Answer:
The respiratory and circulatory systems, for example, work together to provide oxygen to cells and remove carbon dioxide produced by the cells. The circulatory system takes oxygen from the lungs and delivers it to the tissues, then reverses the process with carbon dioxide.
Scientists can't be sure about the early atmosphere and can only draw evidence from other sources. For example, volcanoes release high quantities of carbon dioxide. Iron-based compounds are present in very old rocks that could only have formed if there was little or no oxygen at the time.
