Habitable planet Flashcards
(36 cards)
what is the order and timescales of the precambrian
Hadean 4.6-4 Ga
Archean 4-2.5 Ga
Proterozoic 2.5-0.54 Ga
conditions in precambrian low latitude shallow marine environment
bacterial mats
- green + photosynthesising
- silver bubbles - methane from below (previous mat decaying)
- chunks missing = storms
snails destroy bacterial mats, once gastropods develop they decline rapidly
brief timeline of earths history (precambrian 4Ga - 541Ma)
Hadean
4.5Ga - Giant impact hypothesis
Archean
≈4Ga - Late heavy bombardment, life quickly after
3.8Ga - oldest marine sediments
3.4Ga - oldest bacteria
Proterozoic
2.1 Ga - first photosynthesis
2.45 - 1.85 Ga - GOE -> consequence of photosynthesis
“boring billion” after GOE
720 - 635 Ma - Snowball Earth
635-541 Ma - first animals immediately after snowball earth
What were the main differences on earth >3 Ga (before 3 billion years ago)
Common impacts >K-P event (>150km)
Moon
- 4x closer
- stronger tides
Plate thickness and structure
- thin primordial crust
- no oceanic and continental (until Archean)
- plate tectonics began in Proterozoic
Climatic differences
- weak insolation
- high GHG = high temp = no glaciation
(until GOE decrease methane proportion)
Surface differences
- high radioactivity
- hot metal rich crust
(extreme conditions -> extremophiles)
Atmospheric differences
- no O2 or O3 (ozone) -> high UV
difference between autotroph and heterotroph
where gets carbon for biomass
auto = from CO2
hetero = from organic molecules
difference between phototroph and chemotroph
source of energy
photo = sunlight
chemo = oxidation of chemicals
difference between lithotroph and organotroph
where gets electrons for reducing power
litho = water/H2/rocks
organo = organic molecules
what is likely the sequence of evolution of autotrophic strategies
anaerobic chemolithotrophy
(no oxygen, no sun)
anaerobic anoxygenic phototrophy
(no oxygen, sun, no photosynthesis)
oxygenic phototrophy
(photosynthesising)
what are traits common to all cells and why is this useful to know
- limited building blocks (h2o, C, N, P)
- cell membrane (fatty acid bilayer)
- chiral molecules (left AA, right nucleic acids) (life selected one)
- common suite of organic molecules
- replicate and pass information one generation to the next
useful as ancestors also likely to have had the same
also points to single origin
what is the ‘RNA world’ hypothesis
early life had no DNA, but used RNA for same role
- information storage
- replication
- protein synthesis
- enzymatic activity
what are the 3 hypotheses for the origin of life
- Oparin-Haldane hypothesis
- Panspermia hypothesis
- Hydrothermal hypothesis
explain the opaline-haldane hypothesis
= ‘primordial soup’
energy from lightning could synthesis amino acids easily in early atmosphere, proved by experiments
however, recently with known early composition, very few form + difficult
explain the panspermia hypothesis
= delivery of simple organic molecules from space
from - carbonaceous chondrites, meteorites, comets, dust clouds
e.g. uracil (component of RNA) found in meteorite
all 20 AA found in comet
difficult to test due to contamination
ease of synthesis in early solar system?
explain the hydrothermal vent hypothesis
= localises synthesis of important simple organic molecules to alkaline vents
- heat + fluids = favourable conditions for reactions
- link with extremophiles more common in geological past
how do we estimate dates using molecular clocks
- random mutations that divide a lineage occur at a given rate
- rate of mutation with percentage genetic differences to another species can be used to estimate a date
- different mutation rates though time make more complex
- LUCA estimated in Hadean - remarkable
how do stromatolites form
- grow upwards in columns to reach light
- domal form
- bateria growth covered with sediment leads to another bacterial layer to grow above and process repeats
what are some issues with identifying stromatolites in the geological past
identifying by purely morphological structure can be misleading
e.g.
- Isua, greenland: actually deformation features, identified when cut at 90º
- ‘Taylor-stromatolite’: (convex laminae, concentric circles when cut 90º) -> actually layers of lead based car paint
what are 3 strands of evidence used to distinguish biotic from abiotic in the geological record
- viable context (geological setting)
- biological morphology
- tiers of biological processing (geochemistry)
characteristics of Gunflint Chert, Ontario (1.88 Ga), in the search for early life
- stromatolites (morphology)
- microfossils in chert bands (setting)
-> 12 cyanobacteria species - spheres under E.M. are cells?
- C13 values within range of modern organisms (biological processing)
characteristics of oldest marine sediments Isua, Greenland (3.8 Ga), in the search for early life
- carbon accumulated around apatite grain -> decayed cell later infilled (morphology)
- C13 not exact match to average (ish biological processing)
- marine sediments (setting)
characteristics of Apex Chert, Australis (3.46 Ga) in the search for early life
- chains of round structures resembling cyanobacteria (morphology)
issues:
- 1Ga before cyanobacteria
- random distribution
- not clustered
- no pattern with bedding
- geochem - revealed aluminosilicates
- context - igneous rocks
characteristics of the Strelley Pool formation (3.4 Ga), in the search for early life
- intertidal beach env (setting)
- hollow chain structures, clustered, show decay, stromatolite (morphology)
- -ve 13C (biological processing)
what are the 3 potential pieces of evidence we found and analysed for evidence of early life
3.8 Ga - Isua, Greenland (Earliest marine sediments)
3.4 Ga - Strelley pool, Bus (all 3 factors convincing)
1.8 Ga - Gunflint Chert (gold standard)
what evidence should we look for in discovering photosynthesis in the geological past, and what is an estimate for the onset
- body fossils (cyanobacteria)
- molecular fossils (biomarkers in decay)
- indirect clues of atmospheric oxidation
estimates: 3.8 - 2.35 Ga
