•Cyanobacteria liberated enough O2 to open the way for the evolution of
oxidation reactions as the energy source for the synthesis of ATP.
•When it first appeared, oxygen was poisonous to anaerobic prokaryotes that
•Those prokaryotes evolved the ability to metabolize O2 survived and gained
numerous advantages: aerobic respiration proceeds at more rapid rates and
harvests energy more efficiently.
•An atmosphere rich in O2 also made possible larger cells and more complex
•In contrast to this largely unidirectional change in atmospheric O2
concentration, most physical conditions on Earth have oscillated in response
to the planet’s internal processes.
Earth’s climate has shifted between hot/humid and cold/dry conditions
•Earth’s climate was considerably warmer than it is today, and temperatures
decreased more gradually towards the poles.
•At other times, Earth was colder than it is today; large areas were covered
with glaciers during the end of the Precambrian and during parts of the
Carboniferous and Permian periods.
•For Earth to be in a cold, dry state, atmospheric CO2 levels had to have been
Volcanoes have occasionally changed the history of life
•A few very large volcanic eruptions have had major consequences for life.
•The collision of continents during the Permian period (275 mya) to form a
single, gigantic land mass, Pangaea, caused massive volcanic eruptions.
•The ash ejected by volcanoes into the atmosphere reduced the penetration of
sunlight to Earth’s surface, lowering temperatures, reducing photosynthesis,
and triggering massive glaciations.
Extraterrestrial events have triggered changes on Earth
•A meteorite caused or contributed to a mass extinction at the end of the
Cretaceous period (65 mya).
oThe first clue came from abnormally high concentrations of the
element iridium in a thin layer separating rocks deposited during the
Cretaceous from those deposited during the Tertiary.
oIridium is abundant in some meteorites; rare on Earth’s surfaces.
21.3 What Are the Major Events in Life’s History?
•Life first evolved on Earth about 3.8 billion years ago.
•By about 1.5 billion years ago, eukaryotic organisms had evolved.
•The fossil record of organisms that lived prior to 550 million year ago is
fragmentary; good enough to show that the total number of species and
individuals increased dramatically in late Precambrian times.
Several processes contribute to the paucity of fossils
•Only a tiny fraction of organisms ever become fossils.
•Most organisms live and die in oxygen-rich environments in which they
•They are not likely to become fossils unless moved to an unoxygenated
region; most fossils are destroyed by geological process that transforms