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Earth History.\_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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Earth History

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

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 Geologic

Time Scale

Placing a boundary -

The Cretaceous-Tertiary Boundary

65 Ma

Rocks of the Cretaceous Period (144 to 65 million

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

marine-based animals.

Rocks record the event

An asteroid measuring 6 to 12 miles in diameter struck the Yucatan Peninsula.

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.  

"The Shiva Hypothesis" describes a

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.

“Nemesis Hypothesis”

Nicholas Steno (Danish) 1638-1686

Was the first to suggest that “figured

stones” looked like living organisms

and had in fact once been living.

James Hutton (Scottish) 1726-1797

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

from others.

He is famous for his quote that Earth has

“…no vestige of a beginning – no prospect

of an end.”

Perhaps Hutton’s most lasting contribution to the science of geology is

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.”

Charles Darwin (1809-1882) provided

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


Fossils are the remains of animals and plants, or the record of their presence,

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.

Darwin observed that all living things

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.

Evidence of


Selective pressure –

Environmental changes from

forests to grasslands across Europe

and North America



-speed and strength

-brain size

-lengthened jaws


ancestral lineage

Similarity in bone structure possessed

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?

Vestigial structures- the

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?

Embryology- The early stage

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

individual species.

A virus can evolve faster than the

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.