Abiogenesis, Evolution, and the Formation of Life on Earth

Theories on the Origin of Life (Abiogenesis)

Early Earth Conditions and Oxygenation

The early Earth atmosphere was initially anoxic (lacking free oxygen). Marine life produced oxygen, which gradually surfaced. This process oxidized elements in the sea and atmosphere, transforming the reducing anoxic atmosphere into an oxidative one.

The resulting oxygen led to the formation of the ozone layer (O₃), which filters harmful ultraviolet rays. The establishment of the ozone layer shielded the surface, allowing complex life to originate and thrive.

Hypotheses of Life's Genesis

• Panspermia

  This hypothesis suggests that life did not originate on Earth, but was brought here, perhaps by a meteorite carrying microbial life or organic precursors.

• Oparin and Haldane's Primordial Soup

  According to Oparin and Haldane, life originated in the ocean, where the primary energy source was heat. On early Earth, energy was also provided by electric charges, discharges, and ultraviolet radiation (which both formed and destroyed molecules).

  However, molecules formed on the surface could not accumulate because they were destroyed by these high-energy sources. Molecules formed in the water reacted, resulting in structures called coacervates and micelles. Eventually, these structures included a self-replicating molecule (a nucleic acid), leading to structures called protocells.

  Protocells (still not true cells) were able to divide and copy their internal contents, although they required external matter. An alternative hypothesis suggests that nucleic acid formation was controlled by proteins.

Miller Experiment

The Miller experiment simulated early Earth conditions. Ammonia and methane were mixed and subjected to electric shocks. This process yielded a large amount of organic molecules, demonstrating how organic matter could appear from inorganic components.

Classical Theories of Biological Evolution

Lamarckism: The Law of Use and Disuse

Jean-Baptiste Lamarck believed that changes in body structure are based on the use or disuse of parts. Organisms develop parts more when they are used, and these parts tend to decrease or atrophy when less used.

Environmental characteristics (such as temperature, humidity, and food availability) require adjustments in living creatures, who respond by changing some of their characters.

Acquired Characters

Lamarck's law considers that any animal can transmit to its descendants those characters acquired during its lifetime. Thus, the offspring would inherit characters developed by use or atrophied from disuse. (This theory is now considered obsolete.)

Darwin's Mechanism of Natural Selection

Charles Darwin summarized the mechanisms of biological evolution in the following points:

• Individuals of the same species are not identical; they vary in their characters, and these variations are capable of being transmitted to their descendants.
• Favorable variations are transmitted to descendants and remain, while unfavorable variations tend to be eliminated.
• Favorable variations accumulate over time, eventually leading to the transformation of some species into others (speciation).