Early Life and the Origin of Cells

Early Life

Overview of Stromatolites

  • Definition: Stromatolites are layered sedimentary structures formed from the growth of microbial mats.
  • Mechanism: Microbes within these mats trap and bind sediment clasts and precipitate calcium carbonate.
  • Geological Significance: They are found as trace fossils and serve as important indicators of early life on Earth.
  • Oldest Stromatolites: The oldest known stromatolites have been found in Isua, Greenland, dated to approximately 3.8 billion years ago (Gya).

Domains of Life

  • Bacteria: Includes groups like Spirochaetes, Firmicutes, Actinobacteria, and Cyanobacteria.
  • Archaea: Includes groups such as Euryarchaeota and Crenarchaeota.
  • Eukaryotes: Includes a wide range of organisms such as animals, plants, fungi, and various protists including ciliates and dinoflagellates.

Prokaryote Cell vs. Eukaryote Cell

Prokaryotic Cells

  • Structure: Simple internal structure without a nucleus or internally differentiated organelles.
  • Flagellum: Present in some prokaryotes for movement.

Eukaryotic Cells

  • Structure: Complex internal structure with a defined nucleus.
  • Organelles: Contains mitochondria and chloroplasts, both of which have their own DNA, indicating a unique evolutionary history.

Origin of the Eukaryote Cell

  • Endosymbiosis Hypothesis: Eukaryotic cells are believed to have originated from prokaryotic cells engulfing other prokaryotic cells, which then became organelles.
  • Evidence Supporting the Hypothesis:
    • Mitochondria and chloroplasts have their own DNA, similar to prokaryotic DNA.

Geologic Evidence for Early Life

Types of Fossils

  • Body Fossils: Commonly multicellular organisms.
  • Trace Fossils: Produced by multicellular organisms, indicating movement or presence.
  • Molecular Fossils: Chemical remnants of life, suggesting biological activity.

Key Fossil Discoveries

  1. Apex Chert, Australia:
    • Dated at 3.5 billion years, it contains microbial body fossils.
  2. Gunflint Chert, Canada:
    • Dated at 2 billion years, it contains microbe body fossils.
  3. Negaunee Iron, Michigan:
    • The oldest eukaryotes interpreted as algae colonies, dated at 1.8 billion years.
  4. Cloudina:
    • The oldest shelly organisms, dating back 0.6 billion years.
  5. Ediacaran Biota:
    • Diverse multicellular life from 0.6 billion years ago, regarded as potential early animal kingdoms.

Indirect Evidence for Early Life

  • Isua Greenstone Stromatolites:
    • Dated at 3.8 billion years, indicate life through layered structures formed by photosynthesizing bacteria.
  • Molecular Evidence: Carbon isotopes in these stromatolites show a preference for lighter ^{12}C over heavier ^{13}C, suggesting biological activity.
  • Contentious Claims: Evidence for oxygen in the atmosphere around this time is still disputed and requires careful interpretation.

Early Life Summary

  • Presence of undisputed early prokaryotic life through fossils.
  • Disputed early prokaryotic life evidence called into question based on current understanding.
  • Eukaryotic life began emerging, with distinct fossils of multicellular organisms like Cloudina and the Ediacaran Biota appearing around 0.6 billion years ago.
  • Significant Fossils: Early evidence highlighted include Apex Chert, Isua Stromatolites, Negaunee Iron algae, and the diverse Ediacaran Biota.