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

Chapter 25: The History of Life on Earth

Key Concepts and Questions

• Stages for the origin of life

• Types and significance of fossils

• Dating fossils and geological time

• Major eons and eras

• Endosymbiotic theory

• Mass extinctions and adaptive radiations

• Developmental and regulatory evolution

• Purpose and trends of evolution

How Has Life on Earth Changed Over Time?

Macroevolution and the Fossil Record

Macroevolution refers to evolutionary changes above the species level, documented by the fossil record. The fossil record reveals broad patterns such as the emergence of terrestrial vertebrates, mass extinctions, and the origin of key adaptations (e.g., flight).

• Macroevolution: Evolutionary change at or above the species level.

• Fossil Record: Preserved remains or traces of organisms, showing changes over time.

• Example: Fossils in the Saharan Desert show the transition of whales from land to sea.

CONCEPT 25.1: Conditions on Early Earth Made the Origin of Life Possible

Four Stages for the Origin of Life

Chemical and physical processes on early Earth could produce simple cells through four main stages:

1. Abiotic synthesis of small organic molecules

2. Joining of these molecules into macromolecules

3. Packaging of molecules into protocells (droplets with membranes)

4. Origin of self-replicating molecules

Synthesis of Organic Compounds on Early Earth

• Earth formed about 4.6 billion years ago.

• Early atmosphere had little oxygen, but much water vapor and volcanic compounds.

• Organic compounds may have formed in reducing conditions near volcanoes.

Sources of Organic Molecules

• Hydrothermal Vents: Deep-sea vents with hot water and minerals; "black smoker" vents are extremely hot (300–400°C), while alkaline vents are milder (40–90°C) and more suitable for life’s origin.

• Meteorites: Fragments like the Murchison meteorite contain amino acids, lipids, sugars, and nitrogenous bases.

Abiotic Synthesis of Macromolecules

• All four RNA monomers can be synthesized abiotically in lab conditions.

• RNA polymers form spontaneously on hot sand, clay, or rock, acting as weak catalysts.

Protocells

Protocells are membrane-bound droplets that can maintain an internal chemistry distinct from their environment.

• Formed from fluid-filled vesicles with lipid bilayers.

• Montmorillonite clay increases vesicle formation rate.

• Vesicles can grow, reproduce, metabolize, and maintain internal environments.

Self-Replicating RNA

• First genetic material was likely RNA.

• Ribozymes: RNA molecules that catalyze reactions.

• RNA molecules with different sequences fold into different shapes; copying errors can produce new, more efficient shapes.

• DNA is more stable and accurate for replication than RNA.

CONCEPT 25.2: The Fossil Record Documents the History of Life

Types of Fossils

• Strata: Layers of sedimentary rock.

• Amber: Hardened tree resin preserving entire organisms.

• Mineralized Organic Matter: Minerals replace organic material (e.g., petrified wood).

• Trace Fossils: Footprints, burrows, or other activity traces.

• Frozen Soil, Ice, Acid Bogs: Rare preservation of large bodies.

Benefits and Problems of the Fossil Record

• Shows major changes in organism types over time.

• Many past organisms are extinct; new groups arise from old ones.

• Incomplete due to preservation bias (favoring abundant, long-lived, hard-bodied species).

Dating Rocks and Fossils

• Relative Dating: Order of fossils in strata shows sequence, not actual age.

• Radiometric Dating: Uses decay of radioactive isotopes to determine age.

• Half-life: Time for 50% of parent isotope to decay.

• Carbon-14 dating is useful up to 75,000 years; older fossils dated using isotopes in volcanic rock.

$\text{Half-life} = \text{Time required for 50\% of parent isotope to decay}$

The Geological Record

The geologic record divides Earth's history into four eons: Hadean, Archaean, Proterozoic, and Phanerozoic. The Phanerozoic eon includes the Paleozoic, Mesozoic, and Cenozoic eras.

Eon	Era	Period	Approximate Age (Millions of Years Ago)
Phanerozoic	Cenozoic	Quaternary	0–2.6
Phanerozoic	Mesozoic	Cretaceous	66–145
Phanerozoic	Paleozoic	Permian	252–299
Proterozoic	—	—	541–2500
Archaean	—	—	2500–4000
Hadean	—	—	4000–4600

The Origin of New Groups of Organisms

Mammalian Evolution

• Mammals are tetrapods (four-limbed vertebrates).

• Originated from synapsids, a group of tetrapods.

• Jaw and teeth evolved gradually; bones of the jaw hinge became ear bones (malleus and incus).

• Mammalian teeth are specialized: incisors (tearing), canines (piercing), molars (crushing/grinding).

