2 Guest Vocalists Are Singing First Lines Of Stevies YOU ARE THE SUNSHINE OF MY LIFE

[Boogiedown]

I know this is likely old news to the well-schooled & observant Motowners here, but I just recently took notice of the other-than-Stevie voices that begin singing the opening lines on this record. [[!) And I am quite gobsmacked!
In my defense, in 1973, when YOU ARE THE SUNSHINE OF MY LIFE came out and I was becoming familiar with it, I was listening to it mainly through my AM car radio. Those initial voices , I assumed were simply Stevie [[in typical experimental Stevie fashion) applying some variable inflections with his voice. Sounded well-executed to me, worked fine. Nothing more to contemplate. Did others here make this same mistake??

Likewise in my defense, WHO in the world lets others sing lead lines within a song credited as only them --- let alone the opening ones?? And then, to carry it further, uses two guest vocalists to do so? The possibility never crossed my mind.

I finally caught on to it only recently. The first singer stands out now as less likely being Stevie. But the second singer, a female, still sounds to me like it could be Stevie singing in a higher register. Regardless of intent, a very clever intro Mr. Wonder! I wonder how it came about? Has there ever been an explanation?? A most unusual concept. Quite fun.

I have also now learned who those two non-credited vocalists are, any of the knowledgeable here care to reveal their identities??