Do historians and geologists divide time using the same criteria?

ages, eras

We are accustomed to framing and dividing time in various ways:

  • Days (a day is made up of 24 hours),
  • Years (a year consists of 12 months),
  • Centuries (a century is made up of 100 years),
  • Millennia (a millennium is made up of 10 centuries),
but historians and geologists use different criteria.

Historians divide time into periods based on key milestones in humanity’s journey, such as:
  • The birth of our planet, approximately 4.5 billion years ago.
  • The first life forms in the sea, 500 million years ago.
  • The first life forms on land, 350 million years ago.
  • The emergence of primates, small animals similar to mice that later evolved into various species of monkeys, about 70 million years ago.
  • The formation of the Great Rift Valley, about 20 million years ago.
  • The hominids achieving an upright stance, 4 million years ago.
  • The appearance of early humans, about 2 million years ago, following a long and complex process.
  • Hominids controlling fire, 400,000 years ago.
  • The emergence of Homo Sapiens, 200,000 years ago.
  • The beginning of the Neolithic Age (8000-4000 BC), when humans became farmers and herders.
  • The start of the Copper Age (8000-4000 BC), when humans learned to work copper.
Historical periods (ages) are abstract constructs, chronological categories used by historians to simplify the study of history, and are divided into two main phases:
  1. Prehistory (from the origins of the world until about 3100 BC, the invention of writing), which is further divided into three phases marked by the use of different materials:
    1. The Stone Age, subdivided into:
      • Paleolithic,
      • Mesolithic,
      • Neolithic;
    2. The Copper Age (also known as the Eneolithic or Chalcolithic), especially for the Near East and the Mediterranean;
    3. The Bronze Age;
    4. The Iron Age.
    These transitions did not occur at the same time in all places.
  2. History, which is divided into:
    • Ancient Age (from about 3100 BC to 476 AD, the fall of the Western Roman Empire);
    • Middle Ages (from 476 to 1492, the discovery of America);
    • Modern Age (from 1492 to 1789, the French Revolution);
    • Contemporary Age (from 1789 to today).

Diagram representing the stages of Geologic time as sections of a 24 hour day/clock.
Diagram representing the stages of Geologic time as sections of a 24 hour day/clock.

Geologists use geological eras (periods of varying lengths) to divide the 4.57 billion years from the birth of our planet to the present day. Each geological era represents a phase in Earth’s history characterized by specific organisms, often extinct by the end of that era.
To reconstruct Earth's evolutionary history, geologists rely on tools such as:
  • Radioactivity, to date rocks found on Earth,
  • Fossils, to identify the age of sediments and the evolution of animal and plant species,
  • Rock composition, to understand the conditions under which rocks formed in ancient times.
Geologists have divided Earth's history into five major eras, marked by significant orogenetic events and changes in animal and plant life:
  1. Archeozoic Era (Precambrian, from Cambria, the Romanesque name of Wales, where rocks are best exposed), starting with the oldest formations of Earth's crust (about 4 billion years ago), when the first unicellular life forms (algae and bacteria) appeared.
    This is the longest era, extending until 590 million years ago. It includes the cooling of the Earth, the formation of continents, and the origin of life in the oceans.
  2. Paleozoic Era (Primary), from about 590 to 250 million years ago, characterized by the appearance of ferns and multicellular invertebrates, as well as early amphibians, reptiles, and some fish.
    This era is divided into six geological periods:
    • Cambrian,
    • Ordovician,
    • Silurian,
    • Devonian,
    • Carboniferous,
    • Permian.
  3. Mesozoic Era (Secondary), from about 250 to 65 million years ago, featuring the rise of reptiles (including dinosaurs), the first birds (pterodactyls), and mammals.
    The Mesozoic is divided into three periods:
    • Triassic,
    • Jurassic,
    • Cretaceous.
  4. Cenozoic Era (Tertiary), from about 65 million to 2 million years ago, when flora and fauna began to resemble today’s species, and mammals and primates flourished.
    This era is divided into five periods:
    • Paleocene,
    • Eocene,
    • Oligocene,
    • Miocene,
    • Pliocene.
  5. Neozoic Era (Quaternary), from about 2 million years ago to the present day, marked by multiple ice ages and interglacial periods, and the emergence of the Homo genus, leading to Homo sapiens.
    This era is divided into two periods:
    • Pleistocene,
    • Holocene



Last update: September 30, 2024

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