Evolution

Big Bang

About 13.7 billion years ago a tremendous explosion started the expansion of the universe. This explosion is known as the Big Bang. At the point of this event all of the matter and energy of space was contained at one point. What existed prior to this event is completely unknown and is a matter of pure speculation. The Big Bang produced a fireball of titanic proportions with a temperature measuring in the billions of degrees. In the first milliseconds immediately following the blast, matter and energy burst outward in all directions, literally stretching space. As space expanded, the matter and energy released by the fireball cooled. As matter cooled, highly energetic subatomic particles slowed enough to interact with each other, filling the universe with the simplest elements of Hydrogen and Helium. Driven by gravity and by their own momentum, these gases began to accumulate, first into small pockets and then into larger clumps that gradually grew hotter and brighter and evolved into stars. Stars were formed in this manner throughout the universe, and large groups of stars -- some numbering in the hundreds of billions -- formed the largest structures in the universe, galaxies.

Galaxies

Galaxies tend to occur in groups known as clusters. One such cluster, called the local group, contains our own galaxy, the Milky Way, and our neighbor the Andromeda galaxy. The Milky Way is a spiral galaxy; that is, if viewed from afar it would resemble a spiral. Other major galaxy types are globular clusters, elliptical galaxies, and barred spirals, which have bars of dust, gas, and stars running across the galaxy's center like the spokes of a wheel. Still other galaxies are irregular, with no definite form. Some clusters of galaxies contain thousands of galaxies, and there are roughly 100 billion galaxies in the universe.

Within the galaxies, stars continued to form, burn, explode, die, and re-form. In a remote corner of the Milky Way galaxy about 5 billion years ago, a single gaseous cloud began to contract under the force of gravity. In the process it became less amorphous in shape. Its interior temperature began to rise. At the center of the cloud, a hot, dense sphere of Hydrogen gas held together by gravitation began to radiate. A star had formed. This star was the Sun. Our local star, Sun, is third generation star i.e. it contains heavier elements like carbon, Iron and Uranium that were created by earlier stars which exploded in a Nova explosion when they ran out of their Hydrogen fuel. The heavier elements then seeded the new forming stars and planets. All life contains atoms that were created in the stars and we all are made of 'star dust'.

Solar System

Stars like the Sun form when giant clouds of gas and dust condense into a ball, igniting thermonuclear fusion, which is the conversion of primarily hydrogen and helium into heavier elements. Theory holds that the cloud ought to light up as the density increases during its collapse, which can take hundreds of millions of years. Theorists figure a pre-solar nebula began to collapse about 5 billion years ago, possibly triggered by a shock wave from a nearby exploding star. The Sun likely formed in a crowded environment before wandering to its present lonely location, many astronomers believe. As the cloud collapsed under its own weight and rotated faster, some material was spun into a doughnut-shaped disk. Clumps of dust became rocks. Some were destined to be planets. Others became asteroids, comets and smaller meteoroids. Smaller condensations of matter began to form within the gaseous cloud, slowly revolving around the Sun. These were the protoplanets, whirling accumulations of gas that would, in time, give rise to the planets. The protoplanets grew in size and, through the force of their own gravitation, absorbed other matter in the cloud, including other protoplanets. In this way the number of protoplanets lessened. Finally, the Solar System took its modern shape: spherical planets with their moons locked into orbit around a central sun, located some 30,000 light years away from the center of the Milky Way. Our Solar System was formed 4.6 billion years ago.

Earth

Scientists theorize that Earth began as a waterless mass of rock surrounded by a cloud of gas about 4.6 billions years ago. Radioactive materials in the rock and increasing pressure in Earth's interior produced enough heat to melt the interior of Earth. The heavy materials, such as Iron, sank. The light silicate rocks rose to Earth's surface and formed the earliest crust. The heat of the interior caused other chemicals inside Earth to rise to the surface. Some of these chemicals formed water, and others became the gases of the atmosphere.

The leading theory for the Moon's formation has a Mars-sized object slamming into Earth about 4 billion years ago, shortly after our planet formed. The evidence is partly in the Moon's composition, which is similar to the upper portions of Earth. The shock of the impact strips material from the outer layers of Earth and the impacting object. The mostly iron cores of both bodies meld into Earth's core. Moon was predominantly produced from material of the mantle of the impactor and also includes nearly 20% of Earth's crust blasted by the impact. The blasted material is captured by Earth's gravity and it forms a ring around the Earth. This ring of material would have coalesced to form the moon. In addition, the moon is moving away from the earth (currently at 2 inches per year), as it has been since its creation.

The known history of Earth is divided into four long stretches of time called eons. Starting with the earliest, the eons are Hadean, Achaean, Proterozoic, and Phanerozoic. The first three eons, which together lasted nearly 4 billion years, are grouped into a unit called the Precambrian. The Phanerozoic Eon, when life became abundant, is divided into three eras. They are, from the oldest to the youngest, the Paleozoic, Mesozoic, and Cenozoic eras. Eras are divided into periods, and periods are divided into epochs. These divisions and subdivisions are named for places where rocks of each period were studied. Periods are mostly separated by important changes in the types of fossils found in the rocks. As a result, the lengths of eras, periods, and epochs are not equal.

