Which major gas in today’s atmosphere was introduced by living organisms about 2.7 billion years ago?

Cells tend to have a relatively small and uniform size. why aren’t cells larger?

The concept of keystone species is controversial among ecologists because most organisms are highly interdependent. if each of the trophic levels is dependant on all others how can we say one is most important

Plants consume most of their carbon from a. the soil b. the water c. the roots d. the air