Not an answer
Only a guess.
The behavior doesn't seem limited to one destination. Little kids will put their fingers into any orifice available. Ears are also popular, and navels when sleepy or upset. Keyholes, tubes, other kids' eyes...
It could be that the same impulse to explore, more refined, better articulated and better financed, got Columbus to America.
It could just be a primate thing. You may have heard the elementary school riddle:
Why do gorillas have big nostrils?
Give up?
Because they have big fingers.
Very Lamarckian, if you ask me.
*******
Sometimes when I visit here, I feel like I'm at a very weird cocktail party...
Where was this discussion going again?
Why is there an olive in my manhattan?
Cheers (and I mean it)--
Mary Anne