Chapter 21- The History of Life on Earth
Text Book & Lecture Slide Combined
21.1-How Do Scientists Date Ancient Events?
• Fossils- the preserved remains of ancient organisms
• Paleontologists- scientists who study fossils
• Morphology- the body form
• Age of rocks cannot be determined by just looking at them, but the age of rocks can only be determined
when comparing one another
• Strata- oldest layers in rocks; are known to lie at the bottom, and successively higher strata are
• William Smith concluded that:
o Fossils of similar organisms were found in widely separated places on Earth
o Certain organisms were always found in younger rocks than certain other organisms
o Organisms found in higher, more recent strata were more similar to modern organisms than
were those found in lower, more ancient strata
• Radioactivity- a method of dating rocks; discovered at the beginning of the twentieth century
• Half-Life- a successive time interval in which half of the remaining radioactive material of the
radioisotope decays, either changing into another element or becoming the stable isotope of the same
• For C, production in the upper atmosphere is about equal to its natural decay.
• In an organism, the ratio of14C to 1C stays constant during its lifetime.
• When an organism dies, it is no longer incorporating 14C from the environment.
14 14 12
• The C that was present in the body decays with no replacement and the ratio of C to C
• As soon as an organism dies, it ceases to exchange carbon compounds with its environment
• Isotopes in a sedimentary rock do not contain reliable information about the date of its formation
o Since they are transported over long distances and are deposed in another location
• But igneous rocks (e.g., lava or volcanic ash), that have intruded into layers of sedimentary rock can
• Other radioisotopes are used to date older rocks.
• Decay of potassium-40 to argon-40 is used for the most ancient rocks.
• Radioisotope dating is combined with fossil analysis.
21.2- How have Earth’s Continents and Climates Changed over Time?
• Lithosphere- is Earth’s crust which consists of a number of solid plates each about 40 kilometers thick o Floats on a fluid layer of molten rock or magma; the magma circulates because heat produced
by radioactive decay deep in Earth’s core sets up convection currents in the fluid
• The plates move because 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 (when they occur under
water, such valleys are known as trenches)
• Where plates are pushed apart, ocean basins may form between them
• Continental Drift- movement of the lithospheric plates and the continents they contain
• Throughout Earth’s history, the plates that carry the continents have drifted apart and moved back
together numerous times.
• Plate movement has affected climate, sea level, and the distribution of organisms.
Increase of Oxygen in Atmosphere
• Increase in atmospheric oxygen have been largely unidirectional
• Oxygen first in atmosphere 3.8 bya (billion years ago)
• Oxygen concentrations began to increase significantly about 2.4 billion years ago when some
prokaryotes evolved the ability to split water as a source of hydrogen ions for photosynthesis. The
waste product is O .2
• One lineage of these oxygen-generating bacteria evolved into the cyanobacteria. These organisms
formed rocklike structures called stromatolites.
• The cyanobacteria liberated enough O to al2ow the evolution of oxidation reactions as the energy
source for the synthesis of ATP
• When oxygen first appeared in the atmosphere, it was poisonous to the anaerobic prokaryotes that
inhabited Earth at the time
• Organisms with aerobic metabolism replaced anaerobes in most of Earth’s environments
• As life continued to evolve, the physical nature of the planet was irrevocably changed.
• Living organisms added O to 2he atmosphere & removed CO from it. 2
• An atmosphere rich in O ma2e possible the evolution of larger cells and more complex organisms.
• About 1,500 mya (million years ago), O con2entrations became high enough for large eukaryotic
cells to flourish and diversify.
• By 750–700 mya, O had2increased to levels that could support multicellular organisms.
• To the largely unidirectional change in atmospheric oxygen concentration, most physical conditions on
Earth have oscillated in response to the planet’s internal processes, such as volcanic activity,
continental drift and meteorite impacts
• Extraterrestrial events such as collisions with meteorites, have also left their mark causing mass
• Mass Extinctions- during which a large proportion of the species living at the time disappeared
Earth’s Climate Change
• For Earth to be in a cold, dry state, atmospheric CO lev2ls had to have been usually low • Weather changes rapidly; climates usually change slowly
• Most volcanic eruptions produce only local or short-lived effects, but a few very large volcanic eruptions
have had major consequences for life
• The collision of continents during the late Permian (about 275 mya) created a single, giant land mass
called Pangaea and caused massive volcanic eruptions.
• Ash from the eruptions reduced the penetration of sunlight to Earth’s surface, lowering temperatures,
reducing photosynthesis, and triggering massive glaciations
• Massive volcanic eruptions also occurred as the continents drifted apart during the late Triassic period
and at the end of the Cretaceous
• The first impact to be documented: meteorite 10 km in diameter that caused a mass extinction at the
end of the Cretaceous.
• Abnormally high concentrations of iridium in a thin layer separating the Cretaceous and Tertiary rocks
• Iridium very rare on Earth, abundant in some meteorites.
• 180-km-diameter crater buried beneath the northern coast of the Yucatán Peninsula of Mexico
• Iridium layer = meteorite impact
21.3- What Are the Major Events in Life’s History?
• Life first evolved 3.8 billion years ago; about 1.5 billion years ago, eukaryotic organisms had evolved
• Biota- all of the organisms-animals, plants, fungi, and microorganisms- found in a given area
• Flora- all of the plants found in a given area
• Fauna- all of the animals found in a given area
• Fossils are a major source of information about changes on Earth during the remote past.
• Periods of geological history are marked by mass extinctions or by dramatic increases in diversity called
• Evidence suggests that the major divisions in many animal lineages predate the end of the Precambrian
by more than 100 million years.
• Although the fossil record is fragmentary before 550 mya, it is still good enough to show that the total
number of species and individuals increased dramatically in late Precambrian times.
• The fossil record, though incomplete, is good enough to demonstrate clearly that organisms of
particular types are found in rocks of specific ages and that newer organisms appear sequentially in
• An organism is most likely to become a fossil if its dead body is deposited in an environment that
lacks oxygen. • About 300,000 species of fossil organisms have been described.
• 1.7 million species of present-day biota have been named.
• The actual number of living species is probably at least 10 million.
• Most species exist, on average, for fewer than 10 million years; therefore, Earth’s species must have
turned over many times during geological history.
• Among the nine major animal groups with hard-shelled members, approximately 200,000 species
have been described from fossils.
• During the Precambiran period, life consisted of microscopic prokaryotes; eukaryotes probably evolved
about two-thirds of the way through the era.
o Unicellular eukaryotes & small multicellular animals fed on floating photosynthetic
• Shallow Precambrian seas teemed with life, including protists and algae.
• By the late Precambrian, many kinds of soft-bodied invertebrates had evolved, some of which may
be members of animal lineages that have no living descendants.
• Cambrian period marks the beginning of the Paleozoic era
• By the early Cambrian period (543–490 mya), atmospheric O levels2had nearly reached current levels.
• The continental plates came together in several masses. Gondwana was the largest land mass.
• The rapid diversification of life that took place at this time is referred to as the Cambrian explosion.
• The best fossils of Cambrian animals are found in China.
• A mass extinction occurred at the end of the Cambrian period.
• Fossils tell us only about the hard parts of organisms
o 3 Cambrian fossil beds- the Burgess Shale (British Columbia), Sirius Passet(Northern Greenland)
& Chengjiang (southern China)
o Arthropods are the most diverse group in the Chinese fauna