Transcript Presentation

History of Life on Earth
Chapter 6
Evidence of the Past
How do fossils form?
The heat and pressure from being buried in sediment can sometimes cause the tissues of organisms to release hydrogen and
oxygen, leaving behind a residue of carbon. This process — which is called carbonization, or distillation — yields a detailed
carbon impression of the dead organism in sedimentary rock.
The most common method of fossilization is called permineralization, or petrification. After an organism's soft tissues decay in
sediment, the hard parts — particularly the bones — are left behind. Water seeps into the remains, and minerals dissolved in
the water seep into the spaces within the remains, where they form crystals. These crystallized minerals cause the
remains to harden along with the encasing sedimentary rock.
In another fossilization process, called replacement, the minerals in groundwater replace the minerals that make up the bodily
remains after the water completely dissolves the original hard parts of the organism.
Fossils also form from molds and casts. If an organism completely dissolves in sedimentary rock, it can leave an impression of its
exterior in the rock, called an external mold. If that mold gets filled with other minerals, it becomes a cast. An internal mold
forms when sediments or minerals fill the internal cavity of an organism, such as a shell or skull, and the remains
dissolve.
http://www.fossils-facts-and-finds.com/how_are_fossils_formed.html
can past
we estimate
Evidence ofHow
the
the age of a fossil?
http://www.nature.com/scitable/knowledge/library/
dating-rocks-and-fossils-using-geologic-methods107924044
Radiometric Dating (Absolute Dating)
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Geologists use radiometric dating to estimate how long ago rocks
formed, and to infer the ages of fossils contained within those rocks.
Radioactive elements decay.
The universe is full of naturally occurring radioactive elements.
Radioactive atoms are inherently unstable; over time, radioactive
“parent atoms” decay into stable “daughter atoms.”
When molten rock cools, forming what are called igneous rocks,
radioactive atoms are trapped inside. Afterwards, they decay at a
predictable rate. By measuring the quantity of unstable atoms left in a
rock and comparing it to the quantity of stable daughter atoms in the
rock, scientists can estimate the amount of time that has passed since
that rock formed.
Bracketing the fossils
Fossils are generally found in sedimentary rock—not igneous rock.
Sedimentary rocks can be dated using radioactive carbon, but because
carbon decays relatively quickly, this only works for rocks younger
than about 50 thousand years.
So in order to date most older fossils, scientists look for layers of
igneous rock or volcanic ash above and below the fossil. Scientists
date igneous rock using elements that are slow to decay, such as
uranium and potassium. By dating these surrounding layers, they can
geological time scale
Geologic Time Scale
The standard method used to divide the Earth’s
long natural history into parts.
In our text, time is divided into eras and eras
are divided into subgroups called periods.
Mass extinctions, periods when many species
suddenly become extinct, have occurred
several times throughout Earth’s history.
Scientists do not always know exactly why or
how these happened. The disappearance of
dinosaurs is an example of this.
There was a supercontinent on Earth called
Pangea that moved due to plate tectonics.
Plate tectonics is the theory that explains how
the Earth’s outermost layer, called tectonic
plates, move and change shape. This is a very
gradual process.
precambrian era
570-4600 MYA- Precambrian time covers
the vast bulk of the Earth's history,
starting with the planet's creation about
4.5 billion years ago and ending with the
emergence of complex, multicelled lifeforms almost four billion years later. Early
Earth’s atmosphere did not contain much
oxygen. Therefore, the earliest lifeforms
did not need oxygen . . . these were
single-celled prokaryotes. There is
evidence that cyanobacteria, a type of
photosynthetic bacteria, formed about 3
billion years ago. These helped form a
new layer of gas in the atmosphere called
the ozone and put oxygen into the
atmosphere. The ozone allowed life to
move out of the Earth’s ocean and onto
land. About a billion years after bacteria
formed, multicellular organisms formed.
paleozoic era
245-570 MYA- Paleozoic
mean “ancient life” in greek.
Period of time where life
emerged on the land. Plants,
fungi, and air-breathing
animals colonized land (only
plants that did not appear
during this era were flowering
plants). Included ages of
invertebrates, fishes, and
amphibians. Three mass
extinctions present during
this era. There were six
periods during this era.
mesozoic era
65.5-245 MYA- Pangea cracks
(splits into Laurasia &
Gondwanna), Dinosaurs
appear, first mammals appear,
first birds appear. End of this
period marked with mass
extinction (extinction of
dinosaurs). Flowering plants
appear and Earth starts to look
closer to present-day Earth.
Made up of three periods
cenozoic era
65.5 MYA to present- First large
mammals appear and become dominant,
known as “age of the mammals.” Means
“recent life” in Greek. One mass
extinction during this era. Climate has
changed many times during this period,
two ice ages.
crash course!
https://www.youtube.com/watch?v=92oHNd8v
Fwo&list=PL8dPuuaLjXtMczXZUmjb3mZSU1R
oxnrey&index=5