1. Origins of Life-2
Unit 1 Part 1

2. Origins of Life -2 Objectives
Describe the chemical and physical conditions of the pre-biotic Earth
Discuss several theories of the origins of life on Earth.
Describe how the success of prokaryotic life has changed the chemical conditions of this planet.
Discuss how our understanding of the origins of life on Earth is helping us look for life (or the evidence of past life) elsewhere in the universe (discuss examples Mars, Titan, etc.).

3. Earth
The earth is 4.5 to 4.7 Billion Years Old

4. Major landmarks in biological evolution
Major landmarks in biological evolution. The positions of the stages on the time scale are approximate. Note how the oxygenation of the atmosphere due to cyanobacterial metabolism was a gradual process, occurring over a period of about 2 billion years. Also note that for the bulk of Earth’s history, only microbial life forms existed. Although microfossil evidence is lacking, microchemical evidence for eukaryotic cells goes back as far as 2.7 billion years ago.

6. ? Life No Life How did we get here? 0.5-1 billion years
4.5 billion years ago
~3.5 billion years ago

7. Cellular organization
Inorganic
Organic
Cellular organization
Progenote
Universal Ancestor
Pre-life Earth was a mixture of chemicals
Elements of life H, O, C, N, S, P, (and a little P, Fe, Na, and K)

8. Theories of the Origins of Life
Spontaneous formation of organic molecules
“RNA” world
Solid-phase “Iron-Sulfur” world

9. Theories about the origin of life on Earth
Spontaneous Formation of Organic Molecules (Oparin and Haldane 1920s)
the combination of reduced gasses and electricity (or UV radiation or nuclear radiation) caused the formation of more complicated organic compounds

10. Spontaneous formation evidence
Urey & Miller experiments (1950s)
Created amino acids, sugars, fatty acids, thioesters, and nucleotide bases in the lab
Amino acids and other organic molecules are found in meteorites (shows that it can happen without human intervention)
Detected in space (spectral analysis)

11. Theories
“RNA world”
RNA has two properties that make it an attractive first polymer of life
Contains information (nucleotide bases)
Catalytic (RNA can do some chemical reactions)

12. and major coding molecule
The RNA World
The current “most accepted “theory of life evolving hypothesizes an RNA world
RNA in the early world would have functioned as a self replicating molecule, eventually developing a number of minimal catalytic properties
RNA
self-replicates
Proteins take
over catalysis
Packaging evolves
- RNA codes
and catalyses
DNA becomes
long term storage
and major coding molecule

13. Rybozymes
The first ribozyme discovered was a piece of mRNA that could self-edit – cut out the introns.

14. Order of the Information Macromolecules
DNA
RNA
Proteins
RNA required for protein synthesis (mRNA, tRNA, rRNA)
DNA is a modification of RNA
Where did the first RNA come from?

15. Ribosymes
The first RNA polymers may have formed on clay templates (clay molecules have very regular structures)

16. Origin of Life Stages of RNA World: RNA World -Most accepted theory
-In early world as self-replicating molecule
-Later developing other catalytic properties
Stages of RNA World:
1-Formation of nucleotides (spontaneous formation?)
2-Formation of RNA molecules (on clay template?)
3-RNA replication
RNA molecules that evolve catalytic activities (protein synthesis) had a selective advantage
4-The link between sequence of RNA and sequence of proteins (translation)

17. and major coding molecule
The RNA World
RNA
self-replicates
Proteins take
over catalysis
Packaging evolves
- RNA codes
and catalyses
DNA becomes
long term storage
and major coding molecule

18. Theories
Solid-phase “Iron-Sulfur” world – life formed on solid surfaces
The reaction between Fe+2 and HS- (or H2S) can be coupled to CO2 fixation
This type of reaction can occur in high temperature, high pressure environments
- hydrothermal vents

19. Iron-sulfur world Evidence
Iron and sulfur are important catalytic elements in biology (oxidation/reduction reactions)
Laboratory demonstrations of the production of pyruvate under “prebiotic” conditions

20. Abiotic reactions leading to organic compounds – occur under high temp/high pressure conditions
Inorganic reactions can catalyze the formation of organic molecules

21. Cells Inorganic Organic Cellular organization Progenote
Universal Ancestor
Once macromolecules formed – how did cells form?

