The Origin and Early Evolution of Life

The Appearance of Oxygen on Earth

The presence of pyrite, a mineral found in sedimentary rock, can indicate the presence of oxygen in the atmosphere. In older rock strata, pyrite appears dull and lacks the brilliance found in pyrite from younger strata. This suggests that oxygen levels were lower in the past. By calculating the age of these rocks, we can estimate when oxygen first appeared in the Earth's atmosphere.

Miller's Experiment and the Synthesis of Organic Matter

Stanley Miller conducted an experiment to simulate the conditions of early Earth and investigate the synthesis of organic matter from inorganic compounds. He successfully synthesized amino acids from inorganic molecules like methane, ammonia, and water, demonstrating the possibility of organic matter formation under early Earth conditions. However, Miller's experiment did not fully explain the origin of life itself.

The Beginning of Life

Evidence from Ancient Rocks

The oldest rocks on Earth, found in Isua, Greenland, provide clues about early life. These rocks contain mineral carbonates with varying ratios of carbon isotopes (¹²C and ¹³C). Living organisms preferentially assimilate ¹²C over ¹³C. Therefore, sediments with a high proportion of ¹²C suggest the presence of life in the past. By dating these rocks, we can estimate the time of life's emergence, which is approximately 4 billion years ago.

The Chemical Building Blocks and Energy for Life

Material Composition

The primary elements that make up living organisms are hydrogen (H), oxygen (O), carbon (C), and nitrogen (N). Hydrogen and oxygen are primarily found in water (H₂O), which constitutes up to 98% of our body mass. Water is essential for life because it acts as a good solvent and facilitates chemical reactions.

Carbon is crucial because it can form four covalent bonds, allowing it to create diverse and complex chain molecules. These molecules form the basis of essential organic compounds like carbohydrates, lipids, proteins, and nucleic acids.

Energy for Life

Life requires energy, which is obtained from chemical reactions involving organic matter. Cellular respiration is the process by which organisms break down organic matter synthesized during photosynthesis to produce energy.

Nutritional Strategies

Organisms can be classified based on how they obtain organic matter:

• Autotrophs: Organisms that produce organic matter from inorganic materials (e.g., through photosynthesis).
• Heterotrophs: Organisms that obtain organic matter by consuming other organisms (autotrophs or other heterotrophs).
• Chemoautotrophs: Organisms that obtain energy from inorganic compounds like sulfides (e.g., certain bacteria that do not require oxygen).

The Origin of Carbon and Water on Earth

Terrestrial Carbon

Terrestrial carbon originates from two sources: the Earth's interior, where it accumulated during the planet's formation, and volcanic activity, which releases carbon dioxide (CO₂) into the atmosphere.

Water

There are two main hypotheses for the origin of water on Earth:

• Delivery by asteroids from the early solar system.
• Release from the Earth's interior through volcanic activity.

Early Earth Conditions

Early Earth was characterized by:

• A very hot interior, leading to intense submarine volcanism.
• An atmosphere lacking oxygen.
• A predominantly oceanic environment.

The first signs of life appeared in the oceans under these conditions.