Chapter 25: The History of Life on Earth – Study Notes

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The Urey-Miller experiment was a landmark study that simulated early Earth conditions to test the chemical origins of life.

Purpose: To determine if organic molecules could form under prebiotic Earth conditions.
Method: A mixture of water, methane, ammonia, and hydrogen was exposed to electrical sparks (simulating lightning).
Results: Amino acids and other organic compounds formed, supporting the hypothesis that life's building blocks could arise abiotically.
Significance: Provided experimental evidence for the chemical evolution of life.

RNA is considered a likely candidate for the first genetic material due to several unique properties.

Catalytic Properties: Some RNA molecules (ribozymes) can catalyze chemical reactions, including self-splicing and peptide bond formation.
Self-Replication: RNA can serve as a template for its own replication, a key feature for inheritance.
Structural Diversity: RNA can fold into various shapes, allowing for functional diversity and the potential for natural selection.
Example: The ribosome, essential for protein synthesis, is a ribozyme.

Protobionts are hypothesized precursors to living cells, exhibiting some characteristics of life.

Fossils provide evidence of past life, and radiometric dating allows scientists to estimate their ages.

Fossils: Preserved remains or traces of ancient organisms, found in sedimentary rock layers.
Carbon-14 Dating: Used to date relatively recent fossils (up to ~50,000 years old).
Half-life of Carbon-14: $5730$ years.
Principle: Measures the ratio of to to estimate age.

The evolution of photosynthetic cyanobacteria dramatically increased atmospheric oxygen, transforming Earth's environment.

Timeline: Occurred about 2.4 billion years ago.
Impact: Oxygen accumulated in the atmosphere, enabling aerobic respiration and leading to the extinction of many anaerobic organisms.
Evidence: Banded iron formations in ancient rocks.

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

Mitochondria: Originated from engulfed proteobacteria.
Chloroplasts: Originated from engulfed cyanobacteria.
Evidence:
- Both organelles have their own circular DNA.
- Double membranes consistent with engulfing mechanism.
- Reproduce by binary fission, similar to bacteria.
- Ribosomes resemble those of prokaryotes.

Earth's history is divided into eons and eras to organize major events.

Earth has experienced several mass extinctions, profoundly affecting life.

Extinction Event	Period	Possible Causes
Permian Extinction	End of Paleozoic (~252 mya)	Volcanic eruptions, climate change, ocean anoxia
Cretaceous Extinction	End of Mesozoic (~66 mya)	Asteroid impact, volcanic activity

Two major periods of rapid diversification occurred in animal and plant evolution.

Continental drift has shaped Earth's biogeography and evolutionary history.

Pangea: Supercontinent that existed about 250 million years ago.
Tectonic Plates: Earth's lithosphere is divided into plates that move, causing continental drift, mountain building, and earthquakes.
Impact: Influenced climate, sea levels, and the distribution of organisms.

Adaptive radiation is the rapid evolution of diversely adapted species from a common ancestor.

Triggers: New environmental opportunities, extinction of competitors, or evolution of novel traits.
Example: Darwin's finches on the Galápagos Islands.

Changes in the timing of developmental events can lead to evolutionary novelties.

Heterochrony: Evolutionary change in the rate or timing of developmental events.
Paedomorphosis: Retention of juvenile features in the adult form.
Significance: Can result in new morphological traits and contribute to speciation.