14. Origin of Life Flashcards
When did the Earth start to form?
Earth started to form ~ 4.6 billion years
ago (bya)
When did Life begin on Earth?
Life began ~3.5 billion years ago
What is the Pre-Cambrian period?
The Pre-Cambrian period –
represents the period before advanced life
o Spans about 90% of Earth history, beginning with the formation of Earth 4.6 bya and ending 541 mya with the start of the Paleozoic era
o Makes up of the Hadean, Archeon (Archaean) and Proterozoic Eons
What is the Hadean eon? (4)
– The Hadean eon (~4.6 – 4.0 bya) represents the time before a reliable (fossil) record of life
– Temperatures are extremely hot, much of the Earth was molten because of frequent collisions with other bodies, extreme volcanism and the abundance of short-lived radioactive elements
– Early atmosphere contained almost no oxygen
– Ejected debris following giant impact collision (~4.5 bya) between early Earth and an ancient planet called Theia is thought to have formed the Moon
What is the Archeon Eon? (6)
- Duration = ~4.0 – 2.5 bya
- High Volcanic activity
- No Oxygen
- Life on Earth -> Cyanobacteria -> Stromatolites
- Prokaryotes Only (anaerobic)
- Formation of Continents and Oceans
What is the Proterozoic Eon? (4)
Duration = ~2.5 bya – 541 mya
– It spans for the time from appearance of oxygen in Earth’s atmosphere to just before the proliferation of complex life (such as corals) on the Earth
– Bacteria begin producing oxygen, leading to the sudden rise of life forms
– Eukaryotic cells emerge, including some forms of soft-bodied multicellular organisms
What are some Geological evidences linked to the evolution of atmospheric oxygen? (3)
– Stromatolites
– Banded iron formations
– Red beds
What are Stromatolites?
Stromatolites
– Significance = earliest fossils evidence of life on Earth (one of the most abundant fossils known from Pre-Cambrian)
– Oldest stromatolites are ~ 3.5 bya
– Form in shallow water
– They are column-shaped structures made up of layers of sedimentary rock, that are created by mats of cyanobacteria (also known as blue green algae) living on top of stromatolite
How are Stromatolites formed?
They are column-shaped structures made up of layers of sedimentary rock, that are created by mats of cyanobacteria (also known as blue-green algae) living on top of stromatolite
o Cyanobacteria continue to reproduce on the upper surfaces of mats as these cells are dependent on light for photosynthesis (this leads to generation of oxygen)
o Also, these bacteria secrete sticky mucus over them, which trap sediment
o As sediments begin to settle and accumulate on top of cyanobacteria, they start to block sunlight…so, cyanobacteria would slowly move to top of this sediment layer…the trapped sediment in older layers within stromatolite will harden into rock
What are Banded iron formations?
– Banded iron formation consists of layers of iron oxides [typically either magnetite (Fe3O4) or hematite (Fe2O3)] separated by layers of chert (silica-rich sedimentary rock), jasper or quartz
– Each layer is usually narrow (mm’s to few cm’s)
– The rock has a distinctively banded appearance because of differently colored lighter (chert, jasper, quartz) and darker iron rich layers
– Fairly heavy because they contain iron, but what is most impressive is how old they are – most are ~ 1.8 to 2.6 billion years old
How are Banded iron formations created?
(8)
- iron entered ocean as Fe2+ and Fe3+ ions
- At the same time cyanobacteria released O2 as a waste product of photosynthesis by
- It was soaked up by iron dissolved in oceans to form iron oxides, mainly hematite or magnetite
- These iron oxides settled to ocean floor
- Once dissolved iron ions were depleted in oceans, O2 levels build up in oceans; but early photosynthesizers (cyanobacteria) were poisoned by an overabundance of O2 in oceans
- As a result, O2 levels in ocean fell (during this period chert/jasper/quartz layers deposited on ocean floor)
- Cyanobacteria levels recovered to help increase O2 levels again by photosynthesis, which led to formation of iron oxides with more dissolved iron ions again that will form the next layer on top (and then the whole cycle repeats resulting in alternating layers involving iron oxides and chert/jasper/quartz)
- Eventually O2 in oceans reacted with all dissolved iron ions, so remaining O2 was then free to diffuse into air
What are Red Beds?
How are they formed?
– Like the banded iron formations before them, red beds are huge deposits of iron-rich sediment
– Once in air, O2 could react with iron in sediments on the earth’s surface, which produced rust-colored sedimentary rock known as red beds
– First appeared in geologic records ~2 bya
– Around 500 – 600 mya (towards end of the Proterozoic Eon), atmospheric O2 concentrations reached levels that are about the same as today
Explain how stromatolites, banded iron formations and red beds are geological evidence of free oxygen on early Earth during the Pre-Cambrian period [NOTE that 500 to 600 million years ago is the earliest time when the levels of oxygen in the atmosphere is the same as present day]
Explain how stromatolites led to the existence of banded iron formations and red beds
-
Stromatolites:
Stromatolites are layered structures formed by the growth of microbial communities, particularly cyanobacteria (blue-green algae), in shallow aquatic environments. These microorganisms performed photosynthesis, producing oxygen as a byproduct. While the atmospheric oxygen levels remained low, these early photosynthetic organisms began releasing oxygen into the oceans, leading to the formation of iron-rich minerals and stromatolites. The oxygen produced by these organisms was initially absorbed by dissolved iron in the oceans, preventing it from accumulating in the atmosphere. -
Banded Iron Formations (BIFs):
As the oxygen produced by early photosynthetic organisms started accumulating in the oceans, it began to react with dissolved iron ions. Oxygen reacts with iron to form iron oxides, which are insoluble and precipitate out of the water. This process created the distinctive banded iron formations, where alternating bands of iron-rich minerals and silica-rich minerals were deposited on the ocean floor. These formations indicate the gradual release of oxygen into the oceans and the removal of soluble iron, marking the period of transition from anoxic to oxygen-rich conditions. -
Red Beds:
Red beds, or sedimentary rock layers with a reddish hue, are also evidence of increasing atmospheric oxygen levels. The iron-rich minerals in these rocks underwent oxidation, giving rise to the red coloration. As oxygen levels in the atmosphere and oceans continued to rise, more iron was oxidized and deposited in sedimentary rocks. The presence of red beds signifies the ongoing interaction between oxygen and iron on the Earth’s surface, further supporting the transition to an oxygen-rich environment.
In summary, stromatolites were the first evidence of oxygen production by early photosynthetic organisms. The oxygen they released led to the formation of banded iron formations as oxygen reacted with dissolved iron in the oceans. Over time, as atmospheric oxygen levels increased, the oxidation of iron led to the formation of red beds, providing a sequence of geological evidence that chronicles the Earth’s transformation from an anoxic environment to one rich in oxygen during the Pre-Cambrian period.
What is the Phanerozoic Eon?
The Phanerozoic Eon –
represents the period of Earth’s history with advanced life forms
Spans the interval between ~541 mya and the present
Comprises of the Paleozoic, Mesozoic and Cenozoic Eras
What is the Paleozoic Era? (5)
- Longest period of the Phanerozoic Eon
- 542 to 251 mya
- By the Paleozoic, the composition of the air has reached something like what we breathe now: about 80% nitrogen, about 20% oxygen, and small amounts of carbon dioxide, water vapor, and other gases
- Cambrian Explosion
- Formation of the supercontinent Pangea occurred near end of this era – occurred ~ 299 mya
