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
Methanogens
Release CH4
Archaea only / fermentation (produces EtOH, lactic acid) / anaerobic cellular respuration —> this would have been present pre-oxygenation event
Why did O2 levels rise
some carbon being fixated in rocks (sedimentary rocks) so oxygen produced from photosynthesis started to rise
Symbiotic fusions occur in…
The tree of life
What are the 3 main domains in the tree of life
Bacteria
Archaea
Eucarya
(Eucarya derived from Archaea)
Carl Woese
rRNA (ribosomal RNA) as encoded in the genome of all living things
‘Structural’ component of ribosomes and perfect ‘deep time’ phylogenetic marker
Origins of mitochondria
Eukaryotic cells (except obligate anaerobes) have mitochondria
Lynn Margulis - evidence that mitochondria are relict bacteria
Endosymbiosis theory of eukaryotic origins
Bacterial aspects of mitochondria
Double membrane / contains own circular DNA / divide like bacteria / possess ribosomes - own protein synthesis —> protein synthesis inhibited by same antibiotics as in inhibit protein synthesis in bacteria
Mitochondria have 16S rRNA gene —> derive from alphaproteobacteria (bacteria that lives inside cells)
1500 BYA the mitochondrion was an alphaproteobacteria
Alphaproteobacteria
Can be found in many invertebrates today (insects)
Adapted to life within eukaryotic cells
Obligate aerobes
Evolution of mitochondria since eukaryogenesis
Mitochondria as alphaproteobacterium has significantly changed since this time
Mitochondrial genomes are very small - ribosome formation, tRNAs for translation, proteins for electron transfer chain
Most ancestral mitochondrial genes transferred to nucleur genome and proteins targeted back to mitochondrion
- (Mitochondrial) mtDNA - 37 genes
Eukaryote genomes are very distinct from archaea
Genes in eukaryotes originate from eubacteria and archeae
Core genes for processes - translation tend to be archeal, other genes eubacteria
Archaea as initial cell, acquiring ‘content’ from eubacteria
In this sense, tree of life is a poor metaphor - as its absorbed genetic information from birth eubacteria and archaea
Molecular evolution of eukaryotes
Happened in the presence of oxygen so must’ve happened after the evolution of Cyanobacterial photosynthesis
Ancestor is about 2 BYA for the mitochondria (rise of oxygen was about 2.6BYA)
Looks like aerobic respiration evolved in bacteria then got symbiotically taken into arches and eukaryotes within 200MYA or so
Fossil evidence for evolution of eukaryotes
Fossil evidence - micro eukaryote fossils from 1.7-1.4 BYA in China
Eukaryotes most likely evolved between 2 - 1.7 BYA
What we don’t know About eukaryote evolution
We’re mitochondria acquired late or early —> LECA —> there at the start of life or came after, displaced all other eukaryotes
How did it happen - proto-eukaryote ‘consuming’ a bacterium vs bacterium entering proyoeukaryote
Why did it do it so well —> current mitochondrial function (providing ATP from aerobic metabolism) unlikely to be initial reason (no means of exporting ATP) —> bacterium detoxified cell of O2 (which is potentially damaging to be an anaerobe)
The order of events —> archaea - eukaryote = meny diffenees (mitochonida, nucleus, chromosomes etc)
We don’t know the order of these, drivers, current function mitochondria (ancestral function)
Eukaryotic diversity - Origin of photosynthetic eukaryotes
Photosynthetic eukaryotes have a single main origin
Archaeplastida - various algae
Viridiplantae - includes variety of green algae and plants
Eukaryotic diversity - Origin of chloroplasts
Lynn Marguilis considered the chloroplast to be a case of endosymbiosis - a Cyanobacteria symbiotically fused into a eukaryote cell
Double membrane / own DNA (5-10% size of Cyanobacteria genomes) / own ribosomes, rRNA, tRNA / proteins of Cyanobacteria origin in nucleus, targeted to chloroplast
Cyanobacteria - how was the origin confirmed
16S rRNA —> closely related to chloroplast DNA
Cyanobacteria complexity
More complex than mitochondria
Archaeplastida algal cells have themselves become symbionts of other eukaryotes —> secondary chloroplasts
Symbiosis with Cyanobacteria occurred more than once
More recent evolution of Cyanobacteria symbiont in eukaryotic Amoeba ‘Paulinella’
Derived c. 100MYA via symbiosis
Cyanobacterial genome still very large / interesting as amoebae in this group are natural predators of Cyanobacteria —> predation and then maintance as origin
Many eukaryotes came to carry plasmids, impact of these (5)
Photosynthesis became spread across the eukaryotic tree
Primary edosymbiosis —> archaeplastids
Primary endosymbiosis —> Paulinella
Secondary endosymbiosis —> other algal groups
Lots of primary productivity in oceans and later land
Coccolithophores
Small eukaryote that live in the sea
Coccolithophores photosynthesis
Photosynthetic, make CaCO3 shell from dissolved CO2 —> up to 40% of marine primary productivity
Coccolithophores (effect on global carbon)
Massive carbon sink —> removing CO2
Coccolithophores, what geology do they cause
Makes geology such as the White Cliffs of Dover (lithosphere - makes stones)
bacteriovore / algavore predators
instead of digesting bacteria they farm for sucrose
Where did decomposers evolve from
Micro-eukaryotes
Micro eukaryotic diversification produced simple but functional energy cycling (explanation)
Primary production (photosynthesis across tree) - alongside Cyanobacteria
Predators and decomposers alongside bacteria
When animals and plants arose…
Micro eukaryote life evolved to be parasites
Corals also require symbiodinium alga
Enables construction of coral reefs / major ecosystem
Overall summary
Life is ancient, but processes leading to origin of life not well resolved.
Photosynthesis was a key innovation that changed the world, providing an oxygenated/oxidising environment
Eukaryotes have origins in symbiotic fusion; eukaryotic genomes remain a chimera of eubacteria/archaebacteria
Photosynthesis spread amongst microeukaryotes; nutrient cycling is present (producers, decomposers, predators, parasites)
