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 condi