Why did plants have to adapt to land before animals did?

Question

A. Plants provide the oxygen animals require.

B. Plants are essential as a source of food and shelter for many animals.

C. They provide chlorophyll needed as an energy source for animals.

D. Their root systems anchored the soil.

Answer 1

B. Plants are essential as a source of food and shelter for many animals. Land plants are the "primary producers" at the lowest level of a considerable proportion of all food webs in terrestrial systems.

Some might venture to guess A., however the large amounts of oxygen in the atmosphere are considered to have been first created by the activities of ancient cyanobacteria. For more, see:

http://en.wikipedia.org/wiki/Cyanobacter...