a. No Stone Age ten-year-old would have been living on tender foods like modern potato chips, hamburgers, and pasta. Their meals would have required far more chewing than is ever demanded of a modern child.
How can I explain those 'would have'? The modals in the sentence a. seem to be different from those in the following common past unreal conditional sentences b. and c.
*b. I would have gone to the party, but I was really busy.
c. If I had had enough money, I would have bought a car.*
Cay I say the underlined modals ‘would have’ in a. are also talking about things that didn’t really happen in the past? That is, can I say that it is ‘very similar to the third conditional without 'if clause'? Then, is it real or unreal in the third conditional? Otherwise, is it a comment on a likely truth of the things in the past? I mean it can be explained just as the perfect form of modal 'would' of likely truth like in the following sentence d.
d. The doorbell just rang. That would be your mother!