We know that the atmosphere became more oxygen rich around 1.8 ga because of geologic evidence such as:

Which kind of aquatic ecosystem does the photograph most likely show? a. marsh b. swamp c. lake d. river

Which best describes red blood cells? a. they are colorless b. they protect against disease-carrying microorganisms c. they transport oxygen throughout the body d. they aid in blood clotting

Which of the following is one of the four conditions necessary for natural selection to occur in a population? fewer organisms are born than the habitat can support mutation does not occur organisms have identical characteristics variation is inherited