22. Origin of Cells Organic molecules have tendency to aggregate
membrane-like structures form easily and even happen in laboratory experiments
Microspheres- " coacervates“- form spontenously and can replicate by pinching off new spheres
Proteinous microspheres with catalytic and self- replicating properties could be formed in the lab (Fox 1965, Protobionts, Progenotes)

23. Coacervate growth and division
Collection of aggregated polymers
Grow by adding new polymers
Form a semi-permeable membrane
When they get too big they divide
Demonstrates how polymers aggregate and act like membranes

24. Progenote or Protobiont
Proteinaceous microspheres – contain proteins and lipids but no nucleic acids
Self replicating
Some ability to catalyze reactions
“pre-cells”
Progenotes would eventually pick up RNA and DNA, develop enzymatic capabilities and membrane organization = primitive cell

25. Origin of Life

26. Where did life start? Hydrothermal vents
RNA lineage puts hyperthermophiles nearest to the universal ancestor (see fig 2.3 where are the hyperthermophiles?)
Rich in reduced compounds Fe+2, H2S, H2 etc.
Protected from sterilizing cosmic radiation

27. Where did life start?
“Panspermia” – extraterrestrial – life landed here on a meteorite
Life started earlier than previously thought (when the earth was less than a billion years old)
Organic material is found in meteorites
Life exists on earth in habitats similar to those found (or once existed) on Mars, Europa, Titan

28. Magnetite crystals
Evidence for life on other planets?

29. The first organism must have employed a simple strategy to obtain energy. Primitive metabolism was anaerobic, and likely chemolithotrophic, exploiting the abundant sources of FeS and H2S present.
One possibility is
FeS + H2S ==> FeS2 + H2
The resulting H2 could have been used to drive a primitive ATPase with S0 as a potential electron acceptor
Earliest metabolism
Brock pg 428
Fermentations, and anaerobic respiration probably appeared later along with anoxygenic photosynthesis followed by oxygenic photosynthesis. The latter led to development of an oxic environment, and to great bursts of biological evolution.

30. Oxygen Earliest Evidences: oldest fossils
Oldest photosynthetic microbes B.Y.
- Bacterium-like
- Unicellular
- Evidence for breakdown products of photosynthesis
Cyanobacteria, 3.5 B.Y.
Stomatolites, B.Y.
Evolution of oxygen

31. Oxygen Evidence for O2 production: Banded Iron Formations (BIF)
BIF found in ocean sediments red bands are high in Fe2O3 and Fe3O4 (red bands)- forms when reduced iron reacts with O2

32. Oxygen

33. Oxygen
Only known source of O2 in the atmosphere is oxygen producing photosynthesis
Photosynthesis produced O2 in the oceans that combined with the Fe producing iron oxides-sinking down to ocean floor producing BIF
BIFs occur in geological rock formations dating back to B.Y.

34. Life on Earth

35. Oxygen BIFs occur 3.2-2 B.Y. ago, suddenly disappear.
Red beds, terrestrial formations similar to BIF, but much lower in iron concentrations
Red beds, indication of presence of O2 in the atmosphere
Red beds are present in terrestrial sediments in last 2 B.Y.
Formation of red beds beginning 2B.Y. ago, after all reduced Fe in the oceans had been oxidized

36. Life on Earth Oxygen sinks: Volcanic gases scavenge O2
Aerobic respiration uses O2
Weathering of rocks containing reduced elements such as carbon, sulfur, and iron
Above sinks has not changed since Archean time
But BIFs sink finally became saturated
2 B.Y. ago
Massive iron-rich ores that are used today to make steel are legacy of photosynthetic microbes
Summary: massive change in chemistry of the Earth, mediated entirely by bacteria

37. Why did the availability of oxygen drive evolution (or why are eukaryotes aerobic)?
Hint – see the Nealson 1999 paper.

38. Major landmarks in biological evolution
Major landmarks in biological evolution. The positions of the stages on the time scale are approximate. Note how the oxygenation of the atmosphere due to cyanobacterial metabolism was a gradual process, occurring over a period of about 2 billion years. Also note that for the bulk of Earth’s history, only microbial life forms existed. Although microfossil evidence is lacking, microchemical evidence for eukaryotic cells goes back as far as 2.7 billion years ago.