Archean & Proterozoic Life

SOME TERMINOLOGY

• PROKARYOTE - NO internal organelles or membrane-bound nucleus with chromosomes
• EUKARYOTE - HAVE internal organelles (chloroplasts & mitochondria) & membrane-bound nucleus with chromosomes
• ANAEROBE - cannot survive in an environment with free oxygen
• AEROBE - can survive in an environment with free oxygen
• HETEROTROPH - cannot synthesize food
• AUTOTROPH - can synthesize food by fixing Carbon
• CHEMOSYNTHESIS - use chemical energy to fix Carbon
• PHOTOSYNTHESIS - use sunlight to fix Carbon

ORIGIN OF LIFE

• PROBABLY AT MID-OCEAN RIDGES
• PROPOSED SEQUENCE OF EVENTS INVOLVED IN EVOLUTION OF ORGANISMS FROM INORGANIC MATERIALS
  • 1. Synthesis of abiotic simple organic molecules (amino acids, sugars, fatty acids, and nitrogen & phosphorous compounds)
    • MAY HAVE OCCURRED IN OUTER SPACE (simple organic molecules have been detected in interstellar gases & nebulae and in meteorites)
  • 2. Buildup of complex organic molecules from simple ones (polymerization)
    • PROTEINS FROM AMINO ACIDS; NUCLEIC ACIDS (DNA, RNA) FROM SUGARS AND NITROGEN & PHOSPHATE COMPOUNDS; LIPIDS FROM FATTY ACIDS AND PHOSPHATE COMPOUNDS
  • 3. Assembly of these complex organic molecules into proto-organisms
  • 4. Development of the ability to replicate (inherent in RNA & DNA)
• ALL OF THESE STEPS EXCEPT THE LAST HAVE BEEN DUPLICATED EXPERIMENTALLY
  • 1. Miller/Urey - synthesis of amino acids under primitive Earth conditions
    • ATMOSPHERE OF NH3, CH4, H2O & H2 AND DEVOID OF FREE OXYGEN (the critical element), + ENERGY SOURCE (electric sparks [lightning], ultraviolet radiation, or heat [from Mid-Ocean Ridge])
    • OTHERS HAVE SYNTHESIZED SUGARS & PHOSPHATES (which are used to build DNA) IN THE LAB as well
  • 2. Fox & associates - amino acids readily polymerize into protein-like molecules (proteinoids) at moderate temperatures (as low as 70°C)
    • upon cooling, PROTEINOIDS FORM TINY SPHERES WITH CHARACTERISTICS COMMON TO LIVING CELL MEMBRANES
  • Dreamer & associates - Fatty Acids & Phosphates readily polymerize into Lipid molecules
    • LIPIDS ALSO FORM TINY SPHERES WITH CHARACTERISTICS COMMON TO LIVING CELL MEMBRANES
  • 3. Khorana & associates - synthesized short segments of nucleic acids
  • 4. Carins-Smith & Nisbit have suggested that these early processes may have used clay minerals as templates, particulary for replication

FIRST ORGANISMS

• HETEROTROPHIC ANAEROBIC PROKARYOTES (PROBABLY)
  • metabolized pre-existing organic molecules by fermentation or methanogenesis

C₆H₁₂O₆ -> 2 C₂H₅OH + 2 CO₂ + energy (2 units)

(glucose) -> (ethanol)

• modern Heterotrophs are mostly Aerobic Eukaryotes that metabolize pre-existing organic molecules by oxidation

C₆H₁₂O₆ + 6 O₂ -> 6 CO₂ + 6 H₂O + energy (36 units)

FIRST AUTOTROPHS

• CHEMOSYNTHETIC ANAEROBES (PROBABLY)
  • used heat energy from MOR hot springs

FIRST PHOTOSYNTHETIC AUTOTROPHS

• ANAEROBIC BACTERIA (PROBABLY)
  • like those now found only in restricted environments [anaerobic hot springs in Yellowstone Park]

