Problem 3
Describe two ways in which the sudden appearance of oxygen in the atmosphere about 2.5 billion years ago influenced the development of modern life-forms.
Problem 7
Contrast the eastern and western margins of North America during the Cenozoic era in terms of their relationships to plate boundaries.
Problem 8
Suggest at least one reason plants moved onto land before large animals.
Problem 11
Suggest a geologic reason why the rift valley system of East Africa is so rich in human ancestor fossils.
