Maybe you heard this one. Underneath all the layers of emotion there are only two emotions at the bottom. In the beginning they have no names on them, no thoughts attached, just a feeling of good or a feeling of bad. Fear and love.
That is spiritual talk, not science, but it is pointing at something real. What is actually wired in from day one is two drives. One pulls you toward things. One pushes you away from things. The scientists call them the appetitive and the defensive systems. A newborn does not know either word. All it knows is the pull toward warmth, milk, a face, a body holding it, and the recoil from cold, from hunger, from a loud noise, from the floor dropping out. Everything you later call love or fear grows out of those two moves.
The pushing away drive owns all the startle stuff. Hunger sits near it, loosely, though hunger runs on its own wiring and only turns into fear when nobody comes. The pulling toward drive carries the need for love and touch, and that is not a greeting card. Babies who do not get held do not thrive. They fail.
The specific fears show up later, and they show up on a schedule. Somewhere around six to nine months the baby starts sizing up strangers and backing off from them. Around two, the dark gets scary, and it keeps climbing from four to six. That is when the branches on the tree turn into claws, when the shadow in the corner starts wanting something, when a face too close with too many teeth reads as a threat instead of a smile. The machinery was there the whole time. The world just had to teach it where to aim.
Once you see that it is only these two down there, fear and love, you start to see how the rest of your head got built. Your whole way of thinking is sitting on top of those two. There is nothing wrong with that foundation. It only goes bad when a kid gets neglected, startled, unloved, starved of love, hurt, or abused.
And do not think the old cultures had this figured out any better than we do. They did not. They had bigger problems in front of them every day.