"than was previously supposed" - I don't understand why we leaving "was" here is correct

Question

I've read this sentence:

Their discovery means that archaic humans, who left Africa 1.5 million years earlier than modern people, survived far longer into recent times than was previously supposed.

(New York Times)

I wonder why the finite verb "was" can exist here. My current understanding is that we either use a full clause, like this:

• Their discovery means that archaic humans survived far longer into recent times than (the time) it was previously supposed. OR a phrase, like this:
• Their discovery means that archaic humans survived far longer into recent times than previously supposed. (a past participle phrase)

But we shouldn't mix both. So why the "was"? I suspect some kind of ellipsis from a full clause, but I'm not sure. Because if it's an ellipsis, shouldn't the "was" be deleted as well?

Answer 1

Consider:

The car gets fewer miles per gallon of fuel than (was) previously advertised.

The forms of BE can often be omitted (ellipted) in English without the ellipsis interfering with communication.

The implicit or tacit subject of "was" must be a nominal form of the verb-phrase found in the matrix clause.

That the form "than was" is used there at all is the result of a misguided educational campaign that lasted more than a century.

If "than was" is used in a comparative clause, there's a very good chance the speaker was university educated or that the text was edited by someone who had been taught that "than was" was proper despite the language heading away from it.