Geologic Time Scale

GEOLOGIC TIME SCALE OF THE HISTORY OF LIFE ON EARTH

LEARNING OBJECTIVES:

  • Describe the general features of the different subdivisions that characterized the earliest animals and when they appeared on Earth.

  • Identify the subdivisions of the geologic time scale.

  • Appreciate the beginnings of life and its existence in the present time.

GEOLOGIC TIME

  • Refers to the extensive interval of Earth's geologic history, serving as a "calendar" for Earth's events.

  • Geologic time is subdivided into:

    • Epochs: less than 10 million years

    • Periods: tens of millions of years

    • Eras: hundreds of millions of years

    • Eons: billions of years

GEOLOGIC TIME SCALE (Older ➜ Younger)

  • Eon:

    • Phanerozoic (Today to -541 Ma)

    • Era: Cenozoic

      • Period: Quaternary

      • Epoch: Holocene (Today)

      • Epoch: Pleistocene (11.8 Ka)

      • Epoch: Pliocene

    • Era: Mesozoic

      • Period: Cretaceous (66 Ma)

      • Epoch: Most of Earth's history (Precambrian)

      • Period: Jurassic

      • Period: Triassic (252 Ma)

    • Era: Paleozoic

      • Period: Carboniferous

      • Period: Permian

  • Eon:

    • Precambrian

    • Proterozoic (-2.5 Ga)

    • Archean (-4.0 Ga)

    • Hadean (-4.54 Ga)

EARTH

  • Approximately 4.5 billion years old.

  • Experienced numerous geological and biological changes that have altered its landscape and life forms.

IMPORTANCE OF GEOLOGIC TIMELINE

  • Helps trace how fossils and living organisms have evolved since life began.

  • Scientists study Earth's changing landforms, water bodies, and atmosphere to understand the evolution of species.

  • Common ancestry is suggested by both similarities and differences among present-day organisms, highlighting evolution from a shared origin.

GEOLOGIC TIME SCALE

ERA: MESOZOIC

  • Middle Life

CENOZOIC

  • Recent Life

PERIODS

  • Quaternary: 0-1 Million Years • Rise of Man

  • Tertiary: 62 Million Years • Rise of Mammals

  • Cretaceous: 72 Million Years • Modern Seed Bearing Plants, Dinosaurs

  • Jurassic: 40 Million Years • First Birds

  • Triassic: 49 Million Years • Cycads, First Dinosaurs

  • Paleozoic: Ancient Life

  • Permian: 60 Million Years • First Reptiles

  • Pennsylvanian: 30 Million Years • First Insects

  • Mississippian: 35 Million Years • Many Crinoids

  • Devonian: 60 Years • Plants

  • Silurian: 20 Million Years • Earliest Land Animals

  • Ordovician: 75 Million Years • Early Bony Fish

  • Cambrian: 100 Million Years • Invertebrate animals (Brachiopods)

  • Precambrian: Very few fossils present (bacteria-algae-pollen?)

PRECAMBRIAN SUPEREON (4.6B to 541M years ago)

  • Hadean Eon: (4.6B - 4B years ago)

  • Archean Eon: (4B - 2.5B years ago)

  • Proterozoic Eon: (2.5B - 541M years ago)

EARTH'S EARLY CONDITIONS

  • Life on Earth began more than 600 million years after the planet's formation.

  • Earth cooled from a molten state, forming a solid crust and oceans from atmospheric water vapor.

  • Life emerged in primordial seas, possibly near hot, mineral-rich volcanic vents.

  • Volcanic vents may have acted as catalysts for chemical reactions on the surface of water bubbles, leading to the formation of the first cell membranes.

  • Some bubbles attracted chemicals, forming self-replicating substances. Eventually, these processes combined, resulting in energy-using, living cells.

EARLIEST LIVING ORGANISMS

  • Microscopic bacteria, the earliest organisms, appeared around 3.4 billion years ago.

  • Bacteria adapted by developing photosynthesis after depleting their chemical fuel, enabling the evolution of simple plants and eventually animals.

EARLIEST EVIDENCE OF LIFE

  • Precambrian microfossils resembling algae.

  • Cysts of flagellates.

  • Tubes interpreted as the remains of filamentous organisms.

  • Stromatolites: Sheetlike mats formed by microbial communities.

OXYGENATION OF EARTH'S ATMOSPHERE

  • Around 3 billion years ago, Earth had little to no oxygen, but cyanobacteria began releasing it through photosynthesis about 2.4 billion years ago.

  • Oxygen levels gradually increased, reaching 1% by 2 billion years ago and 21% around 800 million years ago, allowing complex life to evolve.

  • Rising oxygen led to the development of the ozone layer, protecting Earth's surface from harmful solar radiation.

GREAT OXYGENATION

  • The rise of oxygen levels is often referred to as the Great Oxidation Event, where evidence for cyanobacteria arose around 2.4 billion years ago.

  • Atmospheric changes led to significant biological and ecological impacts, allowing for the Cambrian explosion.

LATE PRECAMBRIAN DEVELOPMENTS

  • Evolution of the first multicellular organisms and the emergence of sexual reproduction.

MULTICELLED ANIMALS

  • Ediacarans: Early multicelled animals appeared 600 million years ago on the seabed. They lacked heads, mouths, or digestive organs, with fossils like Dickinsonia resembling doormats. Their fate is unclear; they may have been ancestors of later animals or went extinct without descendants.

