Way back in the times of the Latin language, there were two different verbs, but not with the same meaning as today:
sum, es, esse, fui, meaning "ser, estar, haber". This verb was the main copulative verb and was used also as auxiliary. The Italian language, for instance, still uses the verb essere as an auxiliary the same way as we use haber.
stô, -âs, -âre, stetî, statum, meaning "estar de pie". This is the tricky part. This verb (infinitive: stâre) is the ancestor of modern estar, and is cognate to English stay and