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The first signs of life - probably bacteria – appeared nearly four billion years ago. But animals with shells and bones did not appear until less than 600 million years ago - these were the first living things preserved as fossils. It is mainly with the help of such fossils that geologists have built up a picture of Earth’s history since then. Very little is known about the four billion years before this, called the Precambrian period, which makes up more than 85 per cent of Earth’s history.

The evidence is overwhelming that all life on Earth has evolved from common ancestors in an unbroken chain since its origin. All life tends to increase: more organisms are conceived, born, hatched, germinated from seed, sprouted from spores, or produced by cell division (or other means) than can possibly survive. Each organism so produced varies, however little, in some measurable way from its relatives. In any given environment at any given time, those variants best suited to that environment  will tend to leave more offspring than the others. Offspring resemble their ancestors. Variant organisms will leave offspring like themselves. Therefore, organisms will diverge from their ancestors with time. The term natural selection is shorthand for saying that all organisms do not survive to leave offspring with the same probability. Those alive today have been selected relative to similar ones that never survived or procreated. All organisms on Earth today are equally evolved since all share the same ancient original ancestors who faced myriad threats to their survival. All have persisted since roughly 3.7 billion to 3.5 billion years ago during the Archean Eon (4 billion to 2.5 billion years ago), products of the great evolutionary process with its identical molecular biological bases. Because the environment of Earth is so varied, the particular details of any organism’s evolutionary history differ from those of another species in spite of chemical similarities.

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Fossil, remnant, impression, or trace of an animal or plant of a past geologic age that has been preserved in Earth’s crust. The complex of data recorded in fossils worldwide—known as the fossil record — is the primary source of information about the history of life on Earth.

Only a small fraction of ancient organisms are preserved as fossils, and usually only organisms that have a solid and resistant skeleton are readily preserved. Most major groups of invertebrate animals have a calcareous skeleton or shell (e.g., corals, mollusks, brachiopods, bryozoans). Other forms have shells of calcium phosphate (which also occurs in the bones of vertebrates), or silicon dioxide. A shell or bone that is buried quickly after deposition may retain these organic tissues, though they become petrified (converted to a stony substance) over time. Unaltered hard parts, such as the shells of clams or brachiopods, are relatively common in sedimentary rocks, some of great age.

The hard parts of organisms that become buried in sediment may be subject to a variety of other changes during their conversion to solid rock, however. Solutions may fill the interstices, or pores, of the shell or bone with calcium carbonate or other mineral salts and thus fossilize the remains, in a process known as permineralization. In other cases there may be a total replacement of the original skeletal material by other mineral matter, a process known as mineralization, or replacement. In still other cases, circulating acid solutions may dissolve the original shell but leave a cavity corresponding to it, and circulating calcareous or siliceous solutions may then deposit a new matrix in the cavity, thus creating a new impression of the original shell.

Fossils of hard and soft parts that are too small to be observed by the naked eye are called microfossils. Some fossils are completely devoid of plant and animal parts but show evidence of an organism’s activities. Such traces of organisms, which are appropriately known as "trace fossils", include tracks or trails, preserved waste products, and borings.

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Paleontology is the study of the history of life on Earth as based on fossils. Fossils are the remains of plants, animals, fungi, bacteria, and single-celled living things that have been replaced by rock material or impressions of organisms preserved in rock.

Paleontology, also spelled Palaeontology, scientific study of life of the geologic past that involves the analysis of plant and animal fossils, including those of microscopic size, preserved in rocks.

Paleontologists look at fossils, which are the ancient remains of plants, animals, and other living things. Fossils are mainly formed in two ways.

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Woolly mammoths were closely related to today's Asian elephants. They looked a lot like their modern cousins, except for one major difference. They were covered in a thick coat of brown hair to keep them warm in their home on the frigid Arctic plains. They even had fur-lined ears. 

Their large, curved tusks may have been used for fighting. They also may have been used as a digging tool for foraging meals of shrubs, grasses, roots and other small plants from under the snow.

Though woolly mammoths went extinct around 10,000 years ago, humans know quite a bit about them because of where they lived. The permafrost of the Arctic preserved many woolly mammoth bodies almost intact. When the ground around riverbanks and streams erodes, it often reveals the corpse of a long-dead mammoth that looks much like it did when it died.

Woolly mammoths were around 13 feet (4 meters) tall and weighed around 6 tons (5.44 metric tons), according to the International Union for Conservation of Nature (IUCN). Some of the hairs on woolly mammoths could reach up to 3 feet (1 m) long, according to National Geographic.

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Primitive life forms may have first appeared on Earth about 3800 million years ago. These bacteria lived in the oceans and built up solid mats of calcium carbonate, also known as lime. The deposits from the bacteria are known as stromatolites.

Stromatolites are living fossils and the oldest living lifeforms on our planet. The name derives from the Greek, stroma, meaning “mattress”, and lithos, meaning “rock”. Stromatolite literally means “layered rock”. The existence of these ancient rocks extends three-quarters of the way back to the origins of the Solar System.

With a citizen scientist’s understanding, stromatolites are stony structures built by colonies of microscopic photosynthesising organisms called cyanobacteria. As sediment layered in shallow water, bacteria grew over it, binding the sedimentary particles and building layer upon millimetre layer until the layers became mounds. Their empire-building brought with it their most important role in Earth’s history. They breathed. Using the sun to harness energy, they produced and built up the oxygen content of the Earth’s atmosphere to about 20%, giving the kiss of life to all that was to evolve.

Living stromatolites are found in only a few salty lagoons or bays on Earth. Western Australia is internationally significant for its variety of stromatolite sites, both living and fossilised. Fossils of the earliest known stromatolites, about 3.5 billion years old, are found about 1,000km north, near Marble Bar in the Pilbara region. With Earth an estimated 4.5 billion years old, it’s staggering to realise we can witness how the world looked at the dawn of time when the continents were forming. Before plants. Before dinosaurs. Before humans.

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By around 500 million years ago, bacteria in the oceans had evolved into the earliest fish. These strange creatures had no jaws; they had funnel-like Sucking mouths.

The first animals – including the common ancestor of all animals today – evolved in the sea over half a billion years ago. We have no direct evidence of what they were like.

But by studying animals today, we can work out features they must have shared – small size, soft bodies, and a tendency to stay very still or creep slowly across the ocean floor.

The creatures had bodies built from multiple cells with specialised roles, like organisms before them. Now, those cells could also form sheets called epithelia, allowing structures to develop. Along with increased genetic complexity, this set the scene for big changes.

Earth’s environment was in flux during the Cambrian period, and the Ediacaran period that came before it. Sea levels rose, and chemicals washed into the ocean. In the underwater world, evolution got to work. New creatures emerged that could move further than ever before – and change their environment by burrowing and building. Soon, the new species were living in every habitat across the length and breadth of the ocean.

Credit: Natural History

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