The History of Life
Chapter 17 – Miller · Levine
Fossils
• Paleontologists – scientists who study
fossils
– Infer what past life forms were like
– Classify fossil organisms
• Fossil record – information about past life
– Provides evidence about the history of life on
Earth
– Shows how different groups of organisms have
changed over time
How Fossils Form
• Either the remains of the organism or
some trace of its presence must be
preserved
• Most fossils form in sedimentary rock
• An imprint of soft parts can be made
• The hard parts can become
mineralized
• Can be preserved in rock, ice, amber,
tar, etc.
Relative Dating
• The age of a fossil is determined by comparing its
placement with that of fossils in other layers of
rock
– Older fossils will be in the bottom layers
– More recent fossils will be in the upper layers
• Index fossils – fossils of species that existed for a
short period but had a wide range
• Allows paleontologists to estimate a fossil’s age
compared with that of other fossils
Relative Dating
Radioactive Dating
• Scientists use radioactive
decay to assign absolute
ages to rocks
– Calculate the age of a
sample based on the amount
of remaining radioactive
isotopes it contains
• Half-life – the length of
time required for half of
the radioactive atoms in a
sample to decay
Geologic Time Scale
• Precambrian Time – covers about
88% of Earth’s history
• Paleozoic Era – many vertebrates
and invertebrates
• Mesozoic Era – “Age of the
Dinosaurs”
– Mammals evolved
• Cenozoic Era – “Age of Mammals”
• Eras are subdivided into periods
Geologic Time Scale
Formation of Earth
• Pieces of cosmic debris were probably attracted to
one another
• The early atmosphere probably contained
hydrogen cyanide, carbon dioxide, carbon
monoxide, nitrogen, hydrogen sulfide, and water
• Violent volcanic activity, comets and asteroids
bombarded the surface, oceans did not exist
• About 3.8 billion years ago, Earth’s surface cooled
enough for oceans to form
How Did Life Begin?
• The first organic molecules were able to
form because of the atmosphere being
bombarded by electricity (lightning)
• RNA probably evolved before DNA
The Rise of Oxygen…
• The first life forms must have evolved in
the absence of oxygen (anaerobic
prokaryotes)
• Over time, photosynthetic bacteria evolved,
adding oxygen to the atmosphere
• The ozone layer formed
• The rise of oxygen caused some life forms
to go extinct while other organisms evolved
Endosymbiotic Theory
• Prokaryotic organisms entered the
“ancestral eukaryote”
• Formed a symbiotic relationship
• Some had the ability to perform respiration
– became modern mitochondria
• Others could perform photosynthesis –
became the modern chloroplasts
Endosymbiotic Theory
Macroevolution
• Large scale evolutionary changes that take
place over long periods of time
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Mass extinctions
Adaptive radiation
Convergent evolution
Coevolution
Punctuated equilibrium
Mass Extinctions
• Extinction occurs all the time
• More than 99% of all species that have ever lived
are now extinct
• Mass extinctions occur when large numbers of
species go extinct in a relatively short period of
time
• Most mass extinctions were probably caused by
multiple factors
• Usually followed by mass speciation
Adaptive Radiation
• A single species or a small group of species
has evolved into several different forms that
live in different ways
• Also known as divergent evolution
• Often caused by geographic isolation –
geological change (river, canyon, or
mountain) that isolates segments of a
population
Adaptive Radiation
Convergent Evolution
• Adaptive radiation in groups of different
organisms in different places or at different
times, but in similar environments
• Unrelated organisms resemble one another
• Analogous structures – look and function
similarly but do not share a common
ancestry
– Fins of dolphins, seals, penguins, sharks
Coevolution
• Sometimes organisms that are closely connected
to one another evolve together
• An evolutionary change in one organism may be
followed by a corresponding change in another
organism
– Snails developed thicker shells, so crabs evolved
stronger claws
– Plants produce poisonous chemicals, herbivores
evolved the ability to detoxify these chemicals
Gradualism vs. Punctuated
Equilibrium
• Gradualism –
biological change is a
slow and steady process
• Punctuated
equilibrium – long,
stable periods
interrupted by brief
periods of more rapid
change