Evolution of the earth

Outgassing is the release of gases from Earth’s hot interior through volcanic activity and related processes. During the early history of the planet, water vapour, carbon dioxide, nitrogen and smaller amounts of other gases were released and gradually contributed to the formation of a primitive atmosphere. Later, cooling and condensation allowed water to collect, while biological processes changed the atmospheric composition further. Option A is therefore correct because outgassing supplied much of the material for the early atmosphere. A galactic arm belongs to the structure of a galaxy, the solar core is part of the Sun, and cosmic red shift is an observational effect caused by the motion or expansion of space; none of these is formed by terrestrial outgassing.

As the young Earth became sufficiently hot and partly molten, gravitational differentiation separated materials according to density. Dense elements and minerals, especially iron and nickel, sank toward the centre and contributed to the formation of the core. Less-dense silicate materials moved upward and formed the mantle and crust. This process explains why Earth developed concentric layers rather than remaining a uniform body. Option A is correct because gravity causes denser material to sink and lighter material to rise or remain nearer the surface. Light gases alone could not form the metallic core, while ice pieces and plant remains are not plausible materials for the formation of Earth’s internal layers.

The governing concept is the early evolution of Earth’s solid, liquid and gaseous spheres. As the young planet cooled, a solid crust formed around its hotter interior. This crust provided a relatively stable surface and depressions in which condensed water could collect instead of continually mixing with molten material. The accumulation of water helped the hydrosphere develop, while later interactions among the crust, atmosphere and oceans shaped Earth’s surface environment. Option B is correct because it expresses this sequence. Options A, C and D confuse Earth’s later crustal development with galaxy motion, the origin of the universe or the earlier formation of the Sun.

The governing concept is the distinction between planetary formation and biological emergence. Earth originated through a physical process involving the accumulation of material in the early solar system, heating, differentiation and later cooling into a layered planet. The origin of life refers to a much later transition in which suitable chemical systems became capable of self-reproduction and evolution; its precise pathway remains a scientific research question. Option B is correct because it separates these stages without claiming that they were unrelated. Option A incorrectly treats them as one event, C reverses the sequence, and D is not a scientific explanation of planetary origin.

The governing concept is planetary differentiation. When the early Earth became sufficiently hot and partly molten, materials separated according to density. Heavy elements and substances, especially iron and nickel, moved downward under gravity and concentrated at the centre, forming the core. Lighter silicate materials remained above them and contributed to the mantle and crust. Therefore option A, the core, is correct. The atmosphere is an outer gaseous layer, not a central differentiated part. Oceans occupy surface depressions and formed much later as water accumulated, while a mountain peak is a surface landform produced by geological processes. These alternatives do not result from the sinking of dense material during differentiation. The answer follows directly from the relationship between density, gravity, and Earth’s internal structure.

The governing concept is the development of the hydrosphere during Earth’s early evolution. The hydrosphere includes Earth’s oceans, rivers, lakes, groundwater, ice, and water vapour. Its formation meant that water could exist and remain available on the planet in liquid form, especially after cooling and condensation of atmospheric water vapour. Liquid water is a fundamental requirement for known life because it supports chemical reactions, transport of dissolved substances, and stable environmental processes. Therefore, option B is correct. Metal rain is not the relevant condition, while cities and railways are much later human developments.

Continuous impacts produced heat and kept the surface unstable for a long time. Exam tip: treat impacts as an early heat source.

Density difference arranged Earth into layers. Exam tip: understand differentiation as the basis of internal structure.

The first atmosphere had light gases while the secondary atmosphere formed from gases released from Earth's interior. Exam tip: separate source and loss.

Water vapour cooled and condensed and rainwater formed oceans. Exam tip: arrange process questions chronologically.

The outer crust formed but the interior was still hot. Exam tip: distinguish surface conditions from interior conditions.

Oceans absorbed some gases and changed atmospheric composition. Exam tip: remember interaction between hydrosphere and atmosphere.

Photosynthesis increased atmospheric oxygen which later supported ozone and complex life. Exam tip: treat it as a biological cause.

The direct answer is D: the ozone layer protected life on land from harmful ultraviolet radiation. Ozone is a form of oxygen found mainly in the upper atmosphere. It absorbs much of the Sun's damaging ultraviolet, or UV, radiation. When this protection developed, organisms could survive more safely outside water, so land life could develop. Option D is correct. Option A is wrong because ozone does not remove salt from oceans. Option B is wrong because it does not cool Earth's core; the core is deep inside the planet and is unrelated to ozone's protective function. Option C is wrong because ozone does not stop volcanoes. The reasoning is: sunlight includes UV radiation, strong UV can damage living cells, ozone absorbs much of it, and this makes land environments safer. It did not remove every danger, but it provided important protection. Memory cue: ozone is Earth's UV shield.

Liquid water allowed reactions of simple chemical substances. Exam tip: treat oceans as the chemical background of life.

Formation of simple organic substances prepared the base for life. Exam tip: place chemical evolution before biological evolution.

Movement of heavy materials toward the centre was a result of differentiation. Exam tip: connect core with density and gravity.

Sufficient cooling of the surface was necessary for oceans. Exam tip: understand temperature and state of water together.

Water vapour was in the atmosphere and after rainfall formed hydrosphere on the surface. Exam tip: identify interaction between spheres.

Low free oxygen means early life was not like modern life. Exam tip: link oxygen deficiency with early life.

Volcanic activity released gases from Earth's interior. Exam tip: study volcanoes as a natural source of atmosphere.

The giant impact idea links Moon origin with violent events of early Earth. Exam tip: connect it with planetary evolution.

Lighter materials stayed upward and became associated with crust and outer parts. Exam tip: separate locations of light and heavy materials.

Oxygen availability was necessary for ozone formation. Exam tip: remember the sequence photosynthesis oxygen ozone.

Biosphere developed only when water atmosphere and temperature became favourable. Exam tip: treat biosphere as a result of combined natural conditions.