CH 25 outline ADA.docx

Chapter 25: The History of Life on Earth Overview

• The largest fully terrestrial animal in Antarctica is a 5-mm-long fly.

• Five hundred million years ago, Antarctica was surrounded by warm ocean waters filled with tropical invertebrates.

• The continent was later covered with forests where dinosaurs and predatory "terror birds" hunted.

• Past organisms were significantly different from modern ones.

• Macroevolution: Broad patterns of evolution above the species level, including:

  • Evolution of terrestrial vertebrates.

  • The impact of mass extinctions on life diversity.

  • Key adaptations like flight in birds.

Concept 25.1: Conditions on Early Earth and Origin of Life

• Chemical and physical processes on early Earth fostered the emergence of very simple cells through natural selection.

• Four Main Stages:

  1. Abiotic synthesis of small organic molecules (monomers).

  2. Joining of monomers into macromolecules.

  3. Packaging into protocells, which are droplets with membranes maintaining distinct internal chemistry.

  4. Origin of self-replicating molecules, enabling inheritance.

Speculative Nature of Early Origin of Life

• The scenario is speculative but leads to testable predictions.

• Earth formed about 4.6 billion years ago from dust and rocks around the sun.

• Life was unlikely in the first few hundred million years due to bombardment:

  • Collisions generated heat vaporizing water.

• The first atmosphere was likely reducing, thick with water vapor, nitrogen, carbon dioxide, methane, and ammonia.

• After cooling, water vapor condensed to form oceans. High ultraviolet radiation contributed to organic compound formation.

• Scientists postulated that organic compounds from inorganic precursors formed under early atmospheric conditions (Oparin-Haldane hypothesis).

  • Miller-Urey experiment (1953) tested this hypothesis by recreating early Earth conditions, succeeding in producing amino acids and other organic molecules.

Evidence of Early Organic Compounds

• Various gas mixtures continue to produce organic molecules.

• Meteorites, such as the Murchison meteorite, have revealed amino acids, simple sugars, and nitrogenous bases (uracil).

• Laboratory studies indicate spontaneous synthesis of RNA monomers, linking to the origin of life hypothesis.

• Protocells with similar properties to life can form spontaneously from simple compounds:

  • Formation of vesicles from lipids in water.

  • Vesicles can increase in size and carry out metabolic reactions.

RNA as the First Genetic Material

• RNA likely served as the first genetic material due to its catalytic and self-replicating abilities.

• Laboratory experiments demonstrate RNA evolution under abiotic conditions.

• Evolution through mutations and natural selection in replicating RNA sequences hints at the emergence of life.

Transition from RNA to DNA World

• RNA served a dual role in early life before being superseded by DNA as the genetic archive.

• DNA is more stable and replicates more accurately than RNA.

Concept 25.2: Fossil Record Documentation of Life’s History

• Sedimentary rocks are the richest source of fossils, revealing organisms' sequences over time, albeit not their precise ages.

• The fossil record illustrates significant biological changes, highlighting extinctions and emerging new groups.

• Paleontology aids in predicting fossil locations based on evolutionary relationships and geological dating techniques.

Dating Fossils

• Radiometric dating methods based on radioactive isotopes are essential for determining fossil ages.

• Carbon-14 dating applies to recent fossils; older fossils require isotopes with longer half-lives.

• Fossils between layers of volcanic rock can yield deeper insights into their ages.

Evolution of Tetrapods and Mammals

• Mammals share distinct anatomical traits, allowing them to form substantial fossil records.

• Unique evolutionary features in mammals stem from gradual modifications from their ancestors.

Concept 25.3: Unicellular to Multicellular Life and Land Colonization

• Life's history divided into four eons: Hadean, Archaean, Proterozoic, and Phanerozoic (last half billion years).

• The oldest known fossils, stromatolites, date back to 3.5 billion years ago, indicating the early forms of life.

• Prokaryotes significantly impacted Earth's environment, leading to the rise of atmospheric oxygen during the oxygen revolution.

Changes in Oxygen Levels and Eukaryotic Evolution

• The gradual build-up of oxygen in the atmosphere revolutionized life.

• Eukaryotic cells evolved through endosymbiosis with prokaryotic ancestors leading to complex structures with specialized functions.

Evolution of Multicellularity

• Unicellular eukaryotes led to multicellular forms, culminating in diverse life forms, including plants, fungi, and animals.

• The thawing from severe ice ages allowed for significant diversification events in multicellular eukaryotes.

Adaptive Radiation and Extinction Events

• Adaptive radiations often follow major mass extinctions, allowing surviving species to fill newly available ecological niches.

• Mass extinctions reshape biological diversity, creating significant impacts that require millions of years for recovery.

Concept 25.5: Evolutionary Changes through Developmental Genes

• Variation in developmental genes can lead to significant morphological changes.

• Evolutionary transformations stem from genetic changes influencing organismal development patterns.

Concept 25.6: Nature of Evolution

• Evolution is driven by distinct factors, with no predetermined direction, emerging from natural selection and environmental pressures over time.