Life

It was early in the Archaean Era, 3.8 to 2.5 billion years ago, that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils. In fact, all life during the more than one billion years of the Archaean was bacterial. Life began to flourish, and the autotrophic organisms had tapped a new niche allowing the biomass of Earth at the time to dramatically increase. Carbon Dioxide was constantly being absorbed by these organisms, and after the biological reactions responsible for creating energy in them, Oxygen would be released as a by-product. This meant that Oxygen began to accumulate in the oceans where life existed. This new material would in turn be taken advantage of by the adapting organisms leading to the creation of aerobic organisms, which used Oxygen as a component of their energy creation. The mitochondria are present in both animal and plant cells in today's world, suggesting that the arrival of the mitochondria in the evolutionary chain was slightly before recognizable taxonomical differences between animals and plants. The beginning of the Cambrian era saw a widespread arrival of multi-cellular organisms, particularly in the form of sponges. One major event in time is the development of sexual reproduction. Previous species method of reproduction was simply mitosis, repeated cell division which produced new organisms, and exact copy of their ancestors. Of course, mutations and other factors over time changed their genome causing them to evolve. The first animals in the fossil record appeared between 620 and 550 million years ago in the Vendian Period.

The Cambrian Period, about 544 million years ago, marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of forms appears. Some point to the increase in Oxygen that began around 2 billion years ago supporting a higher metabolic rate and allowing the evolution of larger organisms and more complex body structures. Internal genetic factors were also crucial in the Cambrian Explosion. Recent research suggests that the period prior to the Cambrian explosion saw the gradual evolution of a "genetic tool kit" of genes that govern developmental processes. Once assembled, this genetic tool kit enabled an unprecedented period of evolutionary experimentation -- and competition. Once the body plans that proved most successful came to dominate the biosphere, evolution never had such a free hand again, and evolutionary change was limited to relatively minor tinkering with the body plans that already existed.

During the Ordovician, 505 million years ago, the first plants appeared. But it was not until the late Silurian before they resembled modern plants. The Silurean period, 440 millions years ago, was the time when some plants and animals left the water and colonized the land for the first time. Arthropods were the first animals to adapt to the land by about 420 million years ago. Fossil footprints of arthropods found in Western Australia that were made in the sandy flats surrounding temporary lakes, indicate that these animals may have accompanied the landward march of plants. In most ways they were pre-adapted to life on land. During the Devonian, 410 million years ago, two major animal groups dominated the land. The first Tetrapods, or land-living vertebrates, appeared during the Devonian, as did the first terrestrial arthropods, including wingless insects and the earliest arachnids which had already ventured onto land during the Silurian. In the oceans, brachiopods flourished. In the Carboniferous Period, 360 million years ago, the greatest evolutionary innovations were the amniotic egg which allowed early reptiles to move away from waterside habitats and colonize dry regions. The amniotic egg allowed the ancestors of birds, mammals, and reptiles to reproduce on land by preventing the embryo inside from drying out, so eggs could be laid away from the water. It also meant that in contrast to the amphibians the reptiles lay fewer eggs, they had no larval stage and fertilization was internal. In the Permian Period, 286 million years ago, the most striking transitions in the evolution of life occurred when mammals evolved from one lineage of reptiles.

Roughly 248 million years ago, the Permian-Triassic extinction occurred. This is the largest extinction known. About 95% of all species and about 60% of the genera died out, including many marine animals (like the trilobite). The cause of the Permian extinction might have been global cooling, volcanic eruptions, or a decrease in the continental shelf area during the formation of Pangaea. During the late Triassic, 220 million years ago, the first true mammals appeared. These primitive mammals were tiny and are thought to have been nocturnal. During the Jurassic Period, 213 million years ago, dinosaurs dominated the near-tropical Earth. The gigantic Sauropod dinosaurs, like the Diplodocus and Apatosaurus, diversified. Carnivorous Theropod, like Allosaurus and Compsognathus, were abundant. Bird-like dinosaurs also flourished. Cretaceous Period, 145 million years ago, was the age of the dinosaurs. Huge carnivores like Tyrannosaurus rex and Giganotosaurus appeared, as did Triceratops and many, many others. There was a tremendous diversity in dinosaur species. Mammals were flourishing, and flowering plants developed and radically changed the landscape. The Cretaceous period ended about 65 million years ago, a mass extinction due to impact of an asteroid at Chicxulub in Mexico wiped out the dinosaurs, except for the birds.

