key concept specific environmental conditions are necessary in order for fossils...

12.1 The Fossil Record

KEY CONCEPT

Specific environmental conditions are necessary in

order for fossils to form.


Fossils can form in several ways.

• Permineralization occurs when minerals carried by water

are deposited around a hard structure.


• A natural cast forms when flowing water removes all of the

original tissue, leaving an impression.


• Trace fossils record the activity of an organism.


• Amber-preserved fossils are organisms that become

trapped in tree resin that hardens after the tree is buried.


• Preserved remains form when an entire organism becomes

encased in material such as ice.


• Specific conditions are needed for fossilization.

• Only a tiny percentage of living things became fossils.


Radiometric dating provides an accurate way to estimate

the age of fossils.

• Relative dating estimates the time during which an

organism lived.

– It compares the placement

of fossils in layers of rock.

– Scientists infer the order in

which species existed.


• Radiometric dating uses decay of unstable isotopes.

– Isotopes are atoms of an element that differ in their

number of neutrons.

neutrons protrons


– A half-life is the amount of time it takes for half of the

isotope to decay.

• Radiometric dating uses decay of unstable isotopes.

– Isotopes are atoms of an element that differ in their

number of neutrons.

12.2 The Geologic Time Scale

KEY CONCEPT

The geologic time scale divides Earth’s history based

on major past events.


Index fossils are another tool to determine the age of rock

layers.

• Index fossils can provide the relative age of a rock layer.

– existed only during specific spans of time

– occurred in large geographic areas

• Index fossils include fusulinids and trilobites.


The geologic time scale organizes Earth’s history.

• The history of Earth is

represented in the geologic time

scale.

100

250

550

1000

2000

PRECAMBRIAN TIME

Cyanobacteria

This time span makes up the

vast majority of Earth’s history.

It includes the oldest known

rocks and fossils, the origin of

eukaryotes, and the oldest

animal fossils.


• Eras last tens to hundreds of millions of years.

– consist of two or more periods

– three eras: Cenozoic, Mesozoic, Paleozoic


• Periods last tens of millions of years.

– most commonly used units of time on time scale

– associated with rock systems.

• Epochs last several

million years.

12.3 Origin of Life

KEY CONCEPT

The origin of life on Earth remains a puzzle.

12.3 Origin of Life

Earth was very different billions of years ago.

• There have been many hypotheses of Earth’s origins.

• The most widely accepted hypothesis of Earth’s origins is

the nebula hypothesis.

12.3 Origin of Life

Several sets of hypotheses propose how life began on

Earth.

• There are two organic molecule hypotheses.

– Miller-Urey experiment

– meteorite hypothesis

electrodes

heat source amino acids

water

“atmosphere”

“ocean”

12.3 Origin of Life

• There are different hypotheses of early cell structure.

– iron-sulfide bubbles hypothesis

12.3 Origin of Life

• There are different hypotheses of early cell structure.

– lipid membrane hypothesis

12.3 Origin of Life

• A hypothesis proposes that RNA was the first genetic

material.

– Ribozymes are RNA

molecules that catalyze

their own replication.

– DNA needs enzymes to

replicate itself.

12.4 Early Single-Celled Organisms

KEY CONCEPT

Single-celled organisms existed 3.8 billion years ago.


Microbes have changed the physical and chemical

composition of Earth.

• The oldest known fossils are a group of marine

cyanobacteria.

– prokaryotic cells

– added oxygen to

atmosphere

– deposited minerals


• Fossil stromatolites provide evidence of early colonies of

life.


Eukaryotic cells may have evolved through

endosymbiosis.

• Endosymbiosis is a relationship in which one organism lives

within the body of another.

• Mitochondria and chloroplasts may have developed through

endosymbiosis.


The evolution of sexual reproduction led to increased

diversity.

• Genetic variation is an advantage of sexual reproduction.

• Sexual reproduction may have led to the evolution of

multicellular life.

12.5 Radiation of Multicellular Life

KEY CONCEPT

Multicellular life evolved in distinct phases.


Life moved onto land during the Paleozoic Era.

• Multicellular organisms first

appeared during the

Paleozoic era.

• The era began 544 million

years ago and ended 248

million years ago.

• The Cambrian explosion led

to a huge diversity of animal

species.


• Life moved onto land in the middle of the Paleozoic era.


Reptiles radiated during the Mesozoic era.

• The Mesozoic era is known as the Age of Reptiles.

• It began 248 million years ago and ended 65 million years

ago.

• Dinosaurs, birds, flowering plants, and first mammals

appeared.


Mammals radiated during the Cenozoic era.

• The Cenozoic era began 65 million years ago and continues today.

• Placental mammals and monotremes evolved and diversified.

• Anatomically modern humans appeared late in the era.

12.6 Primate Evolution

KEY CONCEPT

Humans appeared late in Earth’s history.


Humans share a common ancestor with other primates.

• Primates are mammals with flexible hands and feet,

forward-looking eyes and enlarged brains.


• Primates evolved into prosimians and anthropoids.

– Prosimians are the oldest living primates.

– They are mostly small and nocturnal.


– They are subdivided into the New World monkeys, Old

World monkeys, and hominoids.

– Anthropoids are humanlike primates.

– Homonoids are

divided into

hominids, great

apes, and lesser

apes.

– Hominids include

living and extinct

humans.


• Bipedal means walking on two legs.

– foraging

– carrying infants and food

– using tools

• Walking upright has

important adaptive

advantages.


There are many fossils of extinct hominids.

• Most hominids are either the genus Australopithecus or

Homo.

• Australopithecines were a successful genus.

• The Homo genus first evolved 2.4 million years ago.


Modern humans arose about 200,000 years ago.

• Homo sapiens fossils date to 200,000 years ago.

• Human evolution is influenced by a tool-based culture.

• There is a trend toward increased brain size in hominids.

Australopithecus

afarensis

Homo habilis Homo

neanderthalensis

Homo sapiens

key concept specific environmental conditions are necessary in order for fossils...

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