Lecture Twenty Four - Pre-Cambrian Life Flashcards
Describe the archaean Earth.
3.8 billion years ago.
Atmosphere made more H2, H2O, CO2, CO, methane and ammonia.
Describe photosynthesis as life began to form on Earth.
UV light damages replication, therefore life cannot exist in shallow seas near land.
CO2 (volcanic) + bacteria (photosynthetic) = O2 + more bacteria.
The O2 in the atmosphere + UV light = O3 (ozone).
Ozone layer = protected cells which can colonise shallow seas.
What evidence is there fore photosynthesis in early life on Earth.
BIFs = Banded Iron Formations.
Red layers = O2 = Summer.
Grey layers = no O2 = Winter.
Over the next 2.2 billion years the oxygen content in the atmosphere stabilises until we have a constant O2 presence in the atmosphere.
Evidence:
Red beds from 1.6 by a to present day.
What is the earliest evidence for life?
Stromatolites = not fossils, but organosedimentary structures.
Sediment layers trapped by bacterial slimes.
Stromatolites sometimes contain cells.
How can you tell whether a rock is made from organic origins?
Using carbon isotope ratios.
Biochemical pathways use C 12 in preference to C 13, so in our bodies, we have less C 13 than the atmospheres concentration.
Look at the ration of C 13: C12. (δ C 13)
This tells us how different the sample is to the atmospheric ratio.
If the δ C 13 = 0), the C 13 is the same as in the atmosphere.
If the δ C 13 positive, more C 13 than in the atmosphere.
If the δ C 13 is negative, there is less C 13 than in the atmosphere = LIFE.
When were replicating chemicals and membrane bound cells on Earth.
Replicating chemicals - 4.2-3.9 billion years ago.
Membrane bounded cells - 3.8 billion years ago.
Prokaryotic - no nucleus, nuclear material free in cell, then
Eukaryotic - nuclear material in a membrane.
Over one billion years ago, cells were dividing asexually.
By one billion years, cells learnt to reproduce sexually = more variation.
Describe the multicellular life in the Archean and early Protozoic eras.
Soft bodies are the main body type in these periods - fossilisation chance is very very low.
Ideal conditions for fossilisation are fain grained sediments on mud flats at the seas edge.
Where and by whom was the first discovery of muti-cellular life as fossils made?
First discovery of multi-cellular life as fossils from the Eduacara period (560 Ma).
Discovered in 1946.
Why was there such change in organisms ager 2 billion years of life on Earth?
Unicellular life changed to multicellular life after ~ 2 billion years.
Two hypothesis of what triggered the change:
- Snow Ball Earth.
- Acraman Impact.
What is the Snow Ball Earth Hypothesis?
Evidence includes:
Rocks deposited on shot of ancient ocean in Namibia, Africa.
Layer of carbonate (limestone), over tills (glaciers), over carbonates.
Looking at carbon isotope ratios in carbonates: Lower carbonates (under glacial tills) are high in C 13 = high in life - photosynthetic activity. As you approach the tills, C 12 level increases = lower in life as global temperature drops. After glacial layer, high C 12 level initially, the gradual increase in C 13 = gradual increase in life again as global temperature rises.
During the Snowball Earth, only a small number of single celled life survived.
Life slowly recovered after the ice age.
What cooled the Earth?
- Life reducing the CO2 levels.
What warmed the Earth?
- Volcanoes producing CO2, thus creating a greenhouse effect.
What is the pre-cambiran - cambrian boundary?
542 mya.
Just after the Ice Age ended.
