History of Life

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What Made Humans 'the Fat Primate'? A new study suggests that part of the answer may have to do The results open new ways to study both how It turns out they were, because they were This is the first time analysis of A team studied just under marine fossils collected They didn't find the protein, but they did find huge colonies of modern bacteria living inside the Chemists now show that a simple reaction pathway could have given rise to DNA Summaries Headlines. How Multi-Celled Animals Developed. New DNA technology gives significant information on the bones buried in water.

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The DNA The finding Feathers Came First, Then Birds. Present life forms could not have survived at Earth's surface, because the Archean atmosphere lacked oxygen hence had no ozone layer to block ultraviolet light. Nevertheless, it is believed that primordial life began to evolve by the early Archean, with candidate fossils dated to around 3. Earth's only natural satellite , the Moon, is larger relative to its planet than any other satellite in the Solar System. Radiometric dating of these rocks shows that the Moon is 4.

Theories for the formation of the Moon must explain its late formation as well as the following facts. First, the Moon has a low density 3. Second, there is virtually no water or other volatiles on the Moon. Third, the Earth and Moon have the same oxygen isotopic signature relative abundance of the oxygen isotopes. Of the theories proposed to account for these phenomena, one is widely accepted: The giant impact hypothesis proposes that the Moon originated after a body the size of Mars sometimes named Theia [47] struck the proto-Earth a glancing blow.

The collision released about million times more energy than the more recent Chicxulub impact that is believed to have caused the extinction of the non-avian dinosaurs. It was enough to vaporize some of the Earth's outer layers and melt both bodies. The giant impact hypothesis predicts that the Moon was depleted of metallic material, [52] explaining its abnormal composition. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.

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Mantle convection , the process that drives plate tectonics, is a result of heat flow from the Earth's interior to the Earth's surface. These plates are destroyed by subduction into the mantle at subduction zones. During the early Archean about 3. Although a process similar to present-day plate tectonics did occur, this would have gone faster too.

It is likely that during the Hadean and Archean, subduction zones were more common, and therefore tectonic plates were smaller. The initial crust, formed when the Earth's surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. However, it is thought that it was basaltic in composition, like today's oceanic crust , because little crustal differentiation had yet taken place.

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What is left of these first small continents are called cratons. These pieces of late Hadean and early Archean crust form the cores around which today's continents grew. The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites from about 4.

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They show traces of metamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing that rivers and seas existed then. The first are so-called greenstone belts , consisting of low-grade metamorphosed sedimentary rocks. These "greenstones" are similar to the sediments today found in oceanic trenches , above subduction zones. For this reason, greenstones are sometimes seen as evidence for subduction during the Archean. The second type is a complex of felsic magmatic rocks.

These rocks are mostly tonalite, trondhjemite or granodiorite , types of rock similar in composition to granite hence such terranes are called TTG-terranes. TTG-complexes are seen as the relicts of the first continental crust, formed by partial melting in basalt. Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light atmophile elements from the solar nebula, mostly hydrogen and helium.

A combination of the solar wind and Earth's heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted of these elements compared to cosmic abundances. In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of volatiles from the Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as impact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would, therefore, have started to form even as the Earth formed.

Though most comets are today in orbits farther away from the Sun than Neptune , computer simulations show that they were originally far more common in the inner parts of the Solar System. As the Earth cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4. This early formation has been difficult to explain because of a problem known as the faint young Sun paradox.

The carbon dioxide would have been produced by volcanoes and the methane by early microbes.

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Another greenhouse gas, ammonia, would have been ejected by volcanos but quickly destroyed by ultraviolet radiation. One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are many models, but little consensus, on how life emerged from non-living chemicals; chemical systems created in the laboratory fall well short of the minimum complexity for a living organism. The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids , that are the building blocks of life.

An experiment in by Stanley Miller and Harold Urey showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of lightning. Additional complexity could have been reached from at least three possible starting points: self-replication , an organism's ability to produce offspring that are similar to itself; metabolism , its ability to feed and repair itself; and external cell membranes , which allow food to enter and waste products to leave, but exclude unwanted substances.

Even the simplest members of the three modern domains of life use DNA to record their " recipes " and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance, and self-replication. The discovery that a kind of RNA molecule called a ribozyme can catalyze both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA.

Although short, self-replicating RNA molecules have been artificially produced in laboratories, [80] doubts have been raised about whether natural non-biological synthesis of RNA is possible. In this hypothesis, the proto-cells would be confined in the pores of the metal substrate until the later development of lipid membranes.

Another long-standing hypothesis is that the first life was composed of protein molecules. Amino acids, the building blocks of proteins , are easily synthesized in plausible prebiotic conditions, as are small peptides polymers of amino acids that make good catalysts.


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Hence, self-sustaining synthesis of proteins could have occurred near hydrothermal vents. A difficulty with the metabolism-first scenario is finding a way for organisms to evolve. Without the ability to replicate as individuals, aggregates of molecules would have "compositional genomes" counts of molecular species in the aggregate as the target of natural selection. However, a recent model shows that such a system is unable to evolve in response to natural selection. It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step.

Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside. Some clays , notably montmorillonite , have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern, are subject to an analog of natural selection as the clay "species" that grows fastest in a particular environment rapidly becomes dominant , and can catalyze the formation of RNA molecules.

Research in reported that montmorillonite could also accelerate the conversion of fatty acids into "bubbles", and that the bubbles could encapsulate RNA attached to the clay. Bubbles can then grow by absorbing additional lipids and dividing. The formation of the earliest cells may have been aided by similar processes.

A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides , lipids and amino acids. It is believed that of this multiplicity of protocells, only one line survived. Current phylogenetic evidence suggests that the last universal ancestor LUA lived during the early Archean eon, perhaps 3. It was probably a prokaryote , possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as mitochondria or chloroplasts. Like modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis , and enzymes to catalyze reactions.

Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes by lateral gene transfer.

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The Proterozoic eon lasted from 2. The change to an oxygen-rich atmosphere was a crucial development. Life developed from prokaryotes into eukaryotes and multicellular forms.

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The Proterozoic saw a couple of severe ice ages called snowball Earths. The earliest cells absorbed energy and food from the surrounding environment. They used fermentation , the breakdown of more complex compounds into less complex compounds with less energy, and used the energy so liberated to grow and reproduce. Fermentation can only occur in an anaerobic oxygen-free environment. The evolution of photosynthesis made it possible for cells to derive energy from the Sun.

Most of the life that covers the surface of the Earth depends directly or indirectly on photosynthesis. The most common form, oxygenic photosynthesis, turns carbon dioxide, water, and sunlight into food. It captures the energy of sunlight in energy-rich molecules such as ATP, which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product.

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Such extremophile organisms are restricted to otherwise inhospitable environments such as hot springs and hydrothermal vents. The simpler anoxygenic form arose about 3. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2. At first, the released oxygen was bound up with limestone , iron , and other minerals.

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Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast time transformed Earth's atmosphere to its current state. This was Earth's third atmosphere. Some oxygen was stimulated by solar ultraviolet radiation to form ozone , which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere.

It allowed cells to colonize the surface of the ocean and eventually the land: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells.