2 The Early Origins Of Life - Evolution From 4500-600 MYA

Question 1

Process of Abiogenesis - life emerging from chemistry (early earth chemistry)

Answer 1

Early Earth chemistry - are there routes back to ‘building block’ molecules? Urey and Miller 1953

Question 2

What complex molecules were present from 4500 MYA

Answer 2

water / methane / hydrogen / ammonia (small amount of oxygen could also be generated)

Question 3

Building blocks of life (without life)

Answer 3

AA / sugars / lipids and FA

Question 4

Protocells

Answer 4

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)

Question 5

The RNA world hypothesis

Answer 5

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

Question 6

Why was DNA more stable than RNA

Answer 6

H bonds between strands removing charge from outside the molecule

RNA is therefore less stable

Question 7

RNA can be both coding and enzymatic - ribozymes

Answer 7

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

Question 8

When did life originate - Data from C13/12 (ratio in minerals)

Answer 8

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

Question 9

Early planet description (CO2 and O2)

Answer 9

Early planet was CO2 rich and O2 low, O2 generated from UV action on H2O but likely rapidly incoperated into metal oxides (FeO / Fe2O3)

Question 10

Photosynthesis - use of light and infrared radiation

Answer 10

Early use of light energy likely to be non-oxygen if photosynthesis - splitting H2S

Use infrared radiation - low energy input for lower energy reaction

Question 11

Oxygenic photosynthesis

Answer 11

Cyanobacteria - ‘blue green algae’ (technacially bacteria)

Use visable light of Hugh energy to split water in presence of CO2 into carbohydrates

Question 12

Geological evidence of oxygen - crystal deposition patterns

Answer 12

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)

Question 13

What if photosynthesis likely evolved before the rise of O2

Answer 13

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

Question 14

Stromatolites - fossil Cyanobacteria

Answer 14

Cyanobacteria mats can create rock formations —> 3800MYA —> evidence of early photosynthesis

Question 15

Summary of Photosynthesis (the metabolism that changed the world)

Answer 15

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

Question 16

Great oxygenation event (Preston Cloud)

Answer 16

Photosynthesis evolved, early O2 —> oxidised metal and once metals oxidised, free O2 rises

Question 17

What happened 2.45 BYA

Answer 17

Previously everything was anaerobic (unless it was photosynthesis) due to the absence of oxygen

Question 18

Aerobic vs anaerobic efficiency

Answer 18

Aerobic is more efficient —> reverse of photosynthesis / equivalent to combustion of sugar

Aerobic is 20x more efficient than anaerobic

Question 19

Methanogens

Answer 19

Release CH4

Archaea only / fermentation (produces EtOH, lactic acid) / anaerobic cellular respuration —> this would have been present pre-oxygenation event

Question 20

Why did O2 levels rise

Answer 20

some carbon being fixated in rocks (sedimentary rocks) so oxygen produced from photosynthesis started to rise

Question 21

Symbiotic fusions occur in…

Answer 21

The tree of life

Question 22

What are the 3 main domains in the tree of life

Answer 22

Bacteria

Archaea

Eucarya

(Eucarya derived from Archaea)

Question 23

Carl Woese

Answer 23

rRNA (ribosomal RNA) as encoded in the genome of all living things

‘Structural’ component of ribosomes and perfect ‘deep time’ phylogenetic marker

Question 24

Origins of mitochondria

Answer 24

Eukaryotic cells (except obligate anaerobes) have mitochondria

Lynn Margulis - evidence that mitochondria are relict bacteria

Endosymbiosis theory of eukaryotic origins

Question 25

Bacterial aspects of mitochondria

Answer 25

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

Question 26

Alphaproteobacteria

Answer 26

Can be found in many invertebrates today (insects) Adapted to life within eukaryotic cells Obligate aerobes

Question 27

Evolution of mitochondria since eukaryogenesis

Answer 27

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

Question 28

Eukaryote genomes are very distinct from archaea

Answer 28

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

Question 29

Molecular evolution of eukaryotes

Answer 29

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

Question 30

Fossil evidence for evolution of eukaryotes

Answer 30

Fossil evidence - micro eukaryote fossils from 1.7-1.4 BYA in China Eukaryotes most likely evolved between 2 - 1.7 BYA

Question 31

What we don’t know About eukaryote evolution

Answer 31

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)

Question 32

Eukaryotic diversity - Origin of photosynthetic eukaryotes

Answer 32

Photosynthetic eukaryotes have a single main origin Archaeplastida - various algae Viridiplantae - includes variety of green algae and plants

Question 33

Eukaryotic diversity - Origin of chloroplasts

Answer 33

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

Question 34

Cyanobacteria - how was the origin confirmed

Answer 34

16S rRNA —> closely related to chloroplast DNA

Question 35

Cyanobacteria complexity

Answer 35

More complex than mitochondria Archaeplastida algal cells have themselves become symbionts of other eukaryotes —> secondary chloroplasts Symbiosis with Cyanobacteria occurred more than once

Question 36

More recent evolution of Cyanobacteria symbiont in eukaryotic Amoeba ‘Paulinella’

Answer 36

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

Question 37

Many eukaryotes came to carry plasmids, impact of these (5)

Answer 37

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

Question 38

Coccolithophores

Answer 38

Small eukaryote that live in the sea

Question 39

Coccolithophores photosynthesis

Answer 39

Photosynthetic, make CaCO3 shell from dissolved CO2 —> up to 40% of marine primary productivity

Question 40

Coccolithophores (effect on global carbon)

Answer 40

Massive carbon sink —> removing CO2

Question 41

Coccolithophores, what geology do they cause

Answer 41

Makes geology such as the White Cliffs of Dover (lithosphere - makes stones)

Question 42

bacteriovore / algavore predators

Answer 42

instead of digesting bacteria they farm for sucrose

Question 43

Where did decomposers evolve from

Answer 43

Micro-eukaryotes

Question 44

Micro eukaryotic diversification produced simple but functional energy cycling (explanation)

Answer 44

Primary production (photosynthesis across tree) - alongside Cyanobacteria Predators and decomposers alongside bacteria

Question 45

When animals and plants arose…

Answer 45

Micro eukaryote life evolved to be parasites

Question 46

Corals also require symbiodinium alga

Answer 46

Enables construction of coral reefs / major ecosystem

Question 47

Overall summary

Answer 47

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)