Chapter 26: History of Life on Earth and Human Evolution

Instructor: Dr. Subhankar Mandal
Course: BIO 182, General Biology II
Term: Fall 2025

Key Concepts

  • The Fossil Record

  • History of Life on Earth

  • Human Evolution

Origin of Life

  • The Big Bang occurred 13.8 billion years ago (bya) and is believed to have initiated the universe.

  • Our solar system formed 4.6 billion years ago.

  • Life is thought to have emerged between 4.0 and 3.5 billion years ago on Earth.

  • Fossils from 3.5 billion years ago show resemblance to modern cyanobacteria, indicating early life forms.

The Fossil Record

  • Fossils are defined as the preserved remains of past life on Earth.

  • Paleontologists specialize in the study of fossils.

  • Many fossils are found in sedimentary rocks formed from particles of older rocks (sediments).

    • These sediments accumulate due to erosion and pressure create new rock layers.

    • When organisms are buried quickly, their hard parts can be replaced by minerals through processes like permineralization.

  • The depth of rock layers indicates their age; older rock layers tend to be found deeper within the Earth's crust.

History of Life on Earth

  • The geological time scale outlines the timeline of Earth's history from its formation (4.55 billion years ago) to the present.

  • This timeline is divided into four eons:

    • Hadean

    • Archaean

    • Proterozoic

    • Phanerozoic

  • The Precambrian period includes the Hadean, Archaean, and Proterozoic eons.

  • Each eon can be further divided into eras and periods.

Major Time Events

  • The geological time scale includes significant milestones in evolutionary history, as illustrated in Figure 26.4.

  • Notable events include:

    • 7 million years ago: First hominoids appear

    • 135 million years ago: Flowering plants first appear

  • Major milestones timeline:

    • 1.5 billion years ago: Multicellular eukaryotic organisms first appear

    • 1.8 billion years ago: Eukaryotic cells first appear

    • 3.5 billion years ago: Fossils of primitive cyanobacteria found

    • Greek classification of organisms based on these timelines and species evolution observed in fossil records.

Changes in Species

  • Changes in living organisms can be attributed to:

    • Genetic changes within populations

    • Environmental changes affecting survival and reproduction

  • These changes can allow new types of organisms to flourish, but can also lead to mass extinctions, defined as the complete loss of a species or groups of species.

Major Environmental Changes

  • Factors contributing to species changes include:

    • Temperature fluctuations

    • Atmospheric composition (e.g., amounts of oxygen)

    • Landmass shifts

    • Floods and Glaciation events

    • Volcanic eruptions

    • Meteorite impacts

Mass Extinctions

  • There have been five large mass extinction events in Earth’s history associated with major geological transitions. These include:

    • Ordovician, Devonian, Permian, Triassic, and Cretaceous extinction events.

  • The boundaries between geological time periods are often marked by these extinction events.

  • Currently, we are witnessing a sixth mass extinction, attributed to rapid extinction of many species due to human activities.

Proterozoic Eon

Multicellular Eukaryotes

  • Multicellular eukaryotes emerged approximately 1.5 billion years ago.

  • Two theoretical origins of multicellularity include:

    • Individuals forming colonies

    • A single cell dividing and remaining attached to form a multicellular organism.

  • Volvocine green algae exhibit varying degrees of multicellularity, showcasing early evolutionary transitions.

Emergent Properties

  • The formation of different cell types is an emergent property of multicellularity, resulting from complex interactions between cells.

    • Example: Volvox aureus, consisting of 1,000 to 2,000 cells, demonstrates two cell types: somatic and reproductive cells.

First Animals

  • Multicellular invertebrates were the first animals to appear toward the end of the Proterozoic eon, many exhibiting bilateral symmetry that facilitates locomotion.

Phanerozoic Eon

  • Characterized by the extensive proliferation of multicellular eukaryotic life from 543 million years ago to the present.

  • Divided into three principal eras:

    • Paleozoic Era

    • Mesozoic Era

    • Cenozoic Era

Paleozoic Era (543 to 248 million years ago)

  • Comprised of multiple periods:

    • Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.

Cambrian Period

  • Occurring from 543 to 490 million years ago, characterized by warm and wet climates with no ice at the poles.

  • Notable for the Cambrian Explosion, a rapid increase in diversity of animal species, possibly due to:

    • Evolution of shells allowing animals to exploit new environments

    • Increased atmospheric oxygen fostering animal diversity

    • An "evolutionary arms race" between species leading to diversification.

