Is 'would have' always the third conditional?

Question

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!

Answer 1

It's your "modal of likely truth".

In the second sentence of your paragraph 'a' that is obvious, Stone Age children needed to chew their food more than modern children.

In the first sentence the exclusion of all Stone Age children from the group of those eating processed food appears to contradict it but it is of the same form as

"Any Stone Age ten-year-old would have been living on foraged plant matter, fresh hunted meat and, probably, some form of jerky".

This is the likely truth.

Replacing "any" with "no" and a list of Stone Age food with a list of junk food does not change the grammatical form or the likelihood of the statement's being true. It reverses the argument to say what Stone Age children were not eating but saying that Stone Age children did not live on junk food is still the likely truth.