Earth History. http://soest.hawaii.edu/coasts/cgg\_main.html. Using relative dating and radiometric dating, geologists have pieced together A history of planet Earth….the calendar is the Geologic Time Scale. The Geologic Time Scale has irregular episodes (not like
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A history of planet Earth….the calendar is the Geologic Time Scale.
The Geologic Time Scale has irregular episodes (not like
the modern calendar of days, weeks and months).
1. Eons are the longest spans of time covering half a billion years or more.
There are only two eons, the Precambrian (characterized by only single-celled
organisms, or the total absence of life) and the Phanerozoic, meaning
“visible life” indicating that fossils became larger and easier to see.
2. Eras are shorter, covering hundreds of millions of years. The eras consist
of the Paleozoic or “old life” Era, the Mesozoic or “middle life” Era, and the
Cenozoic or “new life” Era.
3. Periods are the most common division of time, usually lasting several tens
of millions of years in duration.
4. Epochs are the shortest division, covering several million to thousands of
years in length.
The Cretaceous-Tertiary Boundary
years ago) are found beneath rocks from the
Tertiary Period (65 to 1.8 million years ago). At
many places in the world where these rocks
are found, the boundary of the two rock units is
marked by clay. Known to geologists as the
“K/T boundary”, this time is characterized by the
extinction of the giant lizards, the dinosaurs. The
clay often contains four special features: small
glass spherules, high concentrations of the element
Iridium, and minute crystals of shocked quartz,
the total extinction of numerous land and
Rocks record the event
The K/T impact resulted in 100 million megatons of energy released in dust, water
vapor, fire, shock waves, sound, and tsunami as well as excavating a crater over
200 kilometers across.
93% of marine reptiles and 56% of land-based reptiles became extinct, however,
non-avian dinosaurs and pterosaurs had 100% extinction. On land, nothing bigger than
25 kilograms survived. Survivors included; snakes, small Lizards, crocodiles, turtles,
salamanders, frogs, mammals and birds.
26 million year cycle of mass extinctions
over the past 540 million years.
One hypothesis is that this corresponds the
solar system oscillating through the
galactic plane as it orbits the Milky Way.
Rampino notes that the last crossing of the
galactic plane occurred a few million years
ago and it has been suggested that this
led to a disturbance of comets in the Oort
Cloud, some of which could now be
approaching the inner solar system.
Another theory holds that our Sun has a
companion star that returns on a regular
cycle and disrupts objects in the Oort Cloud
or the asteroid belt and leads to impacts.
Was the first to suggest that “figured
stones” looked like living organisms
and had in fact once been living.
Strong proponent that geologic processes
alter Earths surface.
Dominant view at the time was that Earth
was unchanging – all rocks had been
formed by precipitation or sedimentation
from a great ocean – called Neptunism.
Hutton argued that geologic time had been
indefinitely long and that Earth was like a
self-renewing machine, as mountains
eroded away, new ones were uplifted,
as the sea covered some lands it receded
He is famous for his quote that Earth has
“…no vestige of a beginning – no prospect
of an end.”
the concept of Uniformitarianism which states that the past history of
Earth is best explained by our observation of modern processes. That is,
geologic principles have been uniform over time.
Sir Archibald Geike (1835-1924) famously summed up uniformitarianism as
“the present is the key to the past.”
the general theory that accounted for
changes seen in the fossil record, the
Theory of Evolution.
His Theory of Evolution, described in the
Origin of Species (1859) and The
Descent of Man (1871), stated that
all living things developed from very
few simple forms through the process
of natural selection. Natural selection
is the tendency for certain populations
of the same species to have the best
chance of surviving and transmitting
their genes to the next generation
because they possess favorable
preserved in the rocks of Earth. Fossilization is the process that turns a once
living thing into a fossil. There are lots of fossils to be found, but only a tiny
number of all the animals and plants that ever lived have been fossilized.
Skin of Dinosaur embryo - 1989
Take a look at your body. Which parts are most likely to become fossilized?
Teeth, hard bones, and nails are the obvious candidates. The same is true of
other animals. The occasional discovery of fossils with soft parts is rare and
exciting – but in most cases it is hard parts that are preserved.
reproduced at high rates and yet no one
group of organisms had been able to
overwhelm Earth’s surface
(other than cockroaches!). In fact, the
actual size of any population tends to
remain fairly constant over time. This
led Darwin to conclude that not all
individuals in a generation will
survive – hence nature must
select those with favorable
variations. Natural selection
was the mechanism he proposed
by which evolution occurred.
Selective pressure –
Environmental changes from
forests to grasslands across Europe
and North America
-speed and strength
by diverse organisms as birds,
swimming mammals, four legged
animals, humans and insect-eating
reptiles. Homologous structures
suggests these animals have evolved
from a common ancestor and that
survival pressure (natural selection) has
preferentially selected for the specific
functions served by each limb.
In four-limbed vertebrates, limb bones
may vary in size and shape but they are
reproductions of one another in terms
of the number and position of specific
bones. How would totally different families
of organisms, under specific and
fundamentally unique selective
pressures, tend to converge on a single
basic blueprint for limb design
unless a common ancestor was involved?
result of evolution under
selective pressure. Vestigial
structures are the “vestiges” of
body parts that had been used by
ancestral forms but are now
Whales possess a vestigial
pelvis and femur originally
designed for walking. Proof
that modern whales have
evolved from walking ancestors
was found in 1994 in the
form of a fossil whale with
front legs designed as flippers
and long hind limbs with
elongate toes for webbed feet.
The boa constrictor also displays
vestiges of legs.
How many vestigial structures do you have?
of development of embryos of
various fish, birds and mammals
display strikingly similar
characteristics. Human embryos
have tails, and gill slits as do
other live forms in their early
stages. It is thought that all
these animals inherited basic
sets of genes from distant
common ancestors that control
early embryologic development.
Later, as development
progresses, other genes
assume control and lead to
medical community is able to design
medicines to fight it. In scientific
parlance, the virus "escapes" drug
Aids evolves…AIDS is caused by a virus called HIV, Human Immunodeficiency
virus. A virus is a living microscopic organism that lives in a cell of another living
thing. Being alive, can evolve or change, and antibodies designed to attack one
form of the virus find themselves useless against the new form. This is why
vaccines have not succeeded in eliminating the common flu or AIDS for example.
Effective vaccines against particular strains of flu virus constantly need to be
updated for this reason.