Evolution of the earth

Hydrogen and helium are the lightest gases. In the early stage of Earth’s development, the planet’s gravity and temperature conditions were not sufficient to retain large amounts of these very light gases, and solar wind could strip them away. Their removal therefore supports the conclusion that the earliest atmosphere was light and unstable rather than a durable atmosphere like the present one. Option D is correct. Option A contradicts the evidence because hydrogen and helium are not heavy gases. Option B incorrectly assumes early atmospheric stability, while option C concerns ocean formation and cannot be directly inferred from the loss of these gases.

Water vapour in the secondary atmosphere became important for hydrosphere formation only when Earth’s surface cooled sufficiently. Cooling allowed the vapour to condense into liquid droplets, which gathered into clouds and produced prolonged rainfall. The rainwater then collected in depressions and lower parts of the surface, gradually forming rivers, lakes and early oceans. This sequence connects the atmosphere with the hydrosphere: atmospheric vapour changed into liquid water and accumulated on the surface. Water vapour did not form the metallic core, stop the solar wind or directly create the first complete ozone layer. Therefore, option A gives the correct scientific basis for the beginning of the hydrosphere.

Lower temperature was necessary for liquid water to stay. Exam tip: connect ocean formation with cooling.

Dissolution of gases caused exchange between atmosphere and hydrosphere. Exam tip: note the interaction of spheres.

Liquid water made simple chemical combinations possible. Exam tip: connect oceans with pre-life chemistry.

The correct answer is option A: simple chemicals, then organic substances, then early life. Chemical evolution describes a proposed sequence in which simple substances present on the early Earth could participate in reactions that produced more complex organic molecules. With further organisation, some of these molecules could contribute to primitive pre-cellular systems and eventually the earliest forms of life. Biological evolution begins after living systems exist; it concerns their variation, adaptation and diversification. Option B is illogical because humans cannot precede the chemical and biological conditions that produced life. Option C mixes ozone formation with an incorrect order and option D combines unrelated features of Earth. The memory cue is “chemistry first, biology next”: non-living chemical complexity must precede biological evolution.

Early life was not like life in modern oxygen-rich atmosphere. Exam tip: connect primitive life with primitive atmosphere.

Photosynthetic organisms changed atmospheric composition. Exam tip: see two-way relation between life and atmosphere.

Increase of free oxygen was necessary before ozone formation. Exam tip: keep the sequence photosynthesis oxygen ozone.

The ozone layer made land surface safer for life. Exam tip: treat ozone as a protective shield.

Oxygen helped both complexity and protection of life. Exam tip: do not limit oxygen only to respiration.

Biosphere developed after favourable Earth systems formed. Exam tip: treat life as a system-based result.

The hydrosphere means the water present on Earth in oceans, rivers, lakes, groundwater, ice and other forms. For water to collect and remain for a long time, Earth needs a sufficiently cool and stable solid surface. A solid crust provides a basin-like foundation on which condensed water can gather instead of remaining only as moving vapour or being affected by a continually molten surface.

If no solid crust had formed, the long-term accumulation of rainwater would have had a weak or unstable foundation. The other choices do not follow: crust formation does not directly make oxygen rise immediately, and it would not cause the ozone layer to form first. Therefore, the best answer is option D, because permanent hydrosphere development depends on a stable solid surface.

Core and atmosphere are results of different processes. Exam tip: match system and process correctly.

Volcanic activity continued even after crust formation. Exam tip: distinguish surface and interior conditions.

The correct answer is option A. Formation of the hydrosphere required both a source of water and a surface cool and stable enough to retain liquid water. Volcanic outgassing and the developing atmosphere supplied water vapour. As Earth cooled and a solid crust formed, the vapour could condense into liquid water. Surface depressions and basins then allowed that water to collect and remain, producing the early hydrosphere. Option B assigns impossible roles: the atmosphere does not form Earth’s core and the surface does not create the solar wind. Option C incorrectly treats ozone and gravity as removable in this way. Option D gives no valid geological process. The key idea is “vapour from above, retention below.”

Early carbon dioxide was linked with natural volcanic sources. Exam tip: keep ancient and modern contexts separate.

Ozone formation was limited in low free oxygen and methane ammonia were primitive gases. Exam tip: identify atmospheric condition from gases.

Gravity was linked with both atmosphere and internal structure. Exam tip: remember the multiple role of gravity.

The giant impact idea shows the violent nature of early planet formation. Exam tip: read it as evidence of planetary evolution.

The direct answer is A: permanent retention of gases would be difficult. Outgassing releases gases from Earth’s interior through volcanic and related processes, so it can provide the source of a secondary atmosphere. However, an atmosphere also needs sufficient gravity to hold those gases near the planet. The reasoning is: outgassing supplies gas; gravity prevents gas from escaping into space; if gravity is too weak, released gases gradually escape; therefore the secondary atmosphere cannot be retained permanently. Option A is correct. Option B is wrong because water vapour cannot simply turn into metal because of weak gravity; phase changes and chemical changes are different ideas. Option C is wrong because weak gravity would not make the core disappear. Option D is wrong because ozone does not form immediately: it requires oxygen and suitable atmospheric conditions, and weak gravity would make retention difficult. Do not confuse atmosphere formation with atmosphere preservation. Memory cue: source plus holding power—outgassing gives the first, gravity gives the second.

Rocks are records of geological history. Exam tip: write the role of rocks in evidence based questions.

Liquid water was important for early chemical evolution. Exam tip: treat oceans as chemical media.

Life depended on the atmosphere and also changed it. Exam tip: understand two way relations of Earth systems.

Ozone protected from harmful ultraviolet rays. Exam tip: connect ozone with protection of land life.