Lecture 3 - Early history of planet Earth

Question 1

What occurred 4,600 million years ago?

Answer 1

• gravitational accumulation of dust & larger objects causes the formation of Earth.
• the mass melts & begins to differentiate into core, mantle & crust.
• water vapour & various gases are out gassed but don’t accumulate, due to the great heat & continual bombardment as a new material is accumulated.
• the moon forms during a major collision.

Question 2

What occurred 3,750 years ago?

Answer 2

• age of the oldest rocks on Earth (Isla Supercrustal Group from Greenland).
• Earth has cooled to the extent that a crust begins to solidify.
• As temperatures continue to fall the oceans & atmosphere can potentially begin to condense out

Question 3

What occurred >3,800 years ago?

Answer 3

• progress retarded by continued bombardment of large objects.
• released energy is sufficient to boil off the oceans & atmosphere (along with any periodic organic compounds)

Question 4

What occurred <3,800 years ago?

Answer 4

• meteorite bombardment decreases in intensity and the planet cools below a threshold that allows oceans & atmosphere to condense out
• organic compounds begin to be synthesised and accumulate

Question 5

What occurred 3,500 years ago?

Answer 5

The earliest fossil evidence for life on Earth

Question 6

How did the Early atmosphere form?

Answer 6

by volcanic outgassing of water vapour + N2 (+ its oxides). CO2, CH4, NH3, H2 & H2S.
- the water vapour gradually condenses to form the oceans and the H2 is lost into space.

Question 7

What occurred after the early atmosphere formed?

Answer 7

eventually there was an Earth hospitable for life - atmosphere was not breathable at the start of the beginning of life

Question 8

When did life begin?

Answer 8

between 3,800 & 3,500 million years ago

Question 9

What is panspermia?

Answer 9

belief that life exists throughout the universe, distributed by space dust

Question 10

What is the first approach to solving the origin of life?

Answer 10

Analyse living prokaryotes & attempt to reconstruct their common ancestor (essentially the simplest conceivable prokaryote)

Question 11

Why is it thought that prokaryotes originated before eukaryotes?

Answer 11

• they appear earlier in the fossil evidence
• they are simpler in virtually every aspect
• there is evidence that eukaryotes evolved from prokaryotes

Question 12

What are fundamental similarities between prokaryotes & eukaryotes?

Answer 12

• the method of transmitting information in triplet code in DNA & translating it into proteins through DNA
• in living organisms all amino acids are laevo-rotatory & in nucleic acids all sugar are dextro-rotatory

Question 13

What is the significance of the fundamental similarities between prokaryotes and eukaryotes?

Answer 13

this means that it is very unlikely that live evolved twice - due to similarities between the two

Question 14

What is the second approach to solving the origin of life?

Answer 14

compare duplicated genes (from before life evolved), potentially enabling us to reach back beyond that ancestor and estimate some of the earliest components of genetic machinery

Question 15

What does LUCA stand for?

Answer 15

last universal common ancester

Question 16

How would you compare duplicated genes?

Answer 16

look in archaea & bacteria for non-functional genes, and study them to find out how old genetic machinery worked

Question 17

What is the 3rd approach to solving the origin of life?

Answer 17

Reconstruct conditions that existed on Earth on these remote times & simulate these experimentally & see what is produced

Question 18

What are the chemicals produced by stimulating conditions on the primitive Earth?

Answer 18

• amino acids - including all of the biologically important ones
• purines/pyrimidines - all 4 bases of RNA formed - but not thymine
• sugars
• porphyrins - molecules which are the forerunners pf important biological compounds like Vitamin B12, chlorophyll
• complex tar-like substances which defy analysis

Question 19

What does space contain that the atmosphere doesn’t?

Answer 19

laevo & dextro rotators forms. This can be seen in some meteorites

Question 20

How is life likely to have evolved?

Answer 20

through basic chemistry on planet Earth

Question 21

What are the variety of possible environments in which life could have formed a number of possible energy sources?

Answer 21

• sun (UV radiation)
• radioactivity
• electrical discharges (e.g. lightening)
• volcanic (hot springs, black smokers etc.)

Question 22

What same basic machinery do all living organisms use to replicate?

Answer 22

information is stored in DNA and transcribed & translated into protein using DNA

Question 23

How was the discovery of what came first - the protein or DNA - solved?

Answer 23

when self-splicing RNA was discovered

Question 24

How did a concoction of chemicals end up in the ocean?

Answer 24

as RNA can self splice - all the organic compounds - e.g. uracil is being used by RNA

Question 25

When are chemicals more abundant?

Answer 25

if they are not used by the replicators - this meant there was now competition for replicators to use other sources, which could’ve led to the development of life.
- there may have also been mutations in the RNA during replication

Question 26

What led to the development of novel metabolic pathways?

Answer 26

when the ‘primordial soup’ ran out

Question 27

What are chemoautotrophs?

Answer 27

energy from oxidising inorganic substance - e.g. H2S, NH3, Fe2+ etc. C source CO2

Question 28

What are chemoheterotroghs?

Answer 28

energy & C source from consuming organic compounds

Question 29

What are photoautotrophs?

Answer 29

energy from CO2

Question 30

What are photoheterotrophs?

Answer 30

energy from light, C source from consuming organic material

Question 31

What was required for the new metabolic pathways?

Answer 31

the synthesis of cytochromes (the basis or oxygen metabolism) & porphyrins & related compounds that are the forerunners of photosynthetic pigment (the chlorophylls)

Question 32

What are Obligate anaerobes?

Answer 32

poisoned by O2, & live exclusively by fermentation or anaerobic respiration

Question 33

What are aerotolerant organisms?

Answer 33

can’t use O2 for growth, but tolerate its presence * live by fermentation

Question 34

What are facultative anaerobes?

Answer 34

use O2 if it’s present but can live by fermentation in an anaerobic environment

Question 35

What are obligate aerobes?

Answer 35

use O2 for cellular respiration & cannot live without it