CH: 26 History of Life on Earth and Human Evolution

Fossils

Preserved remains, impressions, or traces of organisms that lived in the past.

Fossil record

The complete collection of fossils documenting the history of life on Earth.

Importance of fossil record

Allows reconstruction of evolutionary history and long-term biological change.

Sedimentary rock

Rock type where fossils are most common due to gentle formation.

Formation of sedimentary rock

Particles settle in layers, then compact and cement over time.

Why hard parts fossilize easily

Bones and shells resist decay and mineralize readily.

Relative dating

Determining fossil age by comparing positions of rock layers.

Law of superposition

Lower sedimentary layers are older than upper layers.

Limitations of relative dating

Folding, faulting, and erosion can rearrange layers.

Radiometric dating

Determines absolute age using radioactive isotope decay.

Radioactive half-life

Time required for half of a radioactive isotope to decay.

Why different isotopes are used

Different half-lives allow dating of rocks of different ages.

Drivers of long-term evolution

Genetic change and environmental change.

Impact of climate on evolution

Temperature shifts like Ice Ages shaped survival and extinction.

Importance of oxygen to life

Enabled aerobic respiration and complex multicellular life.

Source of early oxygen

Photosynthesis by cyanobacteria.

Stromatolites

Layered rock structures formed by cyanobacteria.

Why stromatolites matter

Evidence of early autotrophic life and oxygen production.

Great Oxygenation Event

Period when atmospheric oxygen levels rose significantly.

Why oxygen was toxic initially

Early life was anaerobic and could not tolerate oxygen.

Advantage of aerobic respiration

Produces more energy than anaerobic metabolism.

Proterozoic Eon

Time period from 2.5 billion to 543 million years ago.

Origin of eukaryotes

Eukaryotic cells arose during the Proterozoic eon.

Endosymbiotic theory

Eukaryotes evolved through symbiosis between bacteria and archaea.

Evidence for endosymbiosis

Mitochondria and chloroplasts have their own DNA.

Bacterial-origin organelles

Mitochondria and chloroplasts.

Endosymbiont

Smaller organism living inside a host cell.

Endocytosis

Process allowing early cells to engulf bacteria.

Origin of multicellularity

Evolved through cell aggregation or cell division with adhesion.

Importance of multicellularity

Allowed increased size, specialization, and complexity.

Cell differentiation

Process where cells specialize for different functions.

Somatic cells

Cells responsible for survival functions.

Reproductive cells

Cells specialized for producing offspring.

First animals

Invertebrates without backbones.

Bilateral symmetry

Mirrored left-right body plan.

Advantage of bilateral symmetry

Improves directional movement and coordination.

Plate tectonics and evolution

Continental movement altered habitats and climates.

Environmental drivers of extinction

Glaciation, flooding, volcanic eruptions, and climate shifts.

Mass extinctions

Rapid events eliminating many species.

First prokaryotes

Arose over 3.5 billion years ago.

Why early life was prokaryotic

Early Earth lacked oxygen.

Prokaryotic domains

Bacteria and Archaea.

Difference between bacteria and archaea

Differ in metabolism, membranes, and genetics.

Autotroph vs heterotroph

Autotrophs produce food; heterotrophs consume others.

Phanerozoic Eon

Most recent eon beginning ~543 mya with abundant visible life.

Meaning of Phanerozoic

“Visible life.”

Phanerozoic eras

Paleozoic, Mesozoic, and Cenozoic.

Paleozoic Era

Diversification of marine life and colonization of land.

Cambrian Period

Warm climate and rapid marine animal diversification.

Cambrian Explosion

Rapid appearance of major animal body plans.

Significance of Cambrian Explosion

Established foundational body plans.

Burgess Shale

Fossil site with exceptional soft-body preservation.

Cause of Burgess Shale preservation

Rapid burial in oxygen-poor conditions.

Ordovician Period

Warm oceans ending in mass extinction.

Ordovician mass extinction

Cooling and glaciation eliminated many marine species.

Silurian Period

Climate stability and early land colonization.

Key Silurian innovation

Vascular plants.

Devonian Period

“Age of Fishes” and first forests.

First tetrapods

Early four-limbed vertebrates.

Carboniferous Period

Swamp forests and coal formation.

Origin of coal

Compressed ancient plant material.

Amniotic egg

Adaptation allowing reproduction away from water.

Permian Period

Pangaea formation and largest mass extinction.

Permian mass extinction

Eliminated 90–95% of marine species.

Likely cause of Permian extinction

Massive volcanism and oxygen depletion.

Mesozoic Era

Age of Dinosaurs.

Triassic Period

First dinosaurs and mammals.

Jurassic Period

Giant dinosaurs and first bird-like animals.

Archaeopteryx

Fossil with dinosaur and bird traits.

Cretaceous Period

Rise of flowering plants and dinosaur dominance.

Angiosperms

Flowering plants.

K–Pg extinction

Asteroid impact causing dinosaur extinction.

Cenozoic Era

Diversification of mammals and flowering plants.

Age of Mammals

Common name for Cenozoic era.

Tertiary Period

Mammal radiation and primate diversification.

Quaternary Period

Ice Ages and human evolution.

Primate evolution

Evolved from arboreal insect-eating mammals.

Defining primate traits

Binocular vision, grasping hands, large brains, social behavior.

Bipedalism

Upright walking on two legs.

Advantages of bipedalism

Efficient travel and free hands for tools.

Australopithecus

Early bipedal hominin with small brain.

Australopithecus afarensis

Species known from “Lucy.”

Paranthropus

Robust hominins with powerful jaws.

Homo habilis

Early Homo species using stone tools.

Homo ergaster

First hominin to leave Africa.

Homo erectus

Widespread tool-using, fire-using hominin.

Homo heidelbergensis

Ancestor of Neanderthals and modern humans.

Homo neanderthalensis

Cold-adapted hominin with large brain.

Denisovans

Hominin group known primarily from DNA.

Homo sapiens

Modern humans appearing ~200,000 years ago.

Out-of-Africa hypothesis

Humans evolved in Africa and spread globally.

Evidence of interbreeding

Modern humans carry Neanderthal and Denisovan DNA.

Human genetic diversity

Very low worldwide variation.

Reason for low diversity

Recent origin and small early populations.