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:
o Fossils of similar organisms were found in widely separated places on
o Certain organisms were always found in younger rocks than certain
o Organisms found in higher, more recent strata were more similar to
modern organisms than were those found in lower, more ancient
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
• 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 O to 2pen 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 O surv2ved and gained
numerous advantages: aerobic respiration proceeds at more rapid rates and
harvests energy more efficiently.
• An atmosphere rich in O also made possible larger cells and more complex
• In contrast to this largely unidirectional change in atmospheric O 2
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 CO le2els 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).
o The 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.
o Iridium 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
rocks. • Many fossil-bearing rocks are deeply buried and inaccessible.
• Number of known fossils is especially large for marine animals that had hard
• Insects and spiders are also relatively well represented in the fossil record.
• The fossil record (incomplete) is good enough to demonstrate clearly that
organisms of particular types are found in rocks of specific ages and that new
organisms appear sequentially in younger rocks.
Precambrian life was small and aquatic
• Life was confined to oceans and all organisms were small.
• Over the Precambrian the shallow seas slowly began to teem with life.
• For most of the Precambrian, life consisted of microscopic prokaryotes
(eukaryotes evolved 2/3 of the way through the era).
• Unicellular eukaryotes and small multicellular animals fed on floating
• Small floating organisms, plankton, were eaten by slightly large animals.
• Other animals ingested sediments on the seafloor and digested the remains
of organisms within them.
• By late Precambrian, many kinds of multicellular soft-bodied animals had
Life expanded rapidly during the Cambrian period
• Cambrian period (452-488 mya) marks the beginning of the Palaeozoic era.
• The O 2oncentration was approaching its current level; the continents had
come together to form several large land masses.
• The largest, Gondwana .
• A rapid diversification of life took place Cambrian explosion.
• Most of the major groups of animals that have species living today appeared
during this period.
• THE ORDOVICIAN (488-444 MYA)
o The continents, located primarily in the S. Hemisphere, still lacked
o Evolutionary radiation of marine organisms during early stages.
o At the end, massive glaciers formed over Gondawa, sea levels were
lowered about 50 meters, and ocean temperatures dropped.
o About 75% percent of the animal species became extinct, probably
because of these major environmental changes.
• SILURIAN (444-416 MYA)
o Northernmost continents coalesced, but the general positions did not
o Marine life rebounded
o Animals able to swim and feed above the ocean bottom appeared for
the first time.
o No new major groups of marine life evolved.
o The tropical sea was uninterrupted by land barriers
o Most marine organisms were widely distributed.
o First vascular plants appeared late in Silurian period; less than 50cm
tall and lacked roots and leaves.
o First terrestrial arthropods appeared at about the same time.
• DEVONIAN (416-359 MYA) o Rates of evolutionary change accelerated.
o The northern land mass (Lau