EARLY MULTICELLULAR CREATURES

  • Poriferans: Simple, soft-bodied creatures without organs or nervous systems.

  • Annelids: Segmented flatworms with fluid-filled bodies, breathing through their skin.

  • Cnidarians: Included jellyfish, corals, and sea anemones, with simple digestive systems and stinging tentacles.

END OF PRECAMBRIAN

  • By the end of the Precambrian, conditions were favorable for the Cambrian explosion of life (~541 million years ago), marking the start of the Phanerozoic Eon.

START OF THE CAMBRIAN PERIOD

  • The Cambrian explosion marks a rapid diversification of life and the emergence of over 35 new animal phyla. This event signifies a critical period of intense biological diversification in Earth's history. The term "Precambrian" persists to distinguish rocks older than Cambrian rocks.

PHANEROZOIC EON (541M years ago to the present)

  • Divided into:

    • Paleozoic Era (541M - 252M years ago)

    • Mesozoic Era (252M - 66M years ago)

    • Cenozoic Era (66M years ago to the present)

2.1 PALEOZOIC ERA (541M - 252M years ago)

  • Earliest Life Forms: The earliest Paleozoic animals lived in the sea.

    • Simple fungi may have existed in freshwater environments, but fossil evidence is lacking.

  • Cambrian Explosion:

    • Began approximately 541 million years ago but possibly started earlier around 575 million years ago with the Ediacara fauna.

    • Marked a rapid evolutionary surge, signifying a critical period of intense biological diversification in Earth's history.

DIVERSIFICATION OF LIFE

  • Life rapidly diversified during the Cambrian and Ordovician periods, occupying nearly all marine environments.

  • Trilobites: Dominant fossils in Cambrian rocks.

  • Brachiopods: Predominant fossils from the Ordovician through the Permian periods.

TERRESTRIAL ADAPTATIONS

  • Organisms adapted to life on land during the Silurian and Devonian periods.

    • Silurian Period: Leafless vascular plants (psilophytes) and centipede-like arthropods were established on land.

    • Devonian Period: Vertebrates transitioned to land as amphibians evolved from air-breathing crossopterygian fish.

CARBONIFEROUS PERIOD INNOVATIONS

  • Evolution of reproduction: Waterborne spores in plants were replaced by seeds from seed ferns.

  • Shell-less eggs in animals were replaced by amniote eggs with protective shells in reptiles.

  • Insects evolved wings, achieving flight.

PERMIAN EXTINCTION

  • Timing: End of the Paleozoic Era.

  • Impact: Major invertebrate groups eliminated; many other groups like ammonoids, brachiopods, and corals were severely affected. An estimated 95% of marine invertebrate species perished, while extinction rates were lower for vertebrates and plants.

  • Causes: Likely factors include changing climate and exceptionally low sea levels.

2.2 MESOZOIC ERA (252M - 66M years ago)

Mass Extinction

  • The Mesozoic Era began following the greatest extinction event in Earth's history, eliminating 95% of marine life and 70% of land life, creating an opportunity for dinosaurs to dominate.

Pangaea Formation

  • Pangaea: A supercontinent formed around 335 million years ago, surrounded by the vast Panthalassa ocean.

  • Pangaea began breaking apart at the end of the Mesozoic Era, shaping today's continents.

TRIASSIC PERIOD

  • Climate: Generally dry during the Triassic period.

  • Dominant Plants: Gymnosperms (cone-bearing, non-flowering plants like conifers).

  • Continental Drift Begins: Pangaea began breaking apart during the Mesozoic Era.

    • Laurasia: Split into North America and Eurasia.

    • Gondwana: Broke into South America, Africa, Australia, Antarctica, and the Indian subcontinent.

JURASSIC PERIOD

  • Climate: Increased humidity, with rising seas flooding exposed land areas.

  • Environmental Impact: Rising seas significantly changed landmass distribution and ecosystems.

CRETACEOUS PERIOD

  • Climate: Alternated between cooling and rising temperatures.

    • Flora: Early angiosperms began to radiate in the early Cretaceous.

    • Spread of Flora: Angiosperms spread toward poles due to even temperature gradients.

OCEAN FORMATION & MASS EXTINCTION

  • Atlantic Ocean began forming as continents separated, leading to The Great Extinction, marking the end of the dinosaurs and the beginning of the Cenozoic Era.

2.3 CENOZOIC ERA (66M years ago to the present)

  • Paleogene Period (66M - 23M years ago)

  • Neogene Period (23M - 2.58M years ago)

  • This era is notable for the prominence of mammals as the largest land animals, yet it is misleading since mammals existed long before this era.

PALEOGENE PERIOD (66M-23M years ago)

  • Transition from the late Mesozoic’s hot, humid conditions to a cooler, drier trend marked by periodic warm phases.

  • Climate and Oceanic Evolution: Cooling driven by the formation of the Antarctic Circumpolar Current; ocean temperatures dropped significantly.

  • Mammal Diversification: Allowed evolution into various modern species, leading to the dominance of mammals and adaptation to various ecological roles.

NEOGENE PERIOD (23M-2.58M years ago)

  • Continued geological changes; the Indian subcontinent's collision with Asia formed the Himalayas.

  • Global cooling initiated the Quaternary Ice Age, marking significant climate change and leading to the adaptation of mammals and birds into modern forms.

  • Grasslands expanded replacing tropical vegetation, leading to the evolution of herbivorous mammals like horses, antelope, and bison.