3 Origin of Life Flashcards
What happened to Earth 4600 mya?
Gravitational accumulation of dust and larger objects forms Earth
The mass melts and differentiates into core, mantle, crust
Water vapour, other gases are outgassed - don’t accumulate due to great heat and continual bombardment as new material accumulated
Moon forms during a major collision
What happened to Earth 3750 mya?
Cooled so that crust begins to solidify (oldest rocks found from this time)
Oceans and atmosphere can begin to condense out and temps fall
Why is the moon classed as a dead planet?
Records a perfect history of cratering
No erosion
Late heavy bombardment that prevented atmosphere forming
What happened to Earth >3800 mya?
Progress retarded by continued bombardment of large objects
Released energy sufficient to boil off oceans and atmosphere + any prebiotic organic compounds
What happened to Earth <3800 mya?
Meteorite bombardment decreases in intensity
Planet cools below threshold allowing oceans and atmosphere to condense out
Organic compounds begin to be synthesised and accumulate
Conditions possibly suitable for life to have originated
When was the earliest fossil evidence for life on Earth?
3500 mya
How were the early atmosphere and oceans formed?
By volcanic outgassing of water vapour, N2 (+oxides), CO2, CH4, NH3, H2, H2S.
water vapour gradually condenses to form the oceans
H2 lost into space
What is panspermia?
The theory that life may have evolved elsewhere in the universe then was transported to Earth by space dust, meteoroids etc
What were the approaches towards solving the origin of life?
1) Analyse living prokaryotes and attempt to reconstruct their common ancestor
2) Compare duplicated genes - to reach back beyond that ancestor and estimate some of earliest components of genetic machinery
3)Reconstruct conditions on Earth in these remote times and simulate these experimentally to see what is produced
Are prokaryotes or eukaryotes believed to have originated first?
Prokaryotes:
Appear earlier in fossil record
Simpler in virtually every aspect
Evidence that eukaryotes evolved from prokaryotes
What are the fundamental similarities between prokaryotes and eukaryotes?
1) Method of transmitting information in triplet code in DNA and translating into proteins through RNA
2) In all living organisms all amino acids are laevo-rotatory and in nucleic acids all sugars are dextro-rotatory
What chemicals are produced by simulating primitive Earth conditions?
Amino acids
Purines/pyrimidines (not thymine)
Sugars
Porphyrins
Complex tar-like substances which defy analysis
Similar chemicals also occur in space but contain laevo- and dextro- rotatory forms
How did life most likely evolve?
Through basic chemistry on Earth
Necessary materials were available
Variety of possible environments in which life could have formed
Number of possible energy sources
What are the possible energy sources that allowed evolution of life?
Sun - UV radiation - no ozone layer at this time due to no oxygen
Radioactivity - gradually declines
Electric discharges - lighting
Volcanic - hot springs, black smokers
What is the current hypothesis of how life evolved?
RNA world:
- Same basic machinery used by all living organisms (DNA, RNA, Protein, translation etc)
- What came first, DNA or proteins?
- Self splicing RNA discovered in 1980s
How would early RNA-based life survive?
Using the chemicals comprising the “primordial soup”
Eventually ran out so had to develop novel metabolic pathways
What are the novel metabolic pathways used by early RNA-based life?
- Chemoautotrophs - energy from oxidising inorganic substances (H2S, NH3, Fe2+ etc) C source CO2
- Chemoheterotrophs - energy and C source from consuming organic compounds
- Photoautotrophs - energy from light, C source CO2
- Photoheterotrophs - energy from light, C source from consuming organic material
What do the novel metabolic pathways require?
Synthesis of cytochromes (basis of O2 metabolism), porphyrins and related compounds that are forerunners of photosynthetic pigments (chlorophylls)
What are obligate anaerobes?
Poisoned by O2
Live exclusively by fermentation or anaerobic respiration
What are aerotolerant organisms?
Cannot use O2 for growth but tolerate its presence
Live by fermentation
What are facultative anaerobes?
Use O2 if present but can live by fermentation in an anaerobic environment
What are obligate aerobes?
Use O2 for cellular respiration and cannot live without it
