Why Яковлев/Яковлевич with the intrusive -л-?

Question

As the title says, why are the surname and patronymic formed from Яков Яковлев and Яковлевич instead of *Яковов and *Яковович? Where did this -л- come from? Is it simply because Яковлев(ич) is more euphonious than *Яковов(ич)? If so, аre there other examples where words or names formed from a root add an -л- through the same process by which it was added to Яков?

Also, am I correct in thinking -л- was chosen for the same reasons -л- shows up in certain first-person verb conjugations, i.e. оставить --> оставлю?

Спасибо заранее.

Answer 1

Surnames and patronimic names were formed with suffixes. The mystery of the surnames Ивлев(Иевлев) and Яковлев is explained in the book by Убенгаун Б.О. Русские фамилии. They were not formed directly from Christian names (Яков, Иов), but from the adjective with the suffix -л(ь)- meaning "belonging to "(Яковль, Иевль). We can still see this suffix in the names of old Russian cities (Ярославль-belonging to Ярослав (the king). They say the usage of two patronimic suffixes helped to avoid pronunciation difficulties.(-овов).

Яков--Яковль--Яковлев--Яковлевич.

Answer 2

Also, am I correct in thinking -л- was chosen for the same reasons -л- shows up in certain first-person verb conjugations, i.e. оставить --> оставлю?

Exactly, this is so called "epenthetic l". It was inserted in Proto-Slavic between a labial consonant and j, and is common for East Slavic languages in all positions, and all Slavic languages in the first syllable.

That's why we have word pairs like земля / земной, рубль / рубить, травля / травить etc.

If so, аre there other examples where words or names formed from a root add an -л- through the same process by which it was added to Яков

Any root ending in a labial (б, п, в, ф, м) would add it if the paradigm requires putting a j after it.

The paradigm which gave Яковлевич is not productive anymore, however you can see it fossilized in the names like Ярославль, Переяслявль, Лихославль etc.

If we take any productive paradigm, say, the one you mentioned, we can see it applies even to the words borrowed long after the Proto-Slavic split: графить / графлю / графлёный etc.