The History of Life

Geological Time Scale

• Hierarchical Structure:

  • Eons

  • Eras

  • Periods

  • Epochs

  • Ages

Understanding Time Units

• Each time unit in geological history has distinct beginnings and endings.

• No Consistent Duration:

  • The number of years in any given unit varies significantly.

• Endings:

  • Every time unit concludes with a mass loss of species as evidenced in the fossil record.

• Beginnings:

  • Each time unit initiates with a mass adaptive radiation in the fossil record.

• Global Markers:

  • Each start and end is marked by specific global events recognizable in strata.

The History of Life

## Hadean Time (4.6 – 3.8 billion years ago)
- Oldest Known Rocks:
- Identified as zircon crystals.
- Planet Characteristics:
- The Earth was volatile and unstable.
- Life Status:
- There is no known life from this period.
- Note: This period is not classified as an Eon, Era, Period, Epoch, or Age.

The Archaean Eon (3.8 – 2.5 billion years ago)

• Atmosphere:

  • Characterized by a reducing atmosphere containing methane, ammonia, and other gases considered toxic.

• Life Forms:

  • Oldest Fossils:

    • Fossil evidence shows the existence of single-celled bacteria approximately 3.5 – 3.8 billion years old.

• Oxygen Appearance:

  • By 2.7 billion years ago, notable levels of O$_2$ began accumulating in the atmosphere.

• Stromatolites:

  • By the end of the Archaean, stromatolites, layered structures produced by the activity of microorganisms, became widespread.

End of the Archaean

• Oxygen Increase:

  • O$_2$ levels significantly increased, fostering an aerobic world, marked by iron ore deposits dated between 2.7 – 2.3 billion years ago.

  • This oxygen surge may have triggered a mass extinction event.

The Proterozoic Eon (2.5 billion years ago – 543 million years ago)

• Oxygen Levels:

  • Atmospheric O$_2$ reached approximately 3%.

• Eukaryotic Life:

  • First eukaryotic cells are identified around 2.2 billion years ago.

• Endosymbiosis Theory:

  • Mitochondria and chloroplasts in eukaryotes are derived from endosymbiotic relationships formed between prokaryotic cells.

• Multicellular Organisms:

  • Emergence dates range from 2.1 to 1.5 billion years ago, reflecting the evolutionary timeline.

  • True plants appeared at least 1 billion years ago.

  • First animal fossils date back to about 600 million years ago.

Phanerozoic Eon (543 million years ago – Present)

• Major Eras:

  • Paleozoic Era

  • Mesozoic Era

  • Cenozoic Era

Paleozoic Era (543 – 248 million years ago)

• Major Periods:

  • Cambrian Period

  • Ordovician Period

  • Silurian Period

  • Devonian Period

  • Carboniferous Period

  • Permian Period

Cambrian Period (543 – 490 million years ago)

• Key Events:

  • Begins with the Cambrian Explosion, a significant adaptive radiation of various animals.

• Notable Site:

  • The Burgess Shale, located at the borders of British Columbia and Alberta in Canada, is a crucial fossil site from this period.

Ordovician Period (490 - 443 million years ago)

• Key Life Forms:

  • First land plants appear alongside numerous types of jawless fishes.

• Mass Extinction Event:

  • The period ended with the first Great Mass Extinction, during which 60 – 70% of all known species were lost.

Silurian Period (443 - 417 million years ago)

• Evolutionary Milestones:

  • The emergence of the first jawed fish, land animals, and vascular plants.

Devonian Period (417 - 354 million years ago)

• Significant Developments:

  • Appearance of the first trees and seed plants.

  • First tetrapods emerge.

• End of the Period:

  • Concludes with the second Great Mass Extinction, resulting in the extinction of more than 70% of known species.

Carboniferous Period (354 - 290 million years ago)

• Key Features:

  • Formation of "Coal Swamps."

  • First reptiles and amniotic eggs evolve.

  • Amniotic egg components include vitellus, air pocket, chorion, albumen, shell, allantois, amnion, and embryo.

Permian Period (290 - 248 million years ago)

• Key Developments:

  • First modern conifers and large vertebrate herbivores appear.

• Mass Extinction Event:

  • Known as the third Great Mass Extinction, leading to an estimated 90 – 95% of all species going extinct.

Mesozoic Era (248 - 66 million years ago)

• Major Periods:

  • Triassic Period

  • Jurassic Period

  • Cretaceous Period

Triassic Period (248 - 206 million years ago)

• Notable Firsts:

  • The emergence of the first dinosaurs and the first mammals.

Jurassic Period (206 - 144 million years ago)

• Significant Events:

  • The appearance of the first birds marks this period.

Cretaceous Period (144 - 66 million years ago)

• Evolution of Life:

  • Introduction of the first flowering plants and diatoms.

  • Peak diversity among dinosaurs occurs.

  • Appearance of the first primate species.

Cenozoic Era (66 million years ago – Present)

• Major Periods:

  • Tertiary Period

  • Quaternary Period

Tertiary Period (66 - 2 million years ago)

• Key Developments:

  • First apes and human-like hominids emerge.

  • Extensive adaptive radiation of mammals occurs, leading to the development of most modern orders of mammals.

  • Flowering plants become dominant, supported by their pollinators: ants, bees, butterflies, and moths.

Quaternary Period (2 million years ago – Present)

• Human Evolution:

  • Marks the emergence of the first humans.

• Definition of Hominins:

  • Hominins are defined as hominoids more closely related to humans than other hominoids.

  • Immediate ancestors of modern humans include:

    • Homo erectus

    • Homo heidelbergensis

    • Homo sapiens

    • Homo neanderthalensis