L5 - A Young Earth Fit For Early Life

Question 1

What eon did life emerge?

Answer 1

• The Archean: meaning “beginning, origin”
• Liquid water was prevalent
• Onset of plate tectonics
• Our first evidence of life dates back to the Archean

Question 2

What was the atmosphere of the planet at this time?

Answer 2

• Reducing atmosphere of methane, ammonia, other gases, which would be toxic to most life on our planet today.
• Anoxic: no free (breathable) oxygen was present
• No ozone layer (O3) layer to shield Earth from UV radiation, and other solar and cosmic radiation
• Higher rates of meteor bombardment than today.

Question 3

How much of Earth’s life history was entirely microbial?

Answer 3

90%

Question 4

What is a biosignature?

Answer 4

• A clear sign that life was there
• A specific molecule, compound, an isotopic signature

Question 5

What are the three types of biosignatures?

Answer 5

1. Fossils
2. Chemical fossils (compound)
3. Isotopic signatures

Question 6

What are fossils?

Answer 6

• Remains/traces of ancient life that have been preserved by natural processes
• Sea shells, imprints of microbes, large skeletons
  
  • Old fossils are mostly imprints of microbes (life with skeletons didn’t exist at the time)

Question 7

What are chemical fossils - molecular biomarkers?

Answer 7

• Natural compounds that can be traced to a particular biological origin
• Effective biomarkers are compounds with specific biological sources, whose structures can be preserved through geologic time
• Lipids are the most stable macromolecules (can remain in environment for millions-billions of years)
• Unstable Isotopes

Question 8

How do lipids work as biomarkers?

Answer 8

• Different types of organisms make different types of lipids
  
  • Lipids preserved in sedimentary rocks offer insights into Earth’s history
  • We’ve characterized enough of these compounds to make inferences of the presence of their biological source at different points in history

Question 9

How do unstable and stable isotopes work as biomarkers?

Answer 9

• Unstable isotopes tell us the age of a compound (radiometric dating)
• Stable isotopes tell us about the source (how that molecule was made)
  
  • Different processes have different stable carbon isotopic signatures (how did that compound get there and who made it?)
• C12 & C13 are stable; C14 are unstable

Question 10

What evidence do we have for old microbial life?

Answer 10

• Their chemical or isotopic signatures will be found in the oldest rocks on Earth

Question 11

Some of the oldest rocks on Earth in Isua, Greenland (3.8 Ga)

Answer 11

• Metamorphic rocks: have undergone a lot of heat and pressure
• Have graphite (very very very pure carbon compound) in them
  
  • Alone, graphite is not a biosignature - won’t tell us if life was there as it can form by cooking organic matter
  • BUT: graphite can keep most of the original stable carbon isotopes
• The isotopic signatures in Isua suggested that this carbon originated from possibly photo-autotropic microorganisms (LIFE)
  
  • OOPS DISPROVED IN 2002

Question 12

First fossilized cells in Apex Chert, Australia (3.5 Ga)??

Answer 12

• One of the few places on the planet where geological evidence of early Earth has been preserved
  
  • It has not been subjected to geological processes (e.g., burial and extreme heating due to tectonic activity) that would have destroyed the isotopic/chemical biosignatures
• Schopf (1993): Found little filaments within the Apex Chert that looked like certain bacteria and declared it new evidence of life
  
  • Arguments that these are not fossils, but actually minerals
  • Schopf (2018): Analyzed microfossils with new carbon isotope analyses (SIMS) which proved that they were consistent with life

Question 13

WHAT ARE STROMATOLITES???

Answer 13

• Least controversial evidence of early life
• Very distinct looking rock-like clay structures made up of layers of bacteria and sediment
• Found in shoreline (wet environments)
• Imagine a rock-croissant
• Found throughout the Archean, more common later in the eon

Question 14

How are stromatolites created?

Answer 14

• Mucus is secreted by bacteria and collects, binds, cements, grains of sediments
• Microbial mats (biofilms): these grains and cells become stuck together along with calcium carbonate
• Very slow growing: 1cm takes 1000 years
• Stromatolites were actually the first micro-scale ecosystems

Question 15

What are the oldest accepted stromatolites?

Answer 15

• Warrawoona Group, Pilbara Craton (Western Australia) 3.5 Ga

Question 16

Where can we find living stromatolites?

