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Lecture

BMEN 515 Lecture Notes - Radionuclide, Continental Drift, Cyanobacteria


Department
Biomedical Engineering
Course Code
BMEN 515
Professor
William Huddleston

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Radioisotopes provide a way to date rocks
Half of the remaining radioactive material of the radioisotope decays
Radioisotope dating have been expanded and refined
The decay of potassium-40 to argon-40 has been used to date most of the ancient events in the evolution of life.
Fossils in the adjacent sedimentary rock that are similar to those in other rocks of known ages provide additional clues.
Radioisotopes dating of rocks, combined with fossil analysis, are the most powerful method of determining geological
age.
How Have Earth’s Continents and Climates Changed over Time?
Earth’s crust consists of a number of solid plates approximately 40km thick, collectively make up the lithosphere. It
floats on a fluid layer of molten rock (magma)
Magma circulates b/c heat produced by radioactive decay deep in Earth’s core sets up convection currents in the fluid.
The plates move b/c magma rises and exerts tremendous pressure.
Where plates are pushed together, either they move sideways past each other, or one plate slides under the other,
pushing up mountain ranges and carving deep rift valleys (underwater known as trenches).
Where plates are pushed apart, ocean basins may form between them.
The movement of the lithospheric plates and the continents they contain is known as continental drift.
Oxygen has steadily increased in Earth’s atmosphere
Increase in atmospheric oxygen has been largely unidirectional.
The atmosphere of early Earth probably contained little or no free oxygen gas.
The increase in atmospheric oxygen came in two big steps; the first step occurred 2.4 billion years ago, when certain
bacteria evolved the ability to use water as the source of hydrogen ions for photosynthesis.
One group of oxygen-generating bacteria, cyanobacteria, formed rocklike structures called stromatolites.
Cyanobacteria liberated enough O2 to open the way for the evolution of oxidation reactions as the energy source for the
synthesis of ATP.
When it first appeared, oxygen was poisonous to anaerobic prokaryotes that inhabited Earth.
Those prokaryotes evolved the ability to metabolize O2 survived and gained numerous advantages: aerobic respiration
proceeds at more rapid rates and harvests energy more efficiently.
An atmosphere rich in O2 also made possible larger cells and more complex organisms.
In contrast to this largely unidirectional change in atmospheric O2 concentration, most physical conditions on Earth have
oscillated in response to the planet’s internal processes.
Earth’s climate has shifted between hot/humid and cold/dry conditions
Earth’s climate was considerably warmer than it is today, and temperatures decreased more gradually towards the
poles.
At other times, Earth was colder than it is today; large areas were covered with glaciers during the end of the
Precambrian and during parts of the Carboniferous and Permian periods.
For Earth to be in a cold, dry state, atmospheric CO2 levels had to have been unusually low.
Volcanoes have occasionally changed the history of life
A few very large volcanic eruptions have had major consequences for life.
The collision of continents during the Permian period (275 mya) to form a single, gigantic land mass, Pangaea, caused
massive volcanic eruptions.
The ash ejected by volcanoes into the atmosphere reduced the penetration of sunlight to Earth’s surface, lowering
temperatures, reducing photosynthesis, and triggering massive glaciations.
Extraterrestrial events have triggered changes on Earth
A meteorite caused or contributed to a mass extinction at the end of the Cretaceous period (65 mya).
o The first clue came from abnormally high concentrations of the element iridium in a thin layer separating rocks
deposited during the Cretaceous from those deposited during the Tertiary.
o Iridium is abundant in some meteorites; rare on Earth’s surfaces.
What Are the Major Events in Life’s History?
Life first evolved on Earth about 3.8 billion years ago.
By about 1.5 billion years ago, eukaryotic organisms had evolved.
The fossil record of organisms that lived prior to 550 million year ago is fragmentary; good enough to show that the
total number of species and individuals increased dramatically in late Precambrian times.
Several processes contribute to the paucity of fossils
Only a tiny fraction of organisms ever become fossils.
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