Lecture 3: Origin of Life

Question 1

What is the name of the asteroid which is thought to have collided with the earth? What is the asteroid named after?

Answer 1

Theia - after a Greek goddess

Question 2

What effect did the collision of Theia with the earth have?

Answer 2

the earth spat out a lot of mantle rock from the earth’s surface that collected together to form the moon

Question 3

What happened to the moon after its collision with earth as time progressed?

Answer 3

It retreated away from the earth which produced angular momentum against the earth thereby slowing the earth’s rotation speed down

Question 4

What impact has the slowing down of the earth’s rotation had on time experienced on earth?

Answer 4

It has caused days to be longer

Question 5

How big roughly is the size of the moon?

Answer 5

It has a core roughly one quarter of the earth’s radius which is relatively large for a satellite planet

Question 6

Why does the moon have a relatively small core?

Answer 6

Because there were a limited number of metal fragments that were splashed out when the moon collided with the earth to form a core

Question 7

What had to have happened before oceans and an atmosphere could form on earth?

Answer 7

The earth’s surface needed to have cooled sufficiently so that it can retain them

Question 8

What are the two potential sources of material/water that could have formed oceans and an atmosphere?

Answer 8

Internal: from inside the earth

Extra-terrestrial impact

Question 9

Describe the location of an internal source of water and how it could have caused the formation of an ocean and atmosphere?

Answer 9

There was water stored in the mantle of the earth that would have been released as it erupted from volcanoes

Question 10

Describe how an extra-terrestrial impact could have caused the formation of an ocean and atmosphere on earth?

Answer 10

In the latter stages of the earth’s planetary accretion, planetesimals may have been scattered in to elliptical orbits as a result of close encounters with larger bodies such as mercury where they then collide with water-rich bodies that could have transported water and other volatiles to earth as they collide

Question 11

What evidence is there which supports the idea of an extra-terrestrial impact?

Answer 11

water rich carbonate chondrites (meteorite) have a similar D/H ratio to earth’s ocean and moon rocks taken from Apollo missions.

Question 12

What source of water is the most likely to have caused the oceans and atmosphere of the earth to form?

Answer 12

Internally

Question 13

What are zircons?

Answer 13

These are extremely small and weathering-resistant minerals found in rocks that contain a clock of their history.

Question 14

What are the two aspects of a Zircon clock?

Answer 14

Radioactive uranium isotopes

Oxygen isotopes

Question 15

What is a D/H ratio?

Answer 15

The ratio of deuterium to hydrogen

Question 16

How does the radioactive uranium isotope record in zircons work?

Answer 16

The two different isotopes decay at different rates based on their half-lives. Their age can then be cross-examined to determine the age of the mineral.

Question 17

How does the oxygen isotope record in zircons work?

Answer 17

Oxygen 18 is a denser isotope than Oxygen 16 which means it is harder for the atmosphere to evaporate and thus it is more commonly found in liquid form in oceans. If there is a high concentration of Oxygen 18 in the Zircon then it suggests that there were oceans present at that time. Moreover, this water must have been there because that is how sedimentary rocks form

Question 18

What was the oldest Zircon identified using the uranium record? Where was it found?

Answer 18

4.4Ga in Western Australia

Question 19

How old was the first actual remaining rock dated to and where was it found?

Answer 19

Quebec in Canada 4.28Ga

Question 20

What type of rock are those rocks which are normally found and dated to as far back as the oldest one found in Canada? What does this mean?

Answer 20

They are mostly igneous which suggests they may have undergone major continental processes

Question 21

What was the atmospheric composition of the earth like around 4Ga?

Answer 21

Carbon dioxide and other greenhouse gases used to be a lot higher in concentration whereas oxygen was much lower.

Question 22

What planets would the earth’s atmospheric composition have mimicked around 4Ga? What would not have been able to exist on earth at this time?

Answer 22

It would have been a lot like mars and venus. This would not have been suitable for life

Question 23

Why was the earth’s atmospheric composition made up more of greenhouse gases around 4Ga?

Answer 23

Because the sun was 30% dimmer and so there needed to be a better insulation of the minimal heat that the earth was actually receiving

Question 24

What and when was the Late Heavy Bombardment?

