Molecule Of Animal Life

Oxygen is thus essential for animal life, and animal life consequently has evolved a host of structures for its acquisition. Because Earth has widely varying oxygen concentrations in different environments, some of the differences shown in the Appendix are related to living in different oxygen regimes. But we must take into account a new variable as well. Not only does oxygen concentration vary from place to place, it has also varied through time. It seems possible that an organism's type of respiratory system could be related to the oxygen levels present on Earth when that organism first evolved. Some animals first appeared when atmospheric oxygen was much higher than now, some under just the opposite condition. Chapter 2 looks at the history of oxygen in Earth's atmosphere and oceans and how this molecule's history has been studied.

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