G proteins and G-protein-linked receptors
A) are found only in animal cells.
B) are found only in bacterial cells.
C) are thought to have evolved very early, because of their similar structure and function in a wide variety of modern organisms whose common ancestors diverged billions of years ago.
D) probably evolved from an adaptation of the citric acid cycle.
E) are not widespread in nature and were unimportant in the evolution of eukaryotes.
Answer: C