CONCEPT 25.3: Key Events in Life’s History

Major Eons and Eras

• Hadean, Archaean, Proterozoic, Phanerozoic

• Phanerozoic: Paleozoic, Mesozoic, Cenozoic

• Major boundaries correspond to extinction events

The First Single-Celled Organisms

• Stromatolites: Layered rocks formed by prokaryotes binding sediment; earliest evidence of life (3.5 billion years ago).

• Prokaryotes were the sole inhabitants for over 1.5 billion years.

Photosynthesis and the Oxygen Revolution

• Oxygen produced by photosynthesis reacted with iron, forming banded iron formations.

• Atmospheric oxygen increased rapidly about 2.4 billion years ago, causing many prokaryotic extinctions and driving survivors to anaerobic habitats or adaptation to aerobic respiration.

The First Eukaryotes and Endosymbiotic Theory

• Oldest eukaryote fossils: 1.8 billion years ago.

• Endosymbiotic Theory: Eukaryotes originated when a prokaryote engulfed another cell, which became a mitochondrion.

• Serial endosymbiosis: Mitochondria evolved before plastids (chloroplasts).

• Evidence: Similarities in membrane proteins, DNA, replication, and ribosomes between mitochondria/plastids and bacteria.

Origin of Multicellularity

• Multicellular eukaryotes evolved from unicellular ancestors, leading to diversification of algae, plants, fungi, and animals.

• Oldest multicellular fossils: red algae (1.2 billion years ago); Ediacaran biota (600 million years ago).

The Cambrian Explosion

• Sudden appearance of many animal phyla in Cambrian period (535–525 million years ago).

• Predation and defense adaptations appeared rapidly.

• DNA evidence suggests animal phyla diverged before the Cambrian.

Colonization of Land

• Prokaryotes colonized land 3.2 billion years ago; fungi, plants, and animals about 500 million years ago.

• Adaptations for land: waxy coatings, vascular systems, mutualisms (mycorrhiza).

• Arthropods and tetrapods were among the first land animals.

CONCEPT 25.4: The Rise and Fall of Groups of Organisms

Plate Tectonics and Continental Drift

• Earth’s crust is composed of plates that move, causing continental drift.

• Formation and breakup of supercontinents (e.g., Pangaea) altered habitats and climate, driving speciation and extinction.

• Allopatric speciation occurs when continents split and populations become isolated.

Mass Extinctions

• Most species that ever lived are now extinct.

• Mass extinctions: rapid, global loss of species due to disruptive changes.

• Five major mass extinctions documented in the fossil record.

Event	Time (mya)	Significance
Permian Extinction	252	96% of marine species extinct; caused by volcanism, climate change, ocean acidification
Cretaceous Extinction	66	50% of marine species, many terrestrial species, and all non-avian dinosaurs extinct; meteorite impact

Consequences of Mass Extinctions

• Recovery of diversity takes millions of years.

• Extinctions change community composition and can eliminate lineages with novel features.

• Adaptive radiations often follow mass extinctions, with rapid diversification of survivors.

Adaptive Radiations

• Rapid evolution of new species to fill ecological niches.

• Triggered by mass extinctions, novel adaptations, or colonization of new regions.

• Examples: Mammals after dinosaur extinction, plants and insects on land.

CONCEPT 25.5: Major Changes in Body Form

Developmental Genes and Evolution

• Genes control rate, timing, and spatial pattern of development.

• Heterochrony: Evolutionary change in timing/rate of developmental events.

• Paedomorphosis: Retention of juvenile features in sexually mature adults.

• Homeotic Genes: Master regulatory genes (e.g., Hox genes) determine body part placement.

• Changes in gene sequence or regulation can produce new morphological forms.

CONCEPT 25.6: Evolution Is Not Goal-Oriented

Nature of Evolutionary Change

• Evolution modifies existing structures; new forms arise by gradual changes.

• Exaptations: Structures evolved for one function, later co-opted for another.

• Trends in evolution result from interactions with the environment, not intrinsic drives.

Summary Table: Major Events in the History of Life

Event	Approximate Time (bya/mya)
Earth forms	4.6 bya
First life (prokaryotes)	3.5 bya
Oxygen revolution	2.4 bya
First eukaryotes	1.8 bya
Multicellular eukaryotes	1.2 bya
Cambrian explosion	535–525 mya
Colonization of land	500 mya
Permian extinction	252 mya
Cretaceous extinction	66 mya

Additional info: These notes synthesize textbook slides and lecture content, expanding on key terms, processes, and evolutionary events relevant to General Biology students.