Mammals

The Paleocene epoch, 63 millions ago, is a crucial time in the history of mammals, it was a world without dinosaurs. Mammals appeared first in the late Triassic, at about the same time as dinosaurs. Throughout the Mesozoic, most mammals were small, fed on insects and lead a nocturnal life, whereas dinosaurs were the dominant forms of life on land. After the abrupt changes about 65 million years ago, when dinosaurs disappeared with the exception of their descendants, the birds, the world was practically without larger sized terrestrial animals. This unique situation was the starting point for the great evolutionary success of the mammals. Only ten million years later, at the end of the Paleocene, they had occupied a large part of the vacant ecological niches, often competing with giant carnivorous birds, especially in South America. By this time, the landscape was teeming with small insectivorous and rodent-like mammals, medium sized mammals were searching the forests for any kind of food they could cope with, the first large (but not yet gigantic) mammals were browsing on the abundant vegetation, and carnivorous mammals were stalking their prey.

Primates

Where and when the first primates - the group to which we belong - appeared remains uncertain, but the oldest confirmed primate fossils date to about 60 million years ago. It is widely agreed that primates emerged from archaic terrestrial and nocturnal insectivores (shrew-like animals) with early primates resembling lemurs or tarsiers and probably lived in trees in tropical or subtropical areas. Many of their characteristic features are well suited for this habitat: hands specialized for grasping, with five digits and, in most primates, opposable thumbs, rotating shoulder joints and stereoscopic (three dimensional) vision. Other traits include a large brain cavity and nails instead of claws. Modern primates range from Prosimians such as the pygmy mouse lemur, through the monkeys, to anthropoid apes such as the gorilla- and humans.

In the Pliocene epoch, 5.3 million years ago, the primates continued to diversify. Humans and Chimpanzees shared their last common ancestor around 7 million years ago, and have since followed separate evolutionary paths. We share about 98.8% of our DNA with Chimpanzees, which are thus our closest relatives amongst the primates. The first known hominids or humanlike primates evolve in eastern Africa about 5.2 million years ago. Hominids feature prominent jaws and most species have large brains relative to those of apes. Most hominids probably lived in groups either in or near forests and some later species made and used tools. The oldest fossils, a jawbone teeth and a toe bone found in Ethiopia, date to 5.3 million years. A younger near complete hominid skeleton named 'Lucy' by its discoverers and a set of remarkably preserved footprints in Hadar, Tanzania revealed more about their appearance and one of their most distinctive traits: even the earliest hominids could walk upright on two legs. This adaptation afforded certain advantages such as the ability to see over the top of high vegetation and to easily carry food or tools and weapons while traveling. The Pleistocene, 10 million to 10,000 years ago, also saw the evolution and expansion of Homo sapiens, and by the close of the Pleistocene, humans had spread through most of the world. A fossil jaw found in Mauer, Germany, of Homo Heidelbergensis dates these early humans to approximately 500,000 years ago. They display physical characteristics of modern humans, with an increased brain capacity, smaller teeth and a face that slopes less than that of other hominid ancestors. About 130,000 years ago modern humans, Homo Sapiens, disperse throughout Africa, the Middle East and Europe. They were characterized by a more gracile skeleton, and higher, domed skull than their European contemporaries, the Neanderthals. Cave paintings suggest that by 40,000 years ago, Paleolithic Period, Cro-Magnon had developed a sophisticated culture; some authors equate this to the appearance of complex spoken language.

The Age of Humans

The final epoch of the Quaternary period, spanning the time from the end of the Pleistocene, 10,000 years ago to the present is the Holocene epoch, also called the Anthropogene, the 'Age of Humans'. The DNA studies suggest that all humans descended from a single African ancestor who lived some 60,000 years ago. The modern humans migrated from Africa to colonize the rest of the world. During the Ice Age, water levels dropped and land bridges connected Africa with Asia, Asia with America, and Indonesian peninsula was very close to Australia. One human migration was from Africa to Australia passing through Arabia, Iran, South Asia, Southeast Asia, and Indonesia. The other migration path took Humans to Central Asia. If Africa was the birth place of humanity then the Central Asia was its nursery. The humans migrated over the Bering Bridge connecting Asia to America about 10,000 years ago. The glaciers started to melt as Earth warmed up about 10,000 years ago, the sea level rose and land bridges disappeared under water, and many coastal areas were flooded and coastal highlands turned into Islands.

Mesolithic Era is marked by the transition from roaming and hunting to an agricultural society and begins around 11,000 years ago. The first evidence for domestication of plants and animals come from temporary campsites and are first seen from 11,000 years ago onwards. Village communities started appearing around 8,000 years ago and were common by 6,000 years ago. In the Neolithic era, cities began to be built around 8,000 to 6,000 years ago. Beginning around 8000 years ago, many human cultures became increasingly dependent on cultivated crops and domesticated animals to secure their supply of food. By 7000 years ago sedentary agriculture was able to support towns such as Jericho in Palestine and Çatal Höyük in Turkey with populations of more than 1,000. By 3500 years ago the first civilizations appeared in the Mesopotamia Tigris- Euphrates valley in Iraq, Nile valley in Egypt, Indus valley in Pakistan, and Yellow river valley in China.

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