Think about it: Since hard-shelled organisms evolved about 550 million years ago, billions of tons of limestone rock have been produced from their shells. Limestone is made of calcium carbonate, with

Think about it: Since hard-shelled organisms evolved about 550 million years ago, billions of tons of limestone rock have been produced from their shells. Limestone is made of calcium carbonate, with the formula CaCO3. 

Based on this, how do you think the amount of atmospheric CO2 has changed in the last 550 million years, and how has this affected Earth’s climate? Explain your answer.