1. earth's first atmosphere was composed of mainly which of the following gases? a. hydrogen and helium b. carbon dioxide and oxygen c. nitrogen and oxygen d. carbon monoxide and o zone 2. during the cambrian explosion that began the paleozoic era, the many new forms of life that evolved a. lived on land b. were invertebrates that lived in the sea c. were vertebrates covered with scales or fur d. were single celled 3. which of the following occurred during the mesozoic era? a. the rise of reptiles b. the rise of plants c. the rise of invertebrates d. the rise of mammals 4. which of the following best describes the climates of europe and north america during 1.8 million years of the quaternary period? a. hot and dry b. cool and rainy periods followed by droughts c. warm and mild d. a series of ice ages followed by periods when the glaciers melted