Why did the Buddha touch the earth at his enlightenment?

At the Buddha's enlightenment, he pointed to the earth or touched the earth, meaning the earth (or world) was adequate witness to his spiritual accomplishment or new status, or, perhaps the only witness that would be acceptable to Mara. Mara accepted the witness. I do not understand the principle or truth of this event. Earth or the world seems to be personified in some way, or endowed with a capacity to accept or reject a plea for witnessing. Explanations and clarifications would be greatly appreciated. Thx.