Eighth Edition
by Harold L. Levin

Chapter 8 - page 8

The Earliest Earth: 2,100,000,000 years of the Archean Eon

Life of the Archean - The Archean Fossil Record

The earliest evidence of life occurs in Archean sedimentary rocks.

Evidence of Archean life consists of:

1. Stromatolites - An organo-sedimentary structure built by photosynthetic cyanobacteria or blue-green algae. They are not true fossils.
   Stromatolites form through the activity of cyanobacteria in the tidal zone. The sticky, mucilage-like algal filaments of the cyanobacteria trap carbonate sediment during high tides.
   Stromatolites are found in carbonate sedimentary rocks and in cherts which replace carbonates.
   They are more abundant in Proterozoic rocks than in Archean rocks.

Examples:
1. Oldest are 3.5 b.y. old, Warrawoona Group, Australia's Pilbara Shield
2. 3 b.y. old Pongola Group of southern Africa
3. 2.8 b.y. old Bulawayan Group of Australia


Sketch of stromatolites, showing top view and side view.
Image courtesy of Pamela Gore.

Modern stromatolites are found today in isolated environments with high salinity, such as Shark Bay, western Australia.


Modern stromatolites, Shark Bay, western Australia

Stromatolites are scarce today, mainly because the microorganisms that build them are eaten by marine snails and other grazing invertebrates. Stromatolites survive only in environments that are too saline or otherwise unsuitable for most grazing invertebrates.

The decline of stromatolites is associated with the evolutionary appearance of new groups of marine invertebrates in the early Paleozoic.

Other evidence of Archean life:

2. Oldest direct evidence of life
   Microscopic cells and filaments of prokaryotes. 11 taxa.
   Associated with stromatolites
   Similar to cyanobacteria living today, which produce oxygen.
   Fossiliferous chert bed associated with the Apex Basalt
   Found in Warrawoona Group, Pilbara Supergroup, western Australia
   3.460-3.465 b.y.
3. Indirect evidence of life in older rocks
   Found in banded iron deposits in Greenland.
   Carbon-13 to carbon-14 ratios are similar to those in present-day organisms.
   3.8 b.y.
4. Algal filament fossils
   Filamentous prokaryotes preserved in stromatolites.
   Found at North Pole, western Australia.
   3.4-3.5 b.y.
5. Spheroidal bacterial structures
   Found in rocks of the Fig Tree Group, South Africa (cherts, slates, ironstones, and sandstones).
   Prokaryotic cells, showing possible cell division.
   3.0 - 3.1 b.y.
   
6. Molecular fossils
   Preserved organic molecules that only eukaryotic cells produce.
   Indirect evidence for eukaryotes.
   In black shales from northwestern Australia.
   2.7 b.y.
   Origin of eukaryotic life is pushed back to 2.7 b.y.

The Origin of Life

The basic materials from which microbial organisms (i.e., life) could have developed initially may have arrived on Earth during the Archean in meteorites called carbonaceous chondrites, which contain organic compounds.

Life requires these elements:

• Carbon
• Hydrogen
• Oxygen
• Nitrogen
• Phosphorus
• Sulfur

Each of these is abundant in the Solar System.

Four essential components of life:

1. Proteins - Chains of simple organic molecules called amino acids, linked together. Proteins are used to build living materials, and as catalysts in chemical reactions in organisms.
2. Nucleic acids - Large complex molecules in the nuclei of cells.
1. DNA (carries the genetic code and can replicate itself)
2. RNA
3. Organic phosphorus compounds - Used to transform light or chemical fuel into energy required for cell activities.
4. A cell membrane to enclose the components within the cell.

The earliest organisms developed in the presence of an atmosphere which lacked oxygen. The organisms must have been anaerobic (i.e., they did not require oxygen for respiration).

Organic molecules could not assemble into larger structures in an oxygenated environment. Oxidation and microbial predators would break down the molecules.

Because the atmosphere lacked oxygen, there was no ozone shield to protect the surface of the Earth from harmful ultraviolet (UV) radiation.

Origin of amino acids

UV radiation can recombine atoms in mixtures of water, ammonia and hydrocarbons, to form amino acids. (The energy in lightning can do the same thing.)

Lab simulation experiments by Miller and Urey in the 1950's.
Formed amino acids from gases present in Earth's early atmosphere:
H₂, CH₄ (methane), NH₃ (ammonia), and H₂O (water vapor or steam), along with electrical sparks (to simulate lightning).

Miller-Urey apparatus. Photo courtesy of Pamela Gore.

This was the first laboratory synthesis of amino acids. A liquid was produced that contained a number of amino acids and other complex organic compounds that comprise living organisms. A main requirement was the lack of free oxygen.

Joining amino acids to form proteins

Amino acids are monomers and have to be joined together to form proteins, which are polymers (or chains). This requires:

1. Input of energy
2. Removal of water

How could this occur?

1. Heating (volcanic activity)
2. At lower temperatures in the presence of phosphoric acid
3. Evaporation
4. Freezing
5. Involve water in a dehydration chemical reaction
6. On surface of clay particles, which have charged surfaces, and to which polar molecules could attach. Metallic ions on clays could concentrate organic molecules in an orderly array, causing them to align and link into protein-like chains.
7. On pyrite, which has a positively charged surface to which simple organic compounds can become bonded. Formation of pyrite yields energy which could be used to link amino acids into proteins.

Proteinoids are protein-like chains produced in the lab by Fox from a mixture of amino acids. Considered to be possibly like the transitional structures leading to proteins billions of years ago.

Similar proteinoids are also found in nature around Hawaiian volcanoes.

Hot aqueous solutions of proteinoids will cool to form microspheres, tiny spheres that have many characteristics of living cells:

• Film-like outer wall
• Capable of osmotic shrinking and swelling
• Budding similar to yeast
• Divide into daughter microspheres
• Aggregate into lines to form filaments, as in some bacteria
• Streaming movement of internal particles, as in living cells

Where Did Life Originate?

Early life may have avoided UV radiation by living:

• Deep beneath the water
• Beneath the surface of rocks (or below sediment - such as stromatolites)

Life probably began in the sea, perhaps in areas associated with submarine hydrothermal vents or black smokers.
Evidence for life beginning in the sea near hydrothermal vents:

• Sea contains salts needed for health and growth.
• Water is universal solvent, capable of dissolving organic compounds, producing a "rich organic broth" or primordial soup.
• Ocean currents mix these compounds, leading to collisions between molecules, leading to combination into larger organic molecules.
• The microbes at these vents are hyperthermophiles that thrive in seawater hotter than boiling point (100^oC). They are Archaea, and can live in fissures deep below the seafloor.
• These microbes derive energy by chemosynthesis, without light, rather than by photosynthesis (suggests origin in deep water in absence of light).
• Hyperthermophiles are Archaea, with DNA different from bacteria.

Feeding Life on Earth - Obtaining Nutrients

Examples of types of feeding modes:

• Fermenters - Organisms which digest or break down chemicals, such as sugar, in the absence of oxygen, to obtain energy. Fermentative anaerobic organisms produce CO₂ and alcohol.
  Example: Yeast
• Autotrophs (=self-feeders) - Organisms that manufacture their own food.
  Examples: sulfur bacteria, nitrifying bacteria, and photoautotrophs (such as plants and photosynthetic bacteria such as cyanobacteria) which use photosynthesis to manufacture food. Photosynthesis produces oxygen as a waste product.
• Heterotrophs - Organisms that can't make their own food, so they must find nutrients in the environment to eat.
  Examples: All animals, including humans.

Evolution of early life and the transition from prokaryotes to eukaryotes

The earliest cells had to form and exist in anoxic conditions (in the absence of free oxygen).
Likely to have been anaerobic bacteria or Archaea.

Some of the early organisms became photosynthetic, possibly due to a shortage of raw materials for energy.
Produced their own raw materials. Autotrophs.
Photosynthesis was an adaptive advantage.

Oxygen was a WASTE PRODUCT of photosynthesis.

Consequences of oxygen buildup in the atmosphere:

1. Development of ozone layer which absorbs harmful UV radiation, and protected primitive and vulnerable life forms.
2. End of banded iron formations which only formed in low, fluctuating O₂ conditions
3. Oxidation of iron, leading to the beginning of red beds - iron oxides (hematite).
4. Aerobic metabolism developed. Uses oxygen to convert food into energy.
5. Development of the eukaryotic cell, which could cope with the oxygen in the atmosphere. Larger than prokaryotes.

Eukaryotes contain a nucleus and organelles (such as chloroplasts and mitochondria) within the cell. Prokaryotes do not.

Prokaryotes reproduce asexually by simple cell division. This restricts their genetic variability. For this reason, prokaryotes have shown little evolutionary change for more than 2 billion years.

Eukaryotes reproduce sexually through the union of an egg and sperm. This combines chromosomes from each parent and leads to genetic recombination and increased variability. A great variety of new genetic combinations.
Led to a dramatic increase in the rate of evolution.

The earliest large cells that appear to be eukaryotes appear in the fossil record about 1.6 - 1.4 billion years ago (in the Proterozoic). Eukaryotes diversified around the time that the banded iron formations disappeared and the red beds appeared, indicating the presence of oxygen in the atmosphere. Origin of eukaryotic life was probably around 2.7 b.y., based on molecular fossils.

Comparison of a prokaryotic cell (small green one on the left) with a larger (yellow) eukaryotic cell.

Table comparing prokaryotic and eukaryotic organisms.

The Endosymbiotic Theory for the Origin of Eukaryotes proposes that billions of years ago, several prokaryotic cells came together to live symbiotically within a host cell as protection from (and adaptation to) an oxygenated environment. These prokaryotes became organelles. Evidence for this includes the fact that mitochondria contain their own DNA.
Example - a host cell (fermentative anaerobe) + aerobic organelle (mitochondrion) + spirochaete-like organelle (flagellum for motility).

Diagram illustrating the Endosymbiotic Theory for the Origin of Eukaryotes.

The appearance of eukaryotes led to a dramatic increase in the rate of evolution, and was ultimately responsible for the appearance of complex multicellular organisms.