Chapter 21: The History of Life on Earth
21.1 How Do Scientists Date Ancient Events?
•Fossils can tell us a great deal about the body form, or morphology, of
organisms that lived there long ago, as well as how and where they lived.
•Earth’s history is largely recorded in its rocks. The oldest layers, or strata, lie
at the bottom, and successively higher strata are progressively younger.
•Observations of fossils contained within sedimentary rocks:
oFossils of similar organisms were found in widely separated places on
oCertain organisms were always found in younger rocks than certain
oOrganisms found in higher, more recent strata were more similar to
modern organisms than were those found in lower, more ancient strata.
Radioisotopes provide a way to date rocks
•Half of the remaining radioactive material of the radioisotope decays
Radioisotope dating have been expanded and refined
•The decay of potassium-40 to argon-40 has been used to date most of the
ancient events in the evolution of life.
•Fossils in the adjacent sedimentary rock that are similar to those in other rocks
of known ages provide additional clues.
•Radioisotopes dating of rocks, combined with fossil analysis, are the most
powerful method of determining geological age.
21.2 How Have Earth’s Continents and Climates Changed over Time?
•Earth’s crust consists of a number of solid plates approximately 40km thick,
collectively make up the lithosphere. It floats on a fluid layer of molten rock
•Magma circulates b/c heat produced by radioactive decay deep in Earth’s core
sets up convection currents in the fluid.
•The plates move b/c magma rises and exerts tremendous pressure.
•Where plates are pushed together, either they move sideways past each other,
or one plate slides under the other, pushing up mountain ranges and carving
deep rift valleys (underwater known as trenches).
•Where plates are pushed apart, ocean basins may form between them.
•The movement of the lithospheric plates and the continents they contain is
known as continental drift.
Oxygen has steadily increased in Earth’s atmosphere
•Increase in atmospheric oxygen has been largely unidirectional.
•The atmosphere of early Earth probably contained little or no free oxygen gas.
•The increase in atmospheric oxygen came in two big steps; the first step
occurred 2.4 billion years ago, when certain bacteria evolved the ability to use
water as the source of hydrogen ions for photosynthesis.
•One group of oxygen-generating bacteria, cyanobacteria, formed rocklike
structures called stromatolites.
•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 quickly
•They are not likely to become fossils unless moved to an unoxygenated region;
most fossils are destroyed by geological process that transforms rocks.
•Many fossil-bearing rocks are deeply buried and inaccessible.
•Number of known fossils is especially large for marine animals that had hard
Chapter 21: the history of life on earth. The oldest layers, or strata, lie at the bottom, and successively higher strata are progressively younger: observations of fossils contained within sedimentary rocks: Radioisotopes provide a way to date rocks: half of the remaining radioactive material of the radioisotope decays. Radioisotope dating have been expanded and refined: the decay of potassium-40 to argon-40 has been used to date most of the ancient events in the evolution of life. Fossils in the adjacent sedimentary rock that are similar to those in other rocks of known ages provide additional clues: radioisotopes dating of rocks, combined with fossil analysis, are the most powerful method of determining geological age. 21. 2 how have earth"s continents and climates changed over time: earth"s crust consists of a number of solid plates approximately 40km thick, collectively make up the lithosphere.