The best natural scientists, when they aren’t busy filling us with awe, are busy reminding us how small and pointless we are.

Stephen Hawking has called humankind “just an advanced breed of monkeys on a minor planet of a very average star.” The biologist and naturalist Edward O. Wilson, in his new book, which is modestly titled “The Meaning of Human Existence,” puts our pygmy planet in a different context.

“Let me offer a metaphor,” he says. “Earth relates to the universe as the second segment of the left antenna of an aphid sitting on a flower petal in a garden in Teaneck, N.J., for a few hours this afternoon.” The Jersey aspect of that put-down really drives in the nail.

Mr. Wilson’s slim new book is a valedictory work. The author, now 85 and retired from Harvard for nearly two decades, chews over issues that have long concentrated his mind: the environment; the biological basis of our behavior; the necessity of science and humanities finding common cause; the way religion poisons almost everything; and the things we can learn from ants, about which Mr. Wilson is the world’s leading expert.