Chapter 19

The History of Life on Earth

The Basic Chemicals of Life

• Oparin and Haldane hypothesized that the early earth contained hydrogen gas, water vapor, ammonia and methane.

• They thought the energy to make these compounds came from lightning, volcanic eruptions, and energy from the sun.

• They didn’t think the early atmosphere contained oxygen gas.

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Forming Complex Molecules

• In 1953, Stanley Miller and Harold Urey tested the hypothesis of the contents of the early atmosphere to see if organic molecules could be synthesized in such an environment.

Forming Complex Molecules

• Their experiment showed that such things as amino acids and fatty acids could be synthesized in such an environment.

• This experiment did not show how life began, it simply showed that simple building blocks can give rise to complex biomolecules when conditions are right.

Forming Complex Molecules

• Another hypothesis looks at the formation of complex molecules from another angle.

• Some researchers think that the first biological molecules arose around hydrothermal vents deep beneath the surface of the ocean.

Forming Complex Molecules

• Still other researchers think that some organic molecules arrived on meteorites or comets.

• This isn’t suggesting that life arose from outer space, it is just a hypothesis regarding the arrival of some organic molecules.

Fossils

• The fossil record includes some of the remains of organisms that have lived on earth in the past.

• It chronicles the diversity of life on earth.

Fossil Formation

• Most fossils are found in sedimentary rock.

• This usually occurs when organisms are rapidly buried in fine sediment.

• The key is that the organisms gets buried before its body decays or is eaten.

• Fossilization is rare. Most organisms that have ever lived have not been fossilized.

Fossil Formation

• Some fossils form when an organism dies, is buried, breaks down, and the hollow space is filled with minerals.

Fossil Formation

• Other fossils form whereby the pores of the organism are filled with minerals and its shape is preserved.

Fossil Formation

• In rare cases, fossils are formed after they have been trapped in sap and that sap hardens into amber.

Dating of Fossils-Relative Age

• The relative age of a fossil can be used to determine when a fossil has been laid down.

• It does not tell the age of the fossil.

• Index fossils are fossils of a common organism that was widespread. They are used to estimate the age of the other strata that contain the same type of fossil.

Dating of Fossils-Absolute Age

• Determining the absolute age of a fossil is done with radiometric dating.

• This process analyzes the chemical composition of the rock surrounding the object.

• Radioactive isotopes within the rock breakdown and form new isotopes which are more stable. Comparing the ratios gives scientists a way to determine how old a rock layer is.

Two Common Examples

• Carbon dating and potassium-argon dating are two common methods of dating fossilized remains.

Geologic Time

• Scientists have divided the earth’s history into the geologic time scale.

• The scale has been shaped by mass extinctions.

• Large divisions include Precambrian time, the Paleozoic, Mesozoic, and Cenozoic Eras.

Evolution of Life-Precambrian

• Precambrian Time lasted from the birth of the earth until about 542 million years ago.

• Single cells prokaryotes, and eukaryotes evolved during this time.

• Multicellularity evolved during this time and allowed for the evolution of modern organisms.

• Oxygen began to accumulate during this time.

Evolution of Life-Precambrian

• The oldest presumed fossils come from marine cyanobacteria called stromatolites and are about 3.5 billion years old.

Evolution of Life-Precambrian

• About 2.4 billion years ago, it is believed that cyanobacteria changed and began adding oxygen to the atmosphere.

• This was poisonous to the organisms that had evolved to live without oxygen.

Evolution of Life-Precambrian

• Many changes occurred in the organisms as a result of the oxygen buildup, and those that evolved a way to deal with it prospered, those that didn’t went extinct.

Evolution of Life-Precambrian

• Other changes as a result of the oxygen buildup (O3 formation) made the earth more hospitable.

• This allowed eukaryotic life to evolve.

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Endosymbiosis

• The origin of organelles is believed to have arisen as a result of larger cells engulfing smaller cells.

• Lynn Margulis is the lady who first proposed this idea.

• It is based on the following evidence: The size and structure of mitochondria/chloroplasts as

compared to some cyanobacteria. The genetic material of chloroplasts and mitochondria

closely resembles that of bacteria. The ribosomes of mitochondria and chloroplasts are

similar in size and structure to bacterial ribosomes. Chloroplasts and mitochondria reproduce via binary

fission and is independent from the cell cycle.

Evolution of Life-Precambrian

• Multicellularity first developed in protists during the Precambrian, and served as an important step in the evolution of current life forms.

• During most of the Precambrian, life was likely limited to prokaryotes and protists.

• The Precambrian ended with a mass extinction and opened up many new ecological niches.

Evolution of Life-Paleozoic Era

• The Cambrian Period is is the first part of the Paleozoic Era.

• “Old Life.”

• It is known as the Cambrian Explosion because it is marked by a rapid diversification of animals.

Evolution of Life-Paleozoic Era

• During this time, marine invertebrates diversified, marine vertebrates evolved, the first land plants evolved, and some arthropods and vertebrates began to colonize land.

Evolution of Life-Mesozoic Era

• “Middle Life.”

• Reptiles, dinosaurs, and birds were the dominant animals during the Mesozoic Era.

• Flowering plants evolved during this time.

Evolution of Life-Cenozoic Era

• “Recent Life.”

• This is the current era.

• During this era, the dinosaurs went extinct, mammals became the dominant life-form on land, humans evolved.