Answer 16

• Hypersaline lakes and marine environments where the extreme salt level prevents animals such as snails from grazing on them
  
  • Shark Bay, Australia
  • Exuma Cays, Bahamas

Question 17

What kind of bacterial make modern stromatolites?

Answer 17

• Cyanobacteria: photosynthetic (obtain energy through photosynthesis) autotroph (makes its own food)
• Found in almost every terrestrial and aquatic habitat

Question 18

What kind of bacteria made ANCIENT stromatolites?

Answer 18

• Anoxygenic phototrophs

Question 19

What is an anoxygenic phototroph?

Answer 19

• Photosynthesis without the production of oxygen
• Use CO2 and light energy
  
  • BUT: they don’t use water an an electron donor (sulfide, iron, hydrogen instead)

Question 20

Examples of early anoxygenic phototrophs?

Answer 20

• Purple sulfur bacteria
• Green sulfur bacteria
• Purple-non sulfur bacteria
  
  • These purple anoxygenic phototrophs that comprise the microbial mats coated the coastlines of the Archean: making it look very purple !!!

Question 21

What kinds of bacteria existed during this time?

Answer 21

• Sulfate-reducing microbes:
  
  • Heterotrophs
  • Live in anaerobic environments (little to no oxygen)
  • Use sulfate as an electron acceptor instead of oxygen

Question 22

How did oxygenic photosynthesis emerge?

Answer 22

• (Uses water, CO2, sunlight and produces glucose, O2 and water)
• Great oxidation event: cyanobacteria (oxygenic, phototrophic microorganisms) induce detectable O2 into the atmosphere through simply existing

Question 23

Why did it take so long for oxygen to build up?

Answer 23

• Great Oxidation Event dated at ~2.5 Ga
• Cyanobacteria dated at ~2.8 Ga \
  
  • Explanation: many sinks for oxygen
    
    • Microorganisms learned to use oxygen that was pumped out by cyanobacteria
    • Banded Iron formations
    • Red beds

Question 24

What are banded iron formations?

Answer 24

• Alternating layers of iron-rich material
• BIFs are abundant in Proterozoic rocks (1.8-2.5 Ga) on all continents
• Thought to have formed through the capture of oxygen (released by Cyanobacteria activity) by iron dissolved in ancient ocean water
  
  • Banded structure thought to occur from fluctuating densities (blooms?) of cyanobacteria in the ocean
    
    • Seasonal fluctuations, storm surges?

Question 25

What are red beds?

Answer 25

- Form when iron is weathered out of rock in the presence of oxygen - Red because of ferric oxides (oxygen and iron interaction) - Earliest ones date back to 2.2 Ga - BIFs and red beds overlap for several million years indicating low levels of atmospheric oxygen at the time

Question 26

Oxygenating Earth's Oceans and Atmospheres?

Answer 26

- By 2 Ga, enough O2 accumulated in atmosphere where anaerobic organisms would have had to find a new niche or learn to use oxygen - O2 is toxic to some anaerobic organisms (METHANOGENS DIE UPON CONTACT TO O2)

Question 27

Formation of the ozone layer?

Answer 27

- Protection from UV radiation - Allow life to expand to regions at and near the Earth's surface

Question 28

Great Oxygenation Event = driver of an increase in diversity?

Answer 28

Towards the end of the Great Oxidation Event, we see an increase in diverse microfossils of bacteria in sedimentary rock, they become more common

Question 29

What is a Eukaryote?

Answer 29

- Membrane bound organelles that prokaryotes do not - DNA w/in membrane-bound nucleus - Cytoskeleton - Complex organelles: contain their own DNA and replicate independently (mitochondria and chloroplasts)

Question 30

What is the first evidence of Eukaryote presence?

Answer 30

- For the first 2.6 Ga years of the planet, prokaryotes (simple-celled microbes) ruled the world - 2.7 Ga: First chemical fossils of Eukaryotes, in Australia (NOT ACTUALLY) - Biomarker for Eukaryotes: Chloestane - A lipid that ONLY EUKARYOTES produce - Not abiotically generated - OOPS actually contamination - ACTUAL ACCEPTED FIRST EVIDENCE ~ 1.9 Ga - Grypania - Gunflint Chert, Minnesota & Ontario - Microfossils (not isotopic/chemical signature)