Evolution of the earth

Earth’s evolution was a sequence of interconnected causes and effects. For example, planetary accretion was followed by internal heating and differentiation, which formed layers; later, outgassing contributed to the atmosphere. Thus, each stage created conditions needed for the next one. Option B is incorrect because these processes occurred over different periods, not simultaneously. Exam tip: present events as ‘cause → effect → next stage’ rather than as an isolated list.

In the molten early Earth, denser iron and nickel sank and concentrated at the centre, while lighter silicates remained above. Nitrogen formation concerns atmospheric evolution, not differentiation. Exam tip: link differentiation with density-based layering.

The ozone layer absorbs much of the Sun’s harmful ultraviolet radiation, while suitable habitats and biological adaptations made life on land possible. Together, these factors help explain the expansion of life from water onto land.

Earth’s core formed through internal differentiation: denser materials moved inward as the early Earth heated and separated into layers. Photosynthesis is a biological process carried out by organisms, so linking it to core formation confuses two different processes.

Photosynthetic organisms, especially early microbes, increased the amount of oxygen in the atmosphere. Higher oxygen levels then created favourable conditions for aerobic respiration and the evolution of more complex life. Thus, the biosphere and atmosphere influenced one another. Solar wind did not form Earth’s core, and an ozone layer could not have preceded the early atmosphere because ozone requires oxygen. Exam tip: in a feedback relationship, look for a change in one Earth system that alters another system and then affects the first system in return.

Option A gives the broad order from Earth’s formation to later atmospheric and biological change. As Earth accreted, heating led to differentiation; the early hydrogen-helium atmosphere was lost, and gases released from within Earth contributed to a later atmosphere. Cooling allowed water to collect as a hydrosphere. Chemical evolution preceded life, and oxygen-producing photosynthesis later raised atmospheric oxygen, allowing the ozone layer to form. These stages overlap in places, so this is a broad sequence rather than a precise timeline.