matter were transformed into a cell and started reproduction by the means of cell division. The history of evolution of life on earth can be summarized as below:

·	Formation of earth 4.6 billion years ago.
·	Simple cells (prokaryotes) evolved 3.6 billion years ago.
·	Cyanobacteria performing photosynthesis since last 3.4 billion years.
·	Sexual reproduction was started 1.2 billion years ago.
·	Fish and proto–amphibians evolved 500 million years ago
·	Amphibians 360 million years ago.
·	Reptiles 300 million years ago.
·	Mammals evolved 200 million years ago.
·	Primates evolved 60 million years ago.
·	Family Hominidae (great apes) evolved 20 million years ago.
·	Genus Homo (including humans and their predecessors) were evolved 2.5 million
	years ago.
·	Anatomically modern humans evolved 250,000 years ago.

In this way life on earth has evolved chronologically. This chronology is better represented in Geological Time Table. Natural selection and other several factors have played vital role in variation and evolution of life.

you can view video on Evolutionary History of Life

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