The most basic instincts are hunger, attachment and fear. These control us as children, and are basic instincts. These are overlain with ongoing imprinting until these are essentially covered, but can arise at any time, should the situation arise.
It is likely that hunger causes overeating if we are under weight, and perhaps slightly overweight; however, if survival is the main factor, instinct should not be driving gross obesity. It is more likely some twist of the mind.
The ego is part of the mind, built up in our minds, which is the reflections of those who went before, mainly learned, or collective ego. It is learned; therefore, impersonal. It is not me, I just carry it, just as all knowledge. It can be changed, unlearned, or learned over. It is impermanent. There is no fixed me.
I am just the awareness that is behind the thoughts. Mostly free of time, but I age, so even awareness changes. I am impermanent. The awareness came at an early age, and memory came after that. Awareness may start developing at conception or shortly after. We humans are not, as individuals, senescent beings, for we would die out without other. We all come from the joining of two parents, and by birth, become totally separate beings, dependent but separate. From then on we are separate, often toughing but still separate. We may join with another to create life, but we remain separate. Through intimacy we can reduce that separation but we can never be "one with everything".