orgins

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The Earth formed

about 4.6 billion years ago.

Conditions on early Earth

made the origin of life possible

• _Hot

• _{Thick with water vapor}

• _{No Oxygen}

• _Lightning

• _{Volcanic action}

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First life forms

– bacteria about 3.8 billion

years ago

• _{The earliest evidence for life on Earth comes from}

3.5-billion-year-old fossils of

stromatolites

, built by ancient

photosynthetic prokaryotes still alive today.

• _{Because these 3.5-billion-year-old prokaryotes used}

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The first life may have evolved

through four stages.

1. The abiotic (nonliving) synthesis of small organic

molecules such as amino acids and nitrogenous bases.

2. The joining of these small molecules into polymers, such as proteins and nucleic acids.

3. The packaging of these molecules into “protocells,”

membrane-enclosed droplets that maintained an internal chemistry different from that of their surroundings.

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Oparin & Haldane (1920s)

• _{In the 1920s, the Russian chemist A. I. Oparin and the}

British scientist J. B. S. Haldane independently proposed that conditions on early Earth could have generated

organic molecules.

• _{Early earth was much different from our modern}

atmosphere is rich in O₂ (which disrupts chemical bonds).

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Miller & Urey (1950s)

• _{In 1953, graduate student Stanley Miller, working under}

Harold Urey, tested the Oparin-Haldane hypothesis.

• _{Experiments show that the abiotic synthesis of organic}

molecules is possible (under conditions of early earth)

• _{Miller identified a variety of organic molecules that are}

common in organisms, including

• _{hydrocarbons (long chains of carbon and hydrogen) and}

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Figure 15.2

Sparks simulating lightning

1 2 3 4 Water vapor “Atmosphere” Electrode Condenser Cold water “Sea” Sample for chemical analysis NH₃ CH₄ H₂

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Macroevolution

is the broad pattern of changes in life on Earth.

The entire 4.6 billion years of Earth’s

history can be divided into three eons

of geologic time.

• _{The Archaean and Proterozoic eons lasted}

about 4 billion years.

• _{The Phanerozoic eon includes the last half}

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Figure 15.4-0 Hadean eon Archean eon Proterozoic eon Phanerozoic eon Origin

of Earth Prokaryotes

Atmospheric oxygen Single-celled eukaryotes Multicellular eukaryotes Animals Colonization of land Present Billions of years ago

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Prokaryotes lived alone on Earth for 1.5 billion years, from 3.5 to 2 billion years ago.

• Bacteria were the first life forms

• _{During this time, prokaryotes transformed the atmosphere.}

• _{Prokaryotic photosynthesis produced oxygen that enriched}

the water and atmosphere of Earth.

• _{Aerobic cellular respiration allowed prokaryotes to flourish.}

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Eukaryotes = true nucleus and

membrane organelles

• _{The oldest fossils of eukaryotes are about 1.8 billion}

years old.

• _{The common ancestor of all multicellular eukaryotes}

lived about 1.5 billion years ago.

• _{The oldest fossils of multicellular eukaryotes are about}

1.2 billion years old.

• _{The first multicellular plants and fungi began to colonize}

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The actual ages of rocks and fossils

mark geologic time

• _{Radiometric dating}_{is based on the decay of radioactive isotopes.}

• _{The rate of decay is expressed as a half-life, the time required for 50%}

of the isotope in a sample to decay.

• _{Carbon-14 is useful for dating relatively young fossils—up to about}

75,000 years old. Radioactive isotopes with longer half-lives are used to date older fossils.

• _{A fossil’s age can be inferred from the ages of the rock layers above}

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Figure 15.5

Time (thousands of years)

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15.6 The fossil record documents the history of life

• _The_{geologic record}_{is based on the sequence and ages of fossils in}

the rock strata.

• _{The most recent Phanerozoic eon}

• _{includes the past 542 million years and}

• _{is divided into three eras:}

• Paleozoic,

• Mesozoic, and

• Cenozoic.

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