L5. Origins: A young earth fit for early life Flashcards
Early Archean
- alien planet
- reducing atmosphere of toxic methane, ammonia, and other gases
- no breathable oxygen available
- no ozone layer to shield Earth from ultraviolet radiation
- high rates of meteor bombardment
What is a Biosignature?
A sign of life, any substance such as an element, isotope, or molecule that provides evidence of life
Different types:
- fossils
- chemical fossils
- isotopic signatures
Fossils (Micro and Macro)
remains or traces of ancient life that have been preserved by natural processes
chemical fossils: molecular biomarkers
natural products that can be traced to a particular biological origin. Specific biological sources preserve better over geologic time (lipids are the most the stable macromolecule)
Carbon Dating
Isotopic dating by measuring amount of carbon 12, 13, 14.
- only works for things that are up to 50,000 years old
- stable carbon isotopic signatures will reflect unique biological processes
Carbon dating and origin of life
3.8 Ga - Isotopic signatures in rock from Greenland suggest that this carbon originated possibly photo-autotrophic microorganisms
3.5 Ga - SIMS Analysis revealed stable carbon isotopic compositions confirmed microscopic fossils are the earliest direct evidence of life on Earth
3.5 Ga - May be fossil evidence of bacteria in rocks in Australia
3.7 Ga - Stromatolites from Greenland
Stromatolites
Rocklike structures made of cyanobacteria bacteria and sediment, found in shorelines. Created through the trapping, binding, and cementation of grains of sediment by microbial mats (other microorganisms living in mats, areas act as micro-scale ecosystems) Found throughout Archean rocks. Not controversial.
Early Anoxygenic Phototrophs
- Purple sulfur bacteria
- Green sulfur bacteria
- Purple nonsulfur bacteria
What other kinds of bacteria existed during this time?
Sulfate-reducing microbes
- heterotrophs
- didn’t need oxygen because used sulfate as an electron acceptor instead
Great Oxidation Event
- Biologically induced appearance of dioxygen in the Earth’s atmosphere through the activity of oxygenic phototrophic microorganisms (cyanobacteria)
- occurred slowly because oxygen sinks initially balanced production
- Once production exceeded consumption/sinks, O2 could accumulate in the atmosphere
Why did it take so long for oxygen to build up?
There are many sinks for oxygen so the O2 being produced was being balanced by sinks
Banded Iron formations (BIFs)
- Abundant in Proterozoic rocks ranging in age from 1.8 to 2.5 billion years old
- though to have formed through the capture of oxygen (from photosynthesis) by iron dissolved in ancient ocean water
- once nearly all the free iron was consumed in seawater, oxygen could gradually accumulate in the atmosphere, allowing an ozone layer to form
- banded structure comes from fluctuating densities of cyanobacteria in the ocean with periodic process maybe due to seasonal fluctuations or storm surges
Red beds
- form when iron is weathered out of rock in the presence of oxygen
- red in colour due to the presence of ferric oxides
Impact of O2 on life on Earth
- O2 is toxic to organism that don’t have protective mechanisms so many died.
- Some organisms developed means to use O2 in respiration to extract more energy from foods (aerobic respiration)
- helped the formation of the ozone layer. Provided protection from UV radiation and allowed life to expand to regions at and near Earth’s surface
- greater diversity
Emergence of Eukaryotes
- for the first billion years, prokaryotes ruled the world
- oldest eukaryotic chemical fossils: 2.7 Ga (not confirmed)
- oldest accepted eukaryote: 1.9 Ga found in rocks in Minnesota and Ontario