6 CO₂ + 6 H₂S + energy -> C₆H₁₂O₆ + 6 S (no free oxygen)

• as old as oldest continental crust (probably)
  • CARBON SPHERES IN 3.8 BY OLD ROCKS OF GREENLAND

SECOND PHOTOSYNTHETIC AUTOTROPHS

• AEROBIC CYANOBACTERIA (BLUE-GREEN ALGAE) (PROBABLY)

6 CO₂ + 6 H₂O + energy -> C₆H₁₂O₆ + 6 O₂ (lots of free oxygen)

• O₂ initially produced by cyanobacteria reacted chemically with previously dissolved (Fe⁺²) iron to form Banded Iron Formations (BIFs) & did not accumulate in the atmosphere
• ~2 billion years ago the dissolved iron reservoir was exhausted & the atmosphere began to be "polluted" by O₂
  • ANAEROBIC ORGANISMS DRIVEN BELOW THE SURFACE
• modern Photosynthetic Autotrophs are mostly Aerobic Eukaryotes

ARCHEAN FOSSIL RECORD

• ARCHEAN FOSSILS ARE NOT ABUNDANT, OCCURRING IN ONLY A FEW PLACES, & CONSIST OF ONLY 2 TYPES - MICROFOSSILS & STROMATOLITES
  • Microfossils - prokaryote bacteria & cyanobacteria only
  • Stromatolites - formed by photosynthetic cyanobacteria & presently are restricted to stressed environments; during the Precambrian occurred in many environments
    • WARRAWOONA GROUP - 3.5-3.4 BY OLD (WESTERN AUSTRALIA) - PRIMITIVE BACTERIA & PRIMITIVE FILAMENTOUS CYANOBACTERIA
    • FIG TREE GROUP - 3.4 BY OLD (SOUTH AFRICA) - PRIMITIVE BACTERIA & UNICELLULAR CYANOBACTERIA
    • PONGOLA SUPERGROUP & BULAWAYAN GROUP - 3.1-2.8 BY OLD (SOUTHERN AFRICA)
    • STROMATOLITES ARE NOT ABUNDANT UNTIL THE EARLY PROTEROZOIC

MAJOR PROTEROZOIC LIFE EVENTS

• EVOLUTION OF MULTICELLED ORGANISMS
  • multicelled algae - Middle Proterozoic
  • multicelled animals - Late Proterozoic

EARLY PROTEROZOIC FOSSIL RECORD

• APPEARANCE OF EUKARYOTES IN THE EARLY PROTEROZOIC
  • Eukaryotes are thought to have developed from symbiotic relationship between previously-independent prokaryotes (see Fig. 12-5)
    • EUKARYOTES ARE LARGER THAN PROKARYOTES
    • MOLECULAR GENETICS INDICATES EUKARYOTES PROBABLY EVOLVED DURING THE ARCHEAN, BUT DIFFERED FROM BACTERIA ONLY IN HAVING A NUCLEUS
  • Organization of DNA into chromosomes in nucleus allowed development of sexual reproduction & the rate of organic evolution increased
  • Eukaryotes did not evolve until the atmospheric O₂ began to accumulate
    • NEARLY ALL EUKARYOTES ARE AEROBES
    • INDICATED BY LARGER SIZE, THICKER CELL WALLS, & CHEMICAL COMPOUNDS UNIQUE TO EUKARYOTES
• EARLY PROTEROZOIC FOSSILS ARE NOT ABUNDANT, OCCURRING IN ONLY A FEW PLACES, & CONSIST OF ONLY 2 TYPES - MICROFOSSILS & STROMATOLITES, LIKE THE ARCHEAN
  • Microfossils - prokaryote bacteria & cyanobacteria only until about 2.1 BY
    • GUNFLINT CHERT - 2.1-1.8 BY OLD (NORTH AMERICA) - DIVERSE ADVANCED BACTERIA & FILAMENTOUS CYANOBACTERIA
    • FIRST APPEARANCE OF ACRITARCHS (CYSTS OF PLANKTONIC EUKARYOTIC ALGAE)
  • Stromatolites become abundant in the Early Proterozoic (see Fig. 12-4)

LATE PROTEROZOIC FOSSIL RECORD

• STROMATOLITES WERE VERY ABUNDANT
  • Restricted to hypersaline environments during the Phanerozoic
• MULTICELLULAR ANIMALS (METAZOANS) EVOLVED AT THE END OF THE PROTEROZOIC
  • Pound Quartzite - <600 my old (Ediacara Hills of Southern Australia) - first discovery of SOFT-BODIED METAZOANS - as impressions
    • CALLED EDIACARA ASSEMBLAGE, OR FAUNA
    • EDIACARA FAUNA OCCURS ON ALL CONTINENTS EXCEPT ANTARCTICA (Namibia, England, Scandinavia, Russia, Siberia, China, & N. America)
    • INCLUDES WHAT ARE THOUGHT TO BE JELLYFISH, & SOFT CORALS, WORMS, & PRIMITIVE ARTHROPODS & ECHINODERMS (see Fig.12-11)
      • Some workers consider Ediacara Fauna as unrelated to modern metazoans
      • Adapted to nutrient-poor environments and low atmospheric oxygen level (5-10% present) - flat with large surface area to volume ratio
  • Small, calcareous, TUBE-SHAPE SHELLED FOSSILS & SIMPLE TRACE FOSSILS are associated with the Ediacara Fauna (see Figs.12-10 & 12-12)
  • Probably resulted from:
    • 1. INCREASING O₂ & CO₂ LEVELS
    • 2. CONTINENTAL FRAGMENTATION
    • 3. END OF GLACIATION
    • 4. FLOODING OF CONTINENTS
    • 5. EVOLUTION OF HERBIVORES TO FEED ON CYANOBACTERIAL MATS
    • 6. EVOLUTION OF PREDATORS
  • SEVERAL GEOCHEMICAL FLUCTUATIONS ASSOCIATED WITH PRECAMBRIAN-CAMBRIAN BOUNDARY (see Fig. 12-13)
    • Sulfur & carbon isotope
      • REFLECT CHANGES IN PRODUCTIVITY & BURIAL OF SULFIDES IN RESTRICTED ENVIRONMENTS
    • Currently being used for stratigraphic correlation

Study Questions

1. What are the differences between heterotrophs & autotrophs, and anaerobes & aerobes?

2. Summarize the sequence of events thought to be involved in the evolution of organisms from inorganic materials.

3. Which events have been duplicated in the lab?

4. What are the natural environments on Earth where they might have occurred?

5. What is the significance of clay minerals in the origin of life on Earth?

6. What was the probable nature of the earliest organisms?

7. What were the probable steps from anaerobic heterotrophs to aerobic photosynthetic autotrophs?

8. What were the changes in the Earth's atmosphere & rocks resulting from evolution of photosynthetic cyanobacteria?

9. What are the differences between prokaryotes & eukaryotes (the differences in their internal structure)?

10. How are eukaryotes thought to have evolved?

11. What was the effect the evolution of eukaryotes on the rate of organic evolution?

12. Summarize the general characteristics of the Archean and Early Proterozoic fossil records (the age & location of and the types of fossils in the important rock units containing these fossil records).

13. What are the differences between the Early Proterozoic & the Archean fossil record.

14. When did eukaryotes evolve & what is the evidence for this event?

15. When did multicellular algae evolve?

16. What are the characteristics of the Ediacarian Fauna & the rock unit in which it was discovered?

17. Summarize the characteristics of the small shelled fossils & trace fossils associated with the Ediacarian Fauna.

18. Summarize the probable causes of the evolution of the Ediacarian Fauna.

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