Chapter 2 — The Origin and Evolution of the Earth

The Big Bang Theory states that all matter existed in one place as a tiny ball with infinite temperature and density. Around 13.7 billion years ago it exploded violently, causing a huge expansion that continues to this day. Within the first 3 minutes atoms began to form, and within 300,000 years the temperature dropped to 4,500 K and the universe became transparent.

Edwin Hubble, in 1920, provided evidence that the universe is expanding by showing that galaxies move further apart over time.

The Nebular Hypothesis, proposed by Immanuel Kant and revised by Laplace in 1796, argued that planets formed from a cloud of material associated with a slowly rotating young sun. Otto Schmidt and Carl Weizascar further revised it in 1950, suggesting the sun was surrounded by a solar nebula of hydrogen, helium, and dust, with friction and collision forming a disc-shaped cloud from which planets accreted.

Planetesimals are small rounded objects that form when a gas cloud condenses and matter around a stellar core develops into tiny bodies through cohesion. These planetesimals then collide and gravitationally attract each other to eventually form larger planetary bodies.

Differentiation is the process by which Earth's interior material separated into distinct layers based on density. Heavier materials like iron sank towards the centre while lighter ones moved to the surface, resulting in the layers: crust, mantle, outer core, and inner core.

First, solar winds stripped away the primordial atmosphere of hydrogen and helium. Second, degassing from Earth's hot interior released water vapour, nitrogen, carbon dioxide, methane, and ammonia to form a new atmosphere. Third, photosynthesis by living organisms modified the atmosphere by adding free oxygen; around 2,000 million years ago oxygen began to flood the atmosphere.

Earth's oceans formed within 500 million years of the planet's formation, making them approximately 4,000 million years old. Water vapour released from the interior condensed as the Earth cooled, and rainwater collected in depressions to form the oceans.

According to Chapter 2, life began to evolve around 3,800 million years ago. The chapter notes that microscopic structures closely related to present-day blue algae have been found in geological formations older than 3,000 million years.

The Earth is 4,600 million years old (4.6 billion years). The chapter states that events between 4,600 million years ago and the present led to the evolution of life on the planet's surface.

Degassing is the process through which gases were outpoured from Earth's interior. Continuous volcanic eruptions released water vapour and gases, contributing to the early atmosphere which largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia, and very little free oxygen.

A light year is a measure of distance, not time — it equals the distance light travels in one year at 300,000 km per second, which is 9.461 × 10¹² km. It is used in Chapter 2 to describe the vast distances involved in galaxy formation and the size of galaxies, whose diameters range from 80,000 to 150,000 light years.

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