Answer 24

Around 4.1-3.9Ga the surfaces of the moon and earth were exposed to a prolonged asteroid shower

Question 25

What is thought to be the reason for the appearance of craters on the moon?

Answer 25

Late Heavy Bombardment

Question 26

What evidence is there for the LHB on the moon?

Answer 26

Rock samples from the moon taken during the space race were dated at around 3.9Ga

Question 27

Why might the LHB have happened?

Answer 27

Because of a resonance between orbits of Jupiter and Saturn hat effectively deflected asteroids to take an elliptical orbit in which they would intercept other planets around the sun such as the moon

Question 28

Why would the LHB have made it difficult for life to survive?

Answer 28

The bombardment would have evaporated a lot of the totality of life

Question 29

When were the oldest sedimentary rocks found so far dated back to and where were they found?

Answer 29

~3.8Ga in Northern Canada

Question 30

What unique feature was present in the sedimentary rock record around 3.8Ga (the oldest sedimentary rock record)?

Answer 30

Striping of what appeared to be oxidised and non-oxidised rock?

Question 31

What did the striping of non-oxidised and oxidised rocks indicate?

Answer 31

There must have been a large increase in the atmospheric concentration of oxygen which leads us to think that there may have been photosynthesis taking place on earth and therefore that plant life existed

Question 32

What is the counter argument to the possibility of life found in the Canadian sedimentary rock record?

Answer 32

The oxygen may not have been produced by the biosphere because there are no micro-structures within the rock that suggest the existence of life at that time

Question 33

What did scientists in Greenland find in an outcrop of rock? What did this POSSIBLY mean?

Answer 33

A dark layer of organic carbon - this suggested presence of life because this is highly produced by plants, but there is a potential it may have been caused by other chemical methods

Question 34

What was found by looking down the microscope at the organic carbon band found in Greenland?

Answer 34

It was in the form of graphite which suggested that there were organisms present.

Question 35

What was found by looking at the carbon isotope analysis of the organic carbon found in Greenland?

Answer 35

There was an expected stronger concentration of the Carbon 12 isotope compared to the heavier Carbon 13 isotope. This is the preferred isotope by plants which suggests photosynthesis was happening and therefore plants were present

Question 36

What are stromatolites?

Answer 36

microstructures that are thought to be created by microorganism communities through excretion and trapping sediment

Question 37

How old were the first stromatolites dated back to ?

Answer 37

3.5Ga

Question 38

What are stromatolites an indication of?

Answer 38

Life

Question 39

What was also found to have possibly existed at the same time as the early stromatolites?

Answer 39

Fossils of cells

Question 40

Who were two key figures in the debate about whether the cell fossils found at the same time as stromatolites were actually fossils of cells?

Answer 40

Schopf - in favour

Brasier et al. - against (non-biologically derived)

Question 41

What evidence of life was found in Swaziland in South Africa and when was it dated back to?

Answer 41

Capturing of cell division process around 3.26 Ga. This is widely accepted as strong evidence of life

Question 42

What was the earliest and latest time from our evidence that life existed on earth?

Answer 42

Earliest= 4.4Ga (Zircons)
Latest = 3.26 Ga (Cell division)

Question 43

Regardless of when life did actually appear on earth, what is so amazing? What does this suggest?

Answer 43

That life was able to emerge so soon after the earth’s formation. This suggests that it is relatively easy for it to occur and so could have easily done so at another planet

Question 44

What is the “Transpermia” idea?

Answer 44

that life was exported here via asteroids and material from other planets so it must have survived the interplanetary journey and been able to end up and survive on earth

Question 45

What evidence found supports the idea of transpermia?

Answer 45

asteroid found in Antarctica that was known to come from another planet. It contained bacteria fossils in it which means it could have brought life here.

Question 46

What is the deep hot biosphere idea?

Answer 46

Bacteria may have emerged from underneath the earth’s surface.

Question 47

What evidence is there for the deep hot biosphere idea?

Answer 47

Life would have been able to survive the LHB if it lived here, and today bacteria can be found at depths of ~3km and may even exceed the bacteria numbers found above ground.