The mystery of the digitally disadvantaged ancestors

Question

I read this short story in 197x. It consists mostly, perhaps entirely, of a conversation, part of which I can summarize/paraphrase:

“Do you ever wonder why we use base twelve?”

“The advantages of using a highly composite number [...].”

“You don't suppose it's because we have twelve fingers?”

“Oh, that's just a coincidence.”

“It's reasonable to suppose so. But in my research I've found that, thousands of years ago, humanity used base ten almost exclusively.”

“What! Why would our ancestors adopt an obviously inferior base? Well, if so, at least they came to their senses.”

“Here's the remarkable thing, though: there's evidence that, back then, most humans had ten fingers.”

When I read it, I took it to mean that a polydactyl mutation had spread to all humans. A more interesting interpretation is that an unrelated six-fingered species, otherwise resembling us enough to be absorbed in human culture and think of themselves as human, had outlived the five-fingered originals. (Compare Poul Anderson's “Turning Point”, and an analogous story about robots by, maybe, Eric Frank Russell? That's a question for another day.)

Author could be Silverberg, or Asimov, or not. (I may be thinking Asimov only because of a posthuman dialogue in “The Last Question”.)

Answer 1

I think this is A Tale of the Ending. It was published in the anthology One Step From Earth and Analog Science Fiction/Science Fact in 1970. Some of the story can be read on Google Books in a later edition of One Step From Earth.

Here’s a summary:

There’s an old Harry Harrison short story, “A Tale of the Ending,” in which the characters come to realize that, although they think of themselves as human, they belong to an alien species that has very, very quietly replaced humanity. 

[...] 

In Harrison’s story, the space aliens realize that there’s been that big change when they notice that humans used to have only five fingers on a hand.  But to a philosopher, the change that matters isn’t the number of fingers; it’s the specialization.  And so if the imaginary pod creatures need to rethink their intellectual ergonomics, so do we.
_{Daily Nous}

Here is the quote from the story (HT @user14111):

+Have you ever considered why we count from a twelve-digit base?+
+Mathematically it is the best. There are but eleven digits and the zero to remember. Yet still capable of infinite amplitude. Divisible as well by one, two, three, four, and six. A fine base+
+That is all?+
+That is enough+
+Have you ever considered that at some time, in the dawn of our race, we must have first started to count and in our simplicity used our fingers as a basic system+
He spread his hands on the table and looked at his dozen fingers.
+Could that not be possible?+