Ordovician Period

  • Lasted from 490 to 443 million years ago, featuring a diverse group of marine invertebrates (e.g., trilobites and brachiopods).

  • The first primitive land plants and arthropods invaded land.

  • An abrupt climate change towards the end of the period led to a mass extinction where over 60% of marine invertebrates became extinct.

Silurian Period

  • From 443 to 417 million years ago, characterized by a relatively stable climate.

  • Introduction of significant new vertebrates and the evolution of early vascular plants, allowing terrestrial colonization.

Devonian Period

  • Spanning 417 to 354 million years ago, marked by a major increase in terrestrial species and the emergence of insects and tetrapods (amphibians).

  • The Age of Fishes was defined by a great flourishing of fish species, with some prolonged extinctions occurring near the period’s end.

Carboniferous Period

  • Occurred 354 to 290 million years ago; rich coal deposits formed from extensive forested swamps.

  • Characterized by the diversification of plants and animals, and the emergence of flying insects and the first amniotic eggs in reptiles.

Permian Period

  • Extended from 290 to 248 million years ago.

  • Continental drift formed the supercontinent Pangaea, leading to drier interior regions and shifts to dominant gymnosperms.

  • At the end of this period, a mass extinction event occurred, resulting in the extinction of 90 to 95% of marine species due to possible glaciations or volcanic eruptions.

Mesozoic Era

  • Defined by the Permian extinction, denoting the transition into the Age of Dinosaurs (248 to 65 million years ago).

  • Characterized by consistently hot climates and dry terrestrial environments.

  • Divided into three periods:

  • Triassic, Jurassic, and Cretaceous periods.

Triassic Period

  • Spanning 248 to 206 million years ago, overrun by reptiles and the first dinosaurs.

  • First true mammals emerged alongside dominant gymnosperms.

Jurassic Period

  • Lasted 206 to 144 million years ago, where gymnosperms continued to dominate and some dinosaurs grew to enormous size.

  • Early bird-like creatures also appeared, along with the presence of mammals, though not prevalent.

Cretaceous Period

  • Extending from 144 to 65 million years ago, dinosaurs remained the dominant terrestrial species.

  • Emergence of earliest flowering plants (angiosperms) occurred.

  • This period ended with another significant mass extinction event, wherein dinosaurs went extinct likely due to meteor impacts or volcanic eruptions.

Cenozoic Era

  • Spans most recent 65 million years, divided into Tertiary and Quaternary periods.

  • Transitional climatic conditions shifted from tropical to a colder, drier climate, often referred to as The Age of Mammals.

  • This era observes an incredible diversification of birds, fish, insects, and flowering plants.

Tertiary Period

  • From 65 to 1.8 million years ago, mammals expanded rapidly, with angiosperms becoming the dominant terrestrial plants.

  • Hominoids (includes humans and relatives such as chimpanzees and gorillas) appeared about 7 million years ago.

Quaternary Period

  • Lasting 1.8 million years ago to present, marked by periodic ice ages and widespread extinctions among mammals.

  • Significant evolutionary changes occurred in hominids, leading to the emergence of Homo sapiens approximately 200,000 years ago.

Human Evolution

  • Inquiry questions related to human evolution include:

    • How closely related are humans to their nonhuman relatives?

    • What evolutionary events led to the rise of modern humans?

    • Level of genetic variation among present-day humans.

    • Are humans still undergoing evolution?

Primate Evolution

  • Primates evolved from a group of small, arboreal, insect-eating mammals approximately 85 million years ago.

  • Characterized by traits including:

    • Binocular vision

    • Nails on digits instead of claws

    • Grasping hands

    • Larger brains relative to body size

    • Complex social structures and parenting behaviors

Taxonomic Groups of Primates

  • Strepsirrhini:

    • Small species like bush babies and lemurs, predominantly nocturnal.

    • Recognized for their wet noses with no fur at the tip.

  • Haplorrhini:

    • Features dry noses and fully forward-facing eyes; includes tarsiers and larger-brained anthropoids.

    • Further divided into:

      • Monkeys

      • Hominoidea: Includes gibbons, orangutans, gorillas, chimpanzees, and humans.

Differentiation between Monkeys and Hominoids

  • Monkeys typically possess tails, while hominoids do not.

  • Hominoids exhibit more mobile shoulder joints, broader rib cages, and shorter spines.

  • Humans are close relatives of chimpanzees and gorillas but did not evolve directly from them; all hominoids share a common ancestor.

Comparison of Genomes

  • Comparative analysis shows that the genomes of humans and chimpanzees differ by only 1.23%.

    • This is substantially less than the differences between mouse and rat genomes.

  • Proteomic analysis reveals that roughly 29% of all proteins are identical, with variations mainly due to single amino acid substitutions.

  • Genetic variability largely arises from chromosomal inversions and duplications, with some differences influencing speech development.

Bipedalism in Hominins

  • The divergence leading to modern humans occurred around 7 million years ago in Africa, differentiating a lineage of humans from other primates.

  • Bipedalism is the key characteristic that differentiates hominins from other apes, and it is associated with significant anatomical changes such as:

    • Foramen magnum being situated more toward the front of the skull

    • Broader pelvis to support bipedal weight

    • Enlarged lower limbs.

Progression of Hominin Species

  • Australopithecus afarensis:

    • First fossil discovered in South Africa; female skeleton known as “Lucy” found in Ethiopia.

    • Smaller than modern humans, with a facial structure and brain size similar to chimpanzees.

    • Exhibits significant sexual dimorphism and was bipedal.

  • Paranthropus:

    • Larger-boned hominins found in the 1930s; identified as a separate genus.

    • Primarily vegetarian with large jaws for grinding roots; went extinct around 1.5 to 2 million years ago.

  • Homo habilis:

    • Fossils found in Olduvai Gorge, Tanzania, estimated to be 2 million years old.

    • Characterized by a larger brain than Australopithecus and associated with stone tools.

  • Homo ergaster:

    • Likely arose from Homo habilis around 2 million years ago.

    • Had human-like facial structure and skull; considered the first to leave Africa.

  • Homo erectus:

    • Large hominin closely resembling modern humans but with heavier bones and slightly smaller brain size.

    • Evidence of tool use and cooking; spread out of Africa after their emergence over a million years ago.

    • Went extinct around 100,000 years ago.

  • Homo heidelbergensis:

    • Similar in form to modern humans, showed tool use for hunting; gave rise to Neanderthals, Denisovans, and Homo sapiens.

  • Homo neanderthalensis (Neanderthals):

    • Discovered in the Neander Valley of Germany; stockier and shorter with a larger brain than Homo sapiens.

    • Artifacts indicate they might have used speech; went extinct 40,000 to 30,000 years ago.

  • Denisovans:

    • Discovered through genetic testing of remains found in Denisova Cave, Siberia; likely distinct from Neanderthals and modern humans.

    • Estimated to have existed around 400,000 years ago and possibly coexisted with Neanderthals and modern humans.

  • Homo sapiens:

    • Our species, identified as “wise man”; slight smaller brain capacity than Neanderthals.

    • Factors leading to their endurance include lower energy needs, increased longevity, and advanced social structures.

Out-of-Africa Model

  • Evidence indicates that Homo sapiens evolved from Homo heidelbergensis in Africa about 200,000 years ago.

  • Some migrated from Africa, leading to the replacement of other hominid species like Neanderthals.

  • An alternative hypothesis, the multiregional hypothesis, suggests parallel evolution of human groups from Homo ergaster across various global regions, though this is less accepted.

Interbreeding Among Hominids

  • DNA studies show that Homo sapiens interbred with related hominid species throughout Europe and Asia.

  • Modern humans of African descent generally carry little Neanderthal or Denisovan DNA, whereas those of European descent have 1 to 4% from Neanderthals, with 20% of the Neanderthal genome represented.

  • Southeast Asians may show approximately 4 to 6% Denisovan DNA.

Are Humans Still Evolving?

  • Human evolution via natural selection continues to manifest. Examples of current adaptations include:

    • Disease resistance (e.g., bubonic plague)

    • Lactose tolerance in populations that consume dairy

    • The prevalence of blue eyes across some European populations over the past 10,000 years.

Genetic Variation in Modern Humans

  • Although humans display striking physical differences globally, genetic variation is relatively low.

  • Humans exhibit lower genetic variance compared to most other mammal species.

  • 99.9% of human genetic variation consists of single nucleotide polymorphisms (SNPs).

  • With a genome of 3 billion base pairs, differences between individuals are about 4.5 million SNPs, equating to only 0.15% variation.

  • Minimal additional genetic variance can also be observed amongst geographically separated human populations.