Evidence of Evolution
- Earth is approximately 4.5 billion years old, and life has evolved gradually through multiple physical, chemical, and biological processes.
- Over this time, geological, climatic, and biological changes have continuously shaped the planet’s surface and influenced the evolution of species.
- These changes occurred on geological timescales, meaning they took millions to billions of years, far beyond human lifespans.
Fossil Record
Fossils
Fossils are preserved remains, traces, or imprints of ancient organisms found in sedimentary rocks.
- The fossil record provides chronological evidence for the evolution of life on Earth.
- Fossils are preserved remains or traces of organisms from past geological ages found in sedimentary rocks.
- By examining fossil layers, scientists can trace changes in species over time, demonstrating how life has evolved.
The Archaeopteryx, with both reptilian (teeth, tail) and avian (feathers, wings) features, demonstrates the transition from reptiles to birds.
AnalogyFossils are like “pages” of Earth’s biological history, incomplete but invaluable records of evolutionary events.
Mechanisms of Change
- Plate tectonics reshaped continents and oceans, isolating populations and driving evolution.
- Volcanic activity caused global climate disruptions, influencing which organisms survived.
- Atmospheric evolution, such as oxygen accumulation during the Great Oxidation Event, allowed aerobic organisms to thrive.
- These long-term processes created the conditions necessary for complex life to evolve.
Role of Carbon Dating and Ice Core Data
- Carbon dating helps determine fossil age by measuring radioactive decay of carbon-14.
- Ice core samples provide evidence of ancient atmospheric gas concentrations, linking climate trends to biodiversity shifts.
The Geological Time Scale
- Earth’s 4.6-billion-year history is divided into hierarchical time intervals: Eons → Eras → Periods → Epochs → Ages
- This division helps scientists interpret biological and environmental changes over immense time scales.
Major Eons
- Hadean (4.5–4.0 billion years ago): Formation of Earth; no life existed.
- Archean (4.0–2.5 billion years ago): First simple life (bacteria).
- Proterozoic (2.5 billion–541 million years ago): Rise of oxygen and multicellular organisms.
- Phanerozoic (541 million years ago–present): Explosion of visible life; rapid diversification of species.
The Phanerozoic Eon is the most studied because it contains abundant fossils that show clear evolutionary sequences.
Major Eras of Biological History
Precambrian (4.6 billion – 541 million years ago)
- Formation of Earth and earliest life.
- Cyanobacteria began photosynthesis, releasing oxygen into the atmosphere (the Great Oxidation Event).
- Emergence of simple multicellular life near the end.
Oxygen accumulation from cyanobacteria created an ozone layer, which allowed life to transition from aquatic to terrestrial environments.
Paleozoic Era (541 – 252 million years ago)
- Cambrian Explosion: Rapid diversification of marine life.
- First vertebrates, land plants, and amphibians appeared.
- Ended with the Permian mass extinction, the largest in Earth’s history.
The trilobite, once abundant in oceans, went extinct at the end of the Paleozoic Era due to drastic environmental changes.
Mesozoic Era (252 – 66 million years ago)
- Known as the Age of Reptiles.
- Dominated by dinosaurs, marine reptiles, and early birds and mammals.
- Ended with the Cretaceous–Paleogene (K–Pg) extinction event, wiping out ~75% of species.
Cenozoic Era (66 million years ago – Present)
- Known as the Age of Mammals.
- Mammals and birds diversified to fill ecological niches left by dinosaurs.
- Humans (Homo sapiens) evolved in the late Quaternary period.
Changes between geological eras often coincide with mass extinctions or major climate shifts.
Epochs as Biological and Environmental Markers
- Epoch transitions reflect major environmental changes that caused extinction or evolution of dominant life forms.
- These boundaries are identified through fossil evidence, sediment analysis, and radiometric dating.
The Cretaceous-Paleogene boundary (66 million years ago) marks the extinction of dinosaurs and rise of mammals.
Common Mistake- Don’t confuse era with epoch.
- Eras span tens to hundreds of millions of years, while epochs are shorter subdivisions within periods.
Fossil Record and Epoch Division
- Fossils act as “biostratigraphic markers”, indicating evolutionary milestones.
- Changes in fossil assemblages mark transitions between epochs, revealing how species adapted after global events.
- The Pentadactyl limb (five-limbed structure in mammals, reptiles, and amphibians) demonstrates divergent evolution from a common ancestor, highlighting adaptive modification over time.
Whale flippers, bat wings, and human hands all share the pentadactyl structure, evidence for homology and evolutionary relationship.
Mass Extinction
- A mass extinction is a global, rapid decrease in biodiversity, where a large number of species go extinct in a relatively short geological period.
- Earth has experienced five major mass extinctions, each reshaping ecosystems and paving the way for new dominant species.
