Earth's Early History

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Fossils and Ancient Life

What is the fossil record?

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Fossils and Ancient Life

The fossil record provides evidence about
the history of life on Earth. It also shows
how different groups of organisms,
including species, have changed over
time.

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Fossils and Ancient Life

The fossil record provides incomplete information
about the history of life.

Over 99% of all species that have lived on Earth
have become extinct, which means that the
species has died out.

Those species that still exist are referred to as
being extant.

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How Fossils Form

How Fossils Form

Most fossils form in sedimentary rock.

Sedimentary rock forms when exposure to the
elements breaks down existing rock into small
particles of sand, silt, and clay.

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How Fossils Form

Fossil Formation

Water carries small rock
particles to lakes and
seas.

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How Fossils Form

Dead organisms are buried by
layers of sediment, which
forms new rock.

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How Fossils Form

The preserved remains may
be later discovered and
studied.

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Interpreting Fossil Evidence

Interpreting Fossil Evidence

Paleontologists determine the age of fossils using
relative dating or radioactive dating.

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Interpreting Fossil Evidence

Relative Dating

In relative dating, the age of a fossil is determined
by comparing its placement with that of fossils in
other layers of rock.

Rock layers form in order by age—the oldest on
the bottom, with more recent layers on top.

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Interpreting Fossil Evidence

Relative Dating

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Interpreting Fossil Evidence

Index fossils are used to compare the relative ages
of fossils.

An index fossil is a species that is recognizable
and that existed for a short period but had a wide
geographic range.

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Interpreting Fossil Evidence

Radioactive Dating

Scientists use radioactive decay to assign an
absolute age to rocks.

Radioactive dating is the use of half-lives to
determine the age of a sample.

A half-life is the length of time required for half of
the radioactive atoms in a sample to decay.

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Interpreting Fossil Evidence

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Geologic Time Scale

The basic divisions of the geologic time
scale are eras and periods.

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17-2 Earth's Early History

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Origin of Eukaryotic Cells

About 2 billion years ago, prokaryotic cells began
evolving internal cell membranes.

The result was the ancestor of all eukaryotic cells.

According to the endosymbiotic theory, eukaryotic
cells formed from a symbiosis among several
different prokaryotes.

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Formation of Earth

Formation of Earth

Hypotheses about Earth’s early history are based on
a relatively small amount of evidence.

Gaps and uncertainties make it likely that scientific
ideas about the origin of life will change.

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Formation of Earth

Evidence shows that Earth was not “born” in a
single event.

Pieces of cosmic debris were probably attracted to
one another over the course of 100 million years.

While Earth was young, it was struck by one or
more objects, producing enough heat to melt the
entire globe.

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Formation of Earth

Once Earth melted, its elements rearranged
themselves according to density.

The most dense elements formed the planet’s core.

Moderately dense elements floated to the surface,
cooled, and formed a solid crust.

The least dense elements formed the first
atmosphere.

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What substances made up Earth's early
atmosphere?

Formation of Earth

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Formation of Earth

Earth's early atmosphere probably
contained hydrogen cyanide, carbon
dioxide, carbon monoxide, nitrogen,
hydrogen sulfide, and water.

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Formation of Earth

Scientists infer that about four billion years ago, Earth
cooled and solid rocks formed on its surface.

Millions of years later, volcanic activity shook Earth’s
crust.

About 3.8 billion years ago, Earth’s surface cooled
enough for water to remain a liquid, and oceans
covered much of the surface.

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The Puzzle of Life's Origin

The Puzzle of Life's Origin

Evidence suggests that 200–300 million years
after Earth had liquid water, cells similar to modern
bacteria were common.

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The Puzzle of Life's Origin

Evolution of RNA and DNA

How could DNA and RNA have evolved? Several
hypotheses suggest:

•Some RNA sequences can help DNA replicate
under the right conditions.

•Some RNA molecules can even grow and
duplicate themselves suggesting RNA might
have existed before DNA.

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Free Oxygen

About 2.2 billion years ago, photosynthetic bacteria
began to pump oxygen into the oceans.

Next, oxygen gas accumulated in the atmosphere.

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Free Oxygen

What occurred when oxygen was added to
Earth's atmosphere?

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Free Oxygen

The rise of oxygen in the atmosphere
drove some life forms to extinction, while
other life forms evolved new, more
efficient metabolic pathways that used
oxygen for respiration.