G proteins and G-protein-linked receptors?

Question

G proteins and G-protein-linked receptors
are not widespread in nature and were unimportant in the evolution of eukaryotes.
are found only in animal cells.
probably evolved from an adaptation of the citric acid cycle.
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.

Answer 1

The last answer is correct.