Earth's Formation and the Emergence of Life

The Birth of Earth

As the terrestrial protoplanet increased in volume, the gravitational pull exerted by the planetesimals that collided with it increased. In these violent clashes, a large amount of heat was released, causing the planet to become partially molten. This merger allowed the denser material to sink, forming the core, while the lighter gases moved outward to form the atmosphere. Intermediate-density materials rose to form the mantle. Once the surface cooled, the abundant water vapor in the atmosphere condensed, forming the oceans.

The Early Atmosphere

The early atmosphere is believed to have formed by the degassing of the planet's interior. It contained large amounts of carbon dioxide and water vapor, along with some nitrogen. The most important difference between the primitive atmosphere and the atmosphere today is that the original had no oxygen. Furthermore, the current carbon dioxide ($ ext{CO}_2$) level is much lower (0.033%). The key reason for these changes is the emergence of living beings.

Origin of Life

1. Formation of Simple Organic Molecules

   Components of the early atmosphere, exposed to solar radiation and electrical discharges, originated amino acids and sugars.

2. Formation of Complex Organic Molecules

   The simple organic molecules combined to form more complex molecules, resulting in the oceans becoming the primordial soup.

3. Formation of Coacervates

   Some of the compounds of the "primordial soup" united to form hollow spheres inside which nucleic acid molecules would be locked, serving as the precursors of the first living beings.

Miller's Experiment

The hypothesis of Oparin and Haldane received strong support in 1953 thanks to the experiment conducted by Stanley Miller, who tried to reproduce in the laboratory the conditions that supposedly occurred on the early Earth. Despite Miller's experiment, many scientists still have serious objections to the Oparin and Haldane hypothesis and propose alternatives.

Life Changes the World: The Precambrian Eon

The Precambrian Eon includes most of Earth's history, from its formation up to 570 Ma (Mega-annum, millions of years ago). It is divided into two vast ages: the **Archaean** and the **Proterozoic**. The boundary between the two is situated 2500 Ma ago. The most important event of the Precambrian was the appearance of life. The oldest fossils found date back 3600 Ma. Around 3000 Ma ago, photosynthetic bacteria evolved, giving rise to limestone-like structures called stromatolites. Stromatolite-forming bacteria played a dual role:

• To provide oxygen to the atmosphere through photosynthetic activity.
• To remove large amounts of carbon dioxide from the atmosphere. The stromatolites were formed using $ ext{CO}_2$ and calcium.

Notable Events of the Paleozoic Era

• The Cambrian Explosion occurs: in less than 40 Ma, virtually all major groups of animals known today appeared.
• Shell-bearing animals appeared, including **trilobites**.
• Life invaded the continents: 450 Ma ago, the first land plants appeared, including ferns.