2 The Early Origins Of Life - Evolution From 4500-600 MYA Flashcards
Process of Abiogenesis - life emerging from chemistry (early earth chemistry)
Early Earth chemistry - are there routes back to ‘building block’ molecules? Urey and Miller 1953
What complex molecules were present from 4500 MYA
water / methane / hydrogen / ammonia (small amount of oxygen could also be generated)
Building blocks of life (without life)
AA / sugars / lipids and FA
Protocells
Lipid membranes - to create a compartment
Long chain FA naturally assemble
Information storage mechanism
Encoded catalysts (in the end were proteins)
Energy storage / transfer (ATP)
The RNA world hypothesis
Early life was based on RNA not DNA
DNA has excellent stability but limited chemical reactivity
But lots of spare capacity for reactivity as bases arent internally paired - charged forces
Why was DNA more stable than RNA
H bonds between strands removing charge from outside the molecule
RNA is therefore less stable
RNA can be both coding and enzymatic - ribozymes
RNA molecules can both encode information (base order) and act as catalysts. Folds to form secondary structures, retain areas of charge and capacity to bond.
RNA can be enzymatic so can fuel chemical reactions - can do protein job and the information job —> that’s why people think that the world was based on RNA
When did life originate - Data from C13/12 (ratio in minerals)
Enzymatic process prefer to use C12, so organic carbon produces by biotic process is C12 enrciched and C13 depleated
So if we find carbon in dated minerals, we can assess whether its biologically produced vs through non-enzymatic chemistry (if there is less C13 its likely to be enzymatic as enzymes prefer using C12) —> use of C12/13 ratio
Early planet description (CO2 and O2)
Early planet was CO2 rich and O2 low, O2 generated from UV action on H2O but likely rapidly incoperated into metal oxides (FeO / Fe2O3)
Photosynthesis - use of light and infrared radiation
Early use of light energy likely to be non-oxygen if photosynthesis - splitting H2S
Use infrared radiation - low energy input for lower energy reaction
Oxygenic photosynthesis
Cyanobacteria - ‘blue green algae’ (technacially bacteria)
Use visable light of Hugh energy to split water in presence of CO2 into carbohydrates
Geological evidence of oxygen - crystal deposition patterns
Iron pyrites (FeS2) and Uraninite (UO2) —> can only form and be stable at low O2
Lost in geological stars formed after 2.3 BYA in coincident fashion —> stopped forming (shows oxygen was now present)
What if photosynthesis likely evolved before the rise of O2
Presence of unoxidised deposit is (or not fully oxidised) metals would absorb O2 generated —> reducing geology. Only when these reducing factors are fully oxidised would O2 start to accumulate.
Fe —> FeO —> Fe2O3 (presence of banded iron formation indicate absorption of O2) 2800-2500MYA
Stromatolites - fossil Cyanobacteria
Cyanobacteria mats can create rock formations —> 3800MYA —> evidence of early photosynthesis
Summary of Photosynthesis (the metabolism that changed the world)
O2 not present in atmosphere until 2.3BYA but O2 was produces but absorbed before that (banded iron 2.5BYA)
Fossil evidence that photosynthesis evolved between 3.8-2.5BYA
Great oxygenation event (Preston Cloud)
Photosynthesis evolved, early O2 —> oxidised metal and once metals oxidised, free O2 rises
What happened 2.45 BYA
Previously everything was anaerobic (unless it was photosynthesis) due to the absence of oxygen
Aerobic vs anaerobic efficiency
Aerobic is more efficient —> reverse of photosynthesis / equivalent to combustion of sugar
Aerobic is 20x more efficient than anaerobic
