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 stand. If you think about it, when you are standing, you are positioned in a given place at a given time, that is precisely the main difference between ser and estar. The latter can be assumed as the same as ser, but in a given moment in time.
So these two verbs are the roots of today's ser and estar. But the DLE says that ser comes from seer, so what verb is that?