I am new here and to linguistics. Recently I have developed a passion and an interest for linguistics, but I am not familiar with it.

So I got into debate with a person from India. He was claiming Avestan is a daughter language of Vedic Sanskrit.

I showed him Wikipedia which says Avestan is sister to Vedic Sanskrit and not its daughter and that all linguistics have confirmed this, but he asked me how we can be sure about that, so I told him that a good historical linguist should be able to disprove the hypothesis easily.

The normal way of showing these things is to look for irreversible sound changes, when sounds have merged together. It shows that Vedic as attested cannot be derived from Avestan as attested and vice versa, which is why the consensus amongst linguists is that the two languages share a recent common ancestor, called Proto-Indo-Iranian.

To which he replied, “What are some of those irreversible sound changes?”

Now due to my insufficient knowledge of linguistics I am stuck here.

Can anyone help me with this?

What are some of the irreversible sound changes and other linguistic evidence (other than the lack of retroflex sounds in Avestan) which prove that Avestan as attested cannot be derived from Vedic and vice versa