The History of LifeChapter 12

Fossils and Ancient Life

A fossil is the preserved remains or evidence of an ancient organism Scientists who study fossils are paleontologists

The fossil record is the grouping of similar organisms from oldest to most recent It can provide evidence about the history of life on Earth

and show how different groups of organisms have changed over time

Extant v/s Extinct

More than 99 percent of all species that have ever lived on Earth have become extinct The term extinct is used to describe a species that no

longer has a living representative

The term extant is used to describe a species that has living representatives

The Formation of Fossils

A fossil can be as large and complete as an entire, perfectly preserved animal, or as small as a tiny fragment of a jawbone or leaf For a fossil to form, either the remains of

the organism or some trace of its presence must be preserved

The formation of any fossil depends on a precise combination of conditions Because of this, the fossil record provides

an incomplete record of the history of life – for every organism that leaves a fossil, many more die without leaving a trace

Section 17-1

Figure 17-2 Formation of a Fossil

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The Formation of Fossils

Relative Dating of Fossils

Sedimentary strata reveal the relative ages of fossils: In relative dating, the order of rock

strata is used to determine the relative age of fossils.

Older fossils are found in deeper layers of strata, and younger fossils are found in superficial layers of strata.

Radioactive Dating of Fossils

Radioactive dating is the use of half-lives to determine the age of a sample Radioactive elements decay, or break down,

into non-radioactive elements at a steady rate called a half-life

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

In radioactive dating, scientists calculate the age of a sample based on the amount of remaining radioactive isotopes it contains

Formation of Earth

Earth’s early atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water About 4 billion years ago, Earth cooled

enough to allow the first solid rocks to form on its surface

About 3.8 billion years ago, Earth’s surface cooled enough to allow water to remain liquid

Could organic molecules evolve under these conditions?

Mixture of gases simulating atmospheres of early Earth

Spark simulating lightning storms

Condensation chamber

Cold water cools chamber, causing droplets to form

Water vapor

Liquid containing amino acids and other organic compounds

Section 17-2The First Organic Moleculeshttp://bcs.whfreeman.com/thelifewire/content/chp03/0301s.swf

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Miller and Urey’s experiments suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive earth.

Miller and Urey produced amino acids, which are needed to make proteins, by passing sparks through a mixture of hydrogen, methane, ammonia, and water.

Aerobic bacteria

Ancient Prokaryotes

Ancient Anaerobic Prokaryote

Primitive Aerobic Eukaryote

Primitive Photosynthetic Eukaryote

Chloroplast

Photosynthetic bacteriaNuclear

envelope evolving Mitochondrion

Plants and plantlike protists

Animals, fungi, and non-plantlike protists

Origin of Eukaryotic Cellshttp://www.sumanasinc.com/webcontent/animations/content/organelles.html

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The endosymbiotic theory, championed by Lynn Margulis, proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms

Patterns of Macroevolution

Macroevolution refers to the large scale evolutionary changes that take place over long periods of time

Six important patterns of macroevolution are:1. Mass extinctions

2. Adaptive radiation

3. Convergent evolution

4. Coevolution

5. Divergent evolution

6. Punctuated equilibrium

Mass Extinctions Extinctions occur all the time

More than 99% of all species that ever lived are extinct today

Usually, extinctions occur at a constant rate Several times, however,

huge numbers of species have disappeared in mass extinctions

Paleontologists think that most mass extinctions in the past were caused by multiple factors Asteroids Volcanic activity Changing position of

continents Changing sea levels

Adaptive Radiation

Adaptive radiation is the RAPID evolution of a group of organisms following some large-scale disturbance.

Adaptive radiation typically occurs when a few organisms make their way to new, often distant areas or when environmental changes cause numerous extinctions, opening up ecological niches for the survivors. Fossil evidence indicates that mammals underwent a

dramatic adaptive radiation after the mass extinctions of dinosaurs 65 million years ago.

Adaptive Radiation in Darwin’s Finches

Convergent Evolution

Convergent evolution describes 2 unrelated species that share similar traits. These similarities are not due to common ancestry, but rather

a result of similar environmental factors (creates analogous structures).

Coevolution

Coevolution describes the evolution of one species in response to new adaptations that appear in another species of which the first shares close interaction.

Divergent Evolution

Divergent evolution is the accumulation of differences between groups which can lead to the formation of new species.

It is usually a result of diffusion of the same species to different and isolated environments which blocks the gene flow among the distinct populations.

These barriers to gene flow allow differentiated fixation of characteristics through genetic drift and natural selection.

Patterns of Evolution

Punctuated Equilibrium

Punctuated equilibrium is a pattern of evolution in which long stable periods (gradualism) are interrupted by brief periods of more rapid change Typically occurs when new niches become available

following a mass extinction

Gradualism v/s Punctuated Equilibrium

Species descended from a common ancestor gradually diverge more and more in morphology as they acquire unique adaptations.

A new species changes most as it buds from a parent species, and then changes little for the rest of its existence.

Section 17-4

that are

can undergo can undergo can undergo can undergo can undergo

in underunderform inin

Species

Unrelated Related

Inter-relationshiops

Similar environments

Intense environmental

pressure

Small populations

Different environments

Coevolution Convergent evolution

ExtinctionPunctuated equilibrium

Adaptive radiation

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Patterns of Macroevolution