What was the Earth like when life appeared on it?

The emergence of life on Earth is one of the most momentous events in the history of our planet. Approximately 3.8 billion years ago, in an environment radically different from today, the first forms of life emerged. Understanding what the Earth was like at that time is crucial to understanding the processes that allowed the transition from chemistry to biology.

The Earth 4,000 Million Years Ago

During the Hadean Eon, which spans from the formation of the Earth about 4.6 billion years ago to 4 billion years ago, our planet was an inhospitable place. Earth was a volcanic world, with a glowing, meteorite-bombarded surface. There was no atmosphere and no oceans as we know them today.

Cooling and Formation of the Crust

As the Earth cooled, about 4 billion years ago, a solid crust began to form. This process was accompanied by the release of volcanic gases that formed a primitive atmosphere. This atmosphere was very different from today’s, dominated by carbon dioxide (CO2), nitrogen (N2) and water vapour, with traces of methane (CH4), ammonia (NH3) and other reducing gases. There was no free oxygen (O2), essential for life as we know it today.

The First Oceans

Condensation of water vapour in the early atmosphere gave rise to the first rains, which further cooled the Earth’s surface and filled depressions to form the first oceans. These early oceans were extremely hot and rich in dissolved minerals. Water acted as a universal solvent, facilitating the chemical reactions that would lead to the emergence of life.

(Photo: NASA/Goddard Space Flight Center/Francis Reddy)

The Primordial Soup and the Synthesis of Organic Molecules

The “primordial soup” hypothesis proposes that in the early oceans, conditions were suitable for the formation of simple organic molecules, the building blocks of life. Experiments such as the Miller-Urey experiment in 1953 demonstrated that under conditions similar to those of the early atmosphere, amino acids, which are the building blocks of proteins, could be synthesized.

Hydrothermal Springs: Breeding Grounds for Life

Another theory suggests that life may have arisen in hydrothermal vents, located at the bottom of the oceans. These underwater vents expelled hot, mineral-rich water, providing the energy and chemical compounds necessary for early biochemical reactions. Hydrothermal vents offer an environment protected from ultraviolet radiation from the Sun and meteorite collisions, which may have favored the emergence of life.

The First Forms of Life

The first life forms on Earth were single-celled prokaryotic organisms, similar to modern-day bacteria. These organisms were anaerobic, meaning they did not need oxygen to survive. They used chemical compounds present in their environment to obtain energy, through processes such as chemosynthesis.

Photosynthesis and the Transformation of the Atmosphere

Approximately 2.5 billion years ago, cyanobacteria, a type of bacteria capable of oxygenic photosynthesis, emerged. This process produces oxygen as a byproduct and marked a drastic change in the Earth’s atmosphere. The accumulation of oxygen in the atmosphere, known as the Great Oxidation Event, radically transformed the planet, making the evolution of more complex life forms possible.

References:

1. Hazen, R. M. (2005). Genesis: The Scientific Quest for Life’s Origins. Joseph Henry Press.
2. Lane, N. (2015). The Vital Question: Energy, Evolution, and the Origins of Complex Life. W. W. Norton & Company.
3. National Aeronautics and Space Administration (NASA). (2020). “The Early Earth and the Origins of Life.”