Which of the following provided the selective pressures that most directly promoted the evolution of photosynthesis?
a. the drop in surface UV radiation caused by the formation of the ozone layer
b. the depletion of dissolved organic material from the oceans
c. the ability of some organisms to use oxygen to generate ATP during respiration
d. an increase in visible light intensity at the surface of the Earth, caused by solar flares
e. an increase in volcanic activity and release of CO2 into the atmosphere
B; Photosynthesis uses light energy to drive the synthesis of organic compounds. As long as there are organic compounds in the ocean, free for the taking, there is no selective pressure to acquire this trick. The ozone layer (choice a) and aerobic respiration (choice c) both followed the generation of O2 by photosynthesis. The availability of light and CO2 (choices d and e) are irrelevant as long as there is a source of organic carbon.
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