Where does “ея” come from?

ея (Russian) comes from Old East Slavic еѣ, from Proto-Slavic jь, from Proto-Balto-Slavic iś, from Old Polish ić, from Proto-Slavic jьti, from Proto-Balto-Slavic eitei, from Proto-Indo-European h₁éyti, from Proto-Indo-European h₁ey- — to go.

Ancestry of “ея”, step by step

ея traces back through two separate lines of ancestry.

via Old East Slavic еѣ

StepLanguageWordMeaning
1Old East Slavicеѣ
2Proto-Slavicthis; he
3Proto-Balto-Slavicthis
4Old Polishto go
5Proto-Slavicjьtito go
6Proto-Balto-Slaviceiteito go
7Proto-Indo-Europeanh₁éytito go, to be going
8Proto-Indo-Europeanh₁ey-to go

via Old Church Slavonic ѥѩ

StepLanguageWordMeaning
1Old Church Slavonicѥѩ
2Proto-Slavic*jь#Pronoun
Every word from Proto-Indo-European h₁ey-Every word from Proto-Indo-European h₁éytiEvery word from Proto-Balto-Slavic eitei