8.4 Chemical Evolution, Life and Climate Change on Earth

By the end of this section, you will be able to:

Lack of Fossil Record: The origin of life on Earth remains elusive due to the constant movement of its crust.
Chemical Evidence: Life existed by approximately 3.9 billion years ago, as indicated by the formation of the oldest surviving rocks.
Stromatolites: By 3.5 billion years ago, sophisticated life forms formed large colonies known as stromatolites.
- Current Presence: Stromatolites are still present and provide evidence of early life (See Figure 8.15).
Fossil Record Limitation: Abundant fossils are mostly preserved from the last 600 million years, less than 15% of Earth's history.

Early Earth Atmosphere: Different from modern times; characterized by abundant carbon dioxide and some methane, but no free oxygen.
Complex Chemical Reactions: In the absence of oxygen, complex reactions led to the formation of amino acids, proteins, and other life's building blocks.
Early Life Forms: Initial life may have consisted of large molecules existing in warm, nutrient-rich seas, relying on organic chemicals for sustenance.

After initial nutrients depleted, evolution began to shape a vast diversity of organisms.
Genetics and Genomics: Tools from genetics help trace the shared genetic code among living organisms.
- Genomic Similarity: Human genomes are 99.9% similar to those of historical figures, and 99% similar to chimpanzees
- Tree of Life: Genetic sequences help shape the 'tree of life', illustrating the relationships among organisms (See Figure 8.16).
- Dominance of microbes: Most of life's diversity occurs at the microbial level.
- Microbial Comparison: There are more microbes in a bucket of soil than stars in the Galaxy.

Blue-Green Algae Development: Key organisms that use carbon dioxide and produce oxygen via photosynthesis, leading to the evolution of plants.
Oxygen in the Atmosphere: Free oxygen began accumulating around 2 billion years ago, linked to geological activity that buried plant carbon.
- Ozone Layer Formation: Increased oxygen led to the formation of the ozone layer, protecting life from solar ultraviolet light.
- Land Colonization: The accumulation of oxygen enabled the colonization of land and the evolution of animals.
- Humans and Oxygen: Humans evolved to use the waste products of plants—oxygen—for metabolic processes.

Essential Role of CO2: Carbon dioxide plays a significant role in trapping heat through the greenhouse effect.
Mechanism of Greenhouse Effect: Light from the Sun reaches Earth, is absorbed, then re-emitted as infrared radiation, which is subsequently trapped by greenhouse gases like CO2.
- Energy Balance: The surface of the Earth must reach a new temperature balance to equal energy received from the Sun.
- Current Impact: The greenhouse effect raises the average surface temperature by approximately 23 °C.
Industrial Influence: Modern industrial activities release carbon dioxide and disrupt the natural balance, leading to a projected doubling of preindustrial CO2 levels.
- Future Projections: By the current century's end, CO2 will likely reach levels twice that of preindustrial times (See Figure 8.18).

Climate Change Indicators: Records show that most of the hottest years have occurred since 2000.
Glacial Retreat: Glaciers are shrinking, and sea ice is becoming thinner.
Rising Sea Levels: Climate change presents a significant threat due to rising sea levels and has led to conservation measures in coastal cities.
Human Contribution: Human activities are responsible for a substantial 100-fold increase in CO2 emissions compared to volcanic eruptions.

Historical Ecological Changes: Throughout history, human activities have dramatically altered ecosystems and climates.
Mass Extinction Events: Humanity’s impact has prompted a significant mass extinction, often referred to as the anthropocene.
Anthropocene Epoch: While not officially recognized, this term encapsulates the idea that human actions significantly influence Earth's ecology and atmosphere.