Roots of "hip" and "cool"

There's a new book out called Hip: The History by John Leland (see and ). I haven't seen the book yet, but some reviews mention Leland's discussion of the African origins of both "hip" and "cool". Here is part of an interview with Leland on the publisher's website:
http://www.harpercollins.com/catalog/book interview xml.asp?isbn=0060528176

When does hip start?
The word traces back to West Africa, to the Wolof verbs hipi , meaning "to see," or hepi , "to open one's eyes." So it begins as a term of enlightenment or awareness. We also get our words dig, jive and honky from Wolof. But hip as we know it begins after the word arrives here, in the complicated dance between former Europeans and former Africans. So I begin the book in 1619, with the arrival of the first Africans in the New World, and carry the story up to the present.
Is hip the same as cool?
Not exactly, or at least not etymologically. Robert Farris Thompson, the Yale art historian, traces the idea of cool back to a 15th century Benin king who was awarded the name Ewuare , meaning "it is cool," because he brought peace to his people. Cool, in the African sense, refers to composure in an individual or stability in a society. So cool is a behavior or a mask. Hip, on the other hand, refers to an awareness or enlightenment. It's the intelligence behind the mask.
I've heard the Wolof conjecture for "hip" et al. before in his introductory chapter () Leland cites Clarence Major's Juba to Jive: A Dictionary of African-American Slang . But tracing "cool" to 15th-century Benin is new to me. I've seen another theory that "cool" was a calque of Mandingo 'slow', literally 'cool' ().

It's always difficult to settle these proposed West African derivations with any certainty, but that doesn't seem to stop a writer like Leland from running with the etymologies as if they've been firmly established.