Lecture 17

Question 1

1) What did Earth form?
2) What was the new star that formed, what additional stuctures formed as a result of this newly formed star?

Answer 1

1) Origins of the universe, circa 12.5 bya. Formation of the Earth, about 4.54 bya, based on data from slowly decaying radioactive isotopes.
2) From accretion of materials making up a disc-shaped nebular cloud of dust and gases released by the supernova of a massive old star.
- A new star, our Sun, formed within this cloud, compacting in on itself and beginning to undergo nuclear fusion and to release heat and light; materials left over in the nebular cloud began to clump and fuse due to collisions and gravitational pull, forming tiny eventually coalesced into planets.

Question 2

What did Earth absorb in its early stages of development?

Answer 2

• Early Earth absorbed rival protoplanets, planet embryos, and planetary debris in a series of titanic collisions that vaporized the incoming material and part of Earth’s materials. Eventually, a rocky mantle with a molten magma interior was formed.

Question 3

1) What were the conditions of early Earth?
2) What did Earth’s early atmosphere composed of?

Answer 3

1) Initially was a fiery hot, with lots of volcanic activity and collisions from planetoids and other debris. Conditions were completely inhospitable for life; and extremely hot surface under intense bombardment from space by masses of accreted materials.
2) Not free 0₂, mostly water vapor, hydrogen, nitrogen, methane, ammonia, and some carbon dioxide.

Question 4

Life requires water. Where did the water come from that formed the oceans of early Earth?

Answer 4

• Collisions with icy comets and asteroids, volcanic out-gassing of water vapor from the planet’s interior.
• At first, the surface of Earth was hot (several hundreds of degrees C), so water from comets, asteroids, and volcanoes was always in the vapor phase (steam).
• Life requires relatively cool temperature (less than 150^° C)

Question 5

1) When did Earth cool enough for water vapor to condense to the liquid state?
2) What is the oldest sedimentary rocks? What do these rocks indicate?

Answer 5

1) As the Earth cooled over millions of years, the water vapor condensed as rain and began accumulating as liquid. This rain, because it was somewhat acidic, containing CO₂, sulfur dioxide, etc., (which form acid when dissolved in water) dissolved some of the rock material of the surface of Earth, leading to formation of salty water that now exists as the Earth’s oceans (ca. 34 parts per thousand salt)
2) The oldest known sedimentary rocks, which form under liquid water, are dated to 3.86 bya; in the Itsaq gneiss complex, in southwestern Greenland. The sedimentary nature of these rocks indicates that at the latest by that time Earth had cooled sufficiently for the water vapor to have condensed and formed the early oceans.

Question 6

What minerals push the date of water formation even further back?

Answer 6

• Crystals of the mineral zircon. Impurities trapped in the crystals and the mineral’s isotopic ratios of oxygen indicate that early Earth may have cooled much earlier than previously believed, with solid crust forming and water condensing into oceans perhaps as early as 4.4 to 4.3 bya.
• What this means is that conditions on Earth might have been compatible with life within a few hundred million years after the formation of our planet.

Question 7

What evidence is provided for microbial life on early earth?

Answer 7

• The fossilized remains of cells and the isotopically “light” carbon abundant in these fossil-bearing rocks are evidence of early microbial life.
• Some ancient rocks contain bacteria-like microfossils, typically as simple rods or cocci.

Question 8

1) What is the dating method?
2) Describe the process

Answer 8

1) Carbon isotope (¹³C/¹²C) fractional analysis.
2) The standard ratio is approximately 5% ¹³C and 95% ¹²C. Biochemical reactions prefer the lighter isotope, discriminating against the heavier isotope. So, cellular carbon becomes enriched in ¹²C, the lighter isotope, by isotopic fractionation. Because this fractionation only occurs biologically, a difference from the normal, the ¹³C/¹²C ratio indicates biological activity. Isotope fractionation data for carbon are presented as: δ ¹³C (0/00), meaning the parts per thousand change in ¹³C compared to the standard.

Question 9

1) What are stromatolites?
2) How were they formed?
3) What do they indicate?

Answer 9

1) Contained in rocks that are 3.5 billion years old or younger. Stromatolites are fossilized microbial mats consisting of layers of filamentous and unicellular Bacteria and trapped sediment.
2) Formed by filamentous phototrophic bacteria.
3) They indicate that microbial life was abundant by 3.5 bya.

Question 10

1) How old were the stromatolites found in Greenland?
2) What does this mean for dating microbial life?

Answer 10

1) The stromatolites found in sedimentary rock in Greenland dated 3.7 billion years old (220 million years older than initially thought).
2) Microbial life is even older than initially thought. Chemical and mineral analyses established that the stromatolites are the fossilized remains of ancient microbes.

Question 11

What is the subsurface origin hypothesis?

Answer 11

• Substantial evidence indicates that life originated at hydrothermal springs on the ocean floor, well below Earth’s surface, where conditions would have been much less hostile and more stable than the surface.
• There, a steady and abundant supply of energy in the form of reduced inorganic compounds, specifically H₂ and H₂S, may have been available at these spring sites.

Question 12

How did this hydrothermal process work? Why was it successful? How were AMP and ATP formed?

Answer 12

• Very warm (90 to 100°C), alkaline, hydrothermal water, flowed up through the crust, and mixed with the cooler, slightly acidic, iron-containing, more oxidized waters; this mixing caused precipitates of colloidal pyrite (FeS), silicates, carbonates, and Mg-containing montmorillonite clays to form. These precipitates built up into structured mounds of gel-like adsorptive surfaces containing pore-filled semi-permeable enclosures.
• The surfaces and pores were rich in minerals such as Fe and Ni sulfides, which catalyzed formation of amino acids, simple peptides, sugars, and nitrogenous bases, and trapped and concentrated these compounds.
• With phosphate from seawater, nucleotides such as AMP and ATP were formed, with their polymerization into RNA catalyzed by montmorillonite clay, which has been shown to catalyze various chemical reactions.
• The flow of reduced inorganic compounds from the crust provided steady sources of electrons for this pre-biotic chemistry, which was fed from ocean water by carbon dioxide, phosphate, iron and other minerals and powered by redox and by pH gradients across the semi-premiable FeS membrane-like surfaces, a pre-biotic proton motive force.

Question 13

1) What is an RNA world?
2) What is RNA - self-catalysis?
3) How did RNA synthesis of early proteins occur?

Answer 13

• Synthesis and build up of organic compounds by this pre-biotic chemistry set the stage for the self-replicating systems, the precursors to cellular life.
  1) One way by which self-replicating systems could have arisen.
  2) Can bind small molecules, such as ATP and other nucleotides, and has catalytic activity, so RNA might have catalyzed its own synthesis from the available sugars, bases and phosphate.
  3) Synthesis of early proteins; binds amino acids; catalyzes synthesis of simple proteins.
• FeS mounds; took over some of the function of a semi-permeable membrane and took over RNA’s catalytic role.

Question 14

1) Is DNA more stable than RNA?
2) How did DNA develope?

Answer 14

1) Yes, more stable than RNA; better repository of genetic (coding) information; arose and assumed the template role for RNA synthesis.
2) Central dogma developed early on in cellular evolution; presumably, this was the best solution to biological information processing.
- Then, a long time of extensive biochemical innovation and experimentation in which much of the structural and functional machinery of these earliest self-replicating systems was invented and refined under natural selection.

Question 15

1) What was another important step in the emergance of cellular life?
2) What purpose does phospholipid membrane vesicles serve?
3) What is LUCA?

Answer 15

1) Synthesis of lipid and phospholipid membrane vesicles.
2) Phospholipid membrane vesicles enclosed the biochemical and replication machinery of early self-replicating cell-like entities. Proteins embedded in the lipids shuttled nutrients and wastes across the lipid membrane. Self-replicating systems contained within lipis membranes set the stage for evolution of energy-conserving processes (PMF, ATP synthesis)
3) From this population of proto-cells arose the Last Universial Common Ancestor. Shortly thereafter cellular life diverged into the ancestors of modern day Bacteria and Archaea.

Question 16

1) How old is LUCA?
2) What diverged from LUCA?
3) When did they diverge?

Answer 16

1) May have existed as early as 4.25 bya.
2) Divergence in lipid biosynthesis and divergence in cell wall biochemistry gave rise to two seperate lineages, Bacteria and Archaea.
3) Ancestors of modern day Bacteria and Archaea probably diverged from LUCA by 3.8 to 3.7 bya.

Question 17

1) What were the first cells? How did they obtain their carbon?
2) What is hydrogen oxidation?
3) What is methanogenesis?
4) What cells came next?

Answer 17

1) Anaerobic autotrophs, obtained their carbon from CO₂ and their energy (electrons with which to reduce carbon dioxide to cellular material) from H₂.
2) 2 H₂ + CO₂ → H₂O + [CH₂O], hydrogen oxidation releases a lot of enegy.
3) 4 H₂ + CO₂ → CH₄ + 2H₂​O, provides energy probably also invented very early on.
4) Chemoorganotrophic Bacteria, based on the build ip of organic materials from the metabolism and reproduction of these early bacteria.

Question 18

1) What is anoxygenic photosynthesis?
2) When did these cells originate?
3) What color are anoxygenic phototrophs?

Answer 18

1) The ability to use solar radiation as an energy source allowed these phototrophs to diversify extensively.
2) Probably began within 500 million years after the first cellular life arose bacteria with this ability are probably the ones that formed the original stromatolites.
3) Purple or green photosynthetics bacteria.
- H₂S + CO₂ → S^° + [CH₂O]

Question 19

1) When did oxygenic photosynthesis begin?
2) What can these cells use in place of H₂S?
3) What is released?

Answer 19

1) H₂O + CO₂ → O₂ + [CH₂O], probably began around 1.3 billion years after the first cellular life arose.
2) The lineage developed a photosystem that could use H₂O in place of H₂S for photosynthetic reduction of CO₂
3) Releases O₂ instead of S⁰ as a waste product; this system is more energetically efficient at trapping the energy if light for use of ATP synthesis. Huge amount of energy available from sunlight; great resource for those cell that discovered how to use it.

Question 20

1) When was the invention of anoxygenic photosynthesis?
2) When was the invention of oxygenic photosynthesis?
3) 0.1% oxygen in atmosphere?
4) 1% oxygen in atmosphere?
5) 10% oxygen in atmosphere?
6) 21% oxygen in atmosphere?

Answer 20

1) 3.2 bya
2) 2.7 bya
3) 2.4 bya
4) 2.0 bya
5) 1.3 bya
6) 0.7 bya