An essay on the origin of life on earth?

Answer 1

The Earth is one of eight major planets and a host of minor planets that circle a fairly average, middle-aged, main-sequence star. It formed from the accretion of residual material from the gravitational collapse of the solar nebula that produced the Sun. Based on their specific distances from the Sun, the planets formed into groups of similar chemical compositions, sizes, and densities. The four smaller inner planets are of higher density and are composed of a mixture of rock and metal. In contrast, the next four planets are massive gas giants with lower densities and larger sizes. The final grouping consists of small, low-density, icy worlds. Unlike the other inner planets, the Earth has remained a geologically active planet and has undergone continuous change since its formation. It is a planet dominated by the presence of liquid water. Throughout its long history, the Earth has evolved from a lifeless world into one that is populated by an uncountable number of species.

Based on radiometric dating analyses of moon rocks and meteorites, the Earth is believed to be about 4.5 billion years old. Many scientists believe that the Earth originated as a cold, undifferentiated body that internally heated up from the energy released through giant impacts, radioactive isotope decay, and the mass of the Earth itself. Once the appropriate melting temperatures were reached, heavy metallic elements sank to the planet's center of gravity, as lighter elements were displaced upward toward the surface. This process may have taken place in as little as 50 million years. It is fairly certain that the present core-mantle-crust structure was in place by 4 billion years ago.

Earth's core consists of both solid and liquid metal, presumably of nickel-iron composition. The outer, liquid metallic core revolves around the inner, solid metallic core and, in the process, generates an electric current. This electric current is responsible for the Earth's magnetic field. Sandwiched between the core and the crust is the mantle, a region of high-density iron- and magnesium-rich silicate rock material. Depending upon specific temperatures and pressures, this material can behave either as a solid or as a liquid.

Scientists closely link the history of the Earth to that of the Moon. The Earth and Moon are geologically similar in many ways, but there are significant differences in their respective elemental abundances, and lunar specimens exhibit an apparent lack of certain volatiles. One origin theory suggests that the Moon is the product of a huge collision between a Mars-sized object and the primordial Earth. The resulting debris from this impact later accreted and formed the Moon. This theory mainly draws its support from computer impact simulations and a great many assumptions, but it lacks the physical evidence necessary to support it.

Finding a common theory of origin for the Earth and Moon has proven to be quite difficult. In fact, when comparing size, density, and internal structure, the Moon seems to have more in common with Mars than it does with the Earth. What is certain is that the Moon has a definite influence on the Earth's environment. The Moon's daily tidal effects on the world's oceans are obvious, but the Moon also has a gravitational influence on the Earth's axial rotation. Without the gravitational pull of the Moon, the Earth's axial tilt would fall outside of its normal range of between 21° and 25° and literally fall over. Without the Moon "holding the Earth in place," the equator of today could easily become the polar regions of tomorrow.

The primitive Earth did not have the same atmosphere that it does today. Many scientists believe that the Earth's original atmosphere may have been a very dense, hot mixture of ammonia and methane. This is consistent with the conditions present during Earth's proto-planetary stage that probably dissipated during the Sun's T-Tauri phase. The first "permanent" atmosphere formed as a product of volcanic degassing. Through this process, carbon dioxide (CO2) gradually became the dominant gas, along with a significant amount of water vapor. As the Earth cooled, water vapor condensed and fell as rain. This rain later filled the lowlands and created the first primitive oceans. Chemical reactions occurring in seawater slowly began to extract CO2 from the atmosphere and form large amounts of carbonate rocks. Somehow, during the first 500 million years of Earth history, life crossed over the threshold between being a collection of complex organic molecules and living organisms. With life-forms such as blue-green algae at work, the CO2-rich atmosphere slowly transformed into a nitrogen and oxygen-rich atmosphere that could support animal life. This transition is clearly marked in the Earth's geological record when previously dark, iron-bearing sediments turned red from oxidation. It is shortly after this geological benchmark that the first marine animals appear in the fossil record.

The formation of supercontinents and continental drift is essentially tied to the internal mechanisms of the Earth's upper mantle. There, convection cells provide the energy necessary to split apart the crust into both large and small sections that can move relative to one another. The direct evidence for the effects of this convective energy is the large number of volcanoes and earthquakes that occur along plate boundaries. No one is certain how long plate tectonics has been a part of Earth history, but it is certainly responsible for the continent-ocean basin relationship, which forms the present crust.

In the geologic past, supercontinents have existed only to be broken apart and distributed across the face of the Earth. This movement is continuous, and the formation of supercontinents seems to be inevitable, as is their eventual breakup. The formation of the world's great mountain chains is the direct result of colliding continents transforming marine sediment into hard rock. New crustal rock is created by volcanic activity at mid-oceanic ridges that pushes plates apart from one another. Such plate movement can also carry older, crustal rock to its destruction in an oceanic trench or be welded together